Maharashtra Board Class 10 Science Chapter 3 Chemical Reactions and Equations Solutions

Std 10 Science Part 1 Chapter 3 Chemical Reactions And Equations Question Answer Maharashtra Board

Class 10 Science Part 1 Chapter 3 Chemical Reactions And Equations Question Answer Maharashtra Board

 

Question 1.Choose the correct option from the bracket and explain the statement giving reasons (Oxidation, displacement, electrolysis, reduction, zinc, copper, double displacement, decomposition)

 

Question 1.a. To prevent rusting, a layer of ........ metal is applied on iron sheets.
Answer: To prevent rusting, a layer of zinc metal is applied on iron sheets. The rusting of iron is an oxidation process. Due to corrosion of an iron a deposit of reddish substance (Fe2O3.H2O) is formed on it. This substance is called rust. To prevent corrosion, a layer of zinc metal (galvanisation) is applied on iron sheets.
In simple words: Applying a layer of zinc (galvanization) on iron prevents rusting because it's an oxidation process, forming reddish rust (Fe2O3.H2O), and zinc protects the iron from corroding.

🎯 Exam Tip: Students should understand that galvanisation is a practical application of electrochemistry for corrosion prevention.

 

Question 1.b. The conversion or ferrous sulphate to ferric sulphate is .... reaction.
Answer: The conversion of ferrous sulphate to ferric sulphate is an oxidation reaction. When ferric ion is formed from ferrous ion, the positive charge is increased by one unit. while this happens the ferrous ion loses one electron. A process in which a metal or its ion loses one or more electrons is called an oxidation. \[ 2FeSO_4 \rightarrow Fe_2(SO_4)_3 \] \[ Fe^{2+} + SO_4^{2-} \rightarrow 2Fe^{3+} + SO_4^{2-} \] Net reaction: \( Fe^{2+} \rightarrow Fe^{3+} + e^- \) Ferrous Ferric
In simple words: The change from ferrous sulphate to ferric sulphate is an oxidation reaction where the ferrous ion loses an electron and its positive charge increases.

🎯 Exam Tip: Focus on the change in oxidation state and electron transfer to identify oxidation reactions accurately.

 

Question 1.c. When electric current is passed through acidulated water ........ of water takes place.
Answer: when electric current is passed through acidulated water decomposition of water takes place. In this reaction. hydrogen and oxygen gas are formed. This decomposition takes place with the help of an electric current, it is also called electrolytic decomposition. \[ 2H_2O_{(l)} \xrightarrow{\text{electric current}} 2H_2\uparrow + O_2\uparrow \]
In simple words: Passing electric current through acidulated water causes it to decompose into hydrogen and oxygen gas, a process known as electrolytic decomposition.

🎯 Exam Tip: Remember that electrolysis is a type of decomposition reaction driven by electrical energy, producing gases from a liquid compound.

 

Question 1.d. Addition of an aqueous solution of ZnSO4 to an aqueous solution of BaCl2 is an example of ........ reaction.
Answer: Addition of an aqueous solution of ZnSO4 to an aqueous solution or BaCl2 is an example or double displacement reaction. \[ BaCl_{2(aq)} + ZnSO_{4(aq)} \rightarrow BaSO_4\downarrow + ZnCl_{2(aq)} \] Barium Zinc Barium Zinc chloride sulphate sulphate chloride Barium chloride reacts with zinc sulphate to form a white precipitate of barium sulphate. white precipitate is formed by exchange of ions \( Ba^{++} \) and \( SO_4^{--} \) between the reactants.
In simple words: Mixing zinc sulphate and barium chloride solutions results in a double displacement reaction, where ions exchange to form a white precipitate of barium sulphate.

🎯 Exam Tip: Double displacement reactions often lead to the formation of a precipitate, gas, or water due to the exchange of ionic components.

 

Question 2.a. What is the reaction called when oxidation and reduction take place simultaneously? Explain with one example.
Answer: The reaction which involves simultaneous oxidation and reduction is called an oxidation-reduction or redox reaction. In a redox reaction, one reactant gets oxidised while the other gets reduced during a reaction. Redox reaction = Reduction + Oxidation In redox reaction, the reductant is oxidized by the oxidant and the oxidant is reduced by the reductant. Example: \( CuO_{(s)} + H_{2(g)} \rightarrow Cu_{(s)} + H_2O \) In this reaction, oxygen is removed from copper oxide therefore it is a reduction of CuO, while hydrogen accepts oxygen to form water that means oxidation of hydrogen takes place. Thus oxidation and reduction reactions occur simultaneously.
In simple words: A redox reaction involves both oxidation (loss of electrons or gain of oxygen) and reduction (gain of electrons or loss of oxygen) happening at the same time, where one substance is oxidized and another is reduced.

🎯 Exam Tip: When identifying redox reactions, always look for simultaneous changes in oxidation states or transfer of oxygen/hydrogen atoms between reactants.

 

Question 2.d. Explain the types of reactions with reference to oxygen and hydrogen. Illustrate with examples.
Answer: With reference to oxygen and hydrogen, there are two types of reaction
1. Oxidation reaction
2. Reduction reaction.
1. Oxidation reaction: Examples:
(1) When carbon burns in air, it forms carbon dioxide. In this reaction carbon accepts oxygen, therefore, this is an oxidation reaction. \[ C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)} \]
(2) When sodium reacts with ethyl alcohol, sodium ethoxide and hydrogen gas is formed. In this reaction, hydrogen is removed from ethyl alcohol, therefore this is an oxidation reaction. \[ 2C_2H_5OH_{(l)} + 2Na \rightarrow 2C_2H_5ONa + H_2\uparrow \] Ethyl Sodium Sodium Hydrogen alcohol ethoxide
(3) Acidified potassium dichromate (\( K_2Cr_2O_7 / H_2SO_4 \)) oxidises ethly alcohol to acetic acid. \[ CH_3-CH_2-OH \xrightarrow{2[O] \text{ K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4} CH_3-COOH + H_2O \]
2. Reduction reaction: Examples:
(1) When hydrogen gas is passed over black copper oxide a reddish coloured layer of copper is formed. In this reaction an oxygen atom removed from CuO to form copper, hence, this is reduction. \[ CuO_{(s)} + H_{2(g)} \rightarrow Cu_{(s)} + H_2O_{(l)} \] Copper oxide Copper
(2) when hydrogen gas is passed over red hot coke, methane is obtained. Here, hydrogen is added to coke (carbon). Hence, this is reduction. \[ C_{(s)} + 2H_{2(g)} \rightarrow CH_{4(g)} \] Carbon Hydrogen gas Methane
In simple words: Oxidation is a reaction where a substance gains oxygen or loses hydrogen, while reduction is where it loses oxygen or gains hydrogen. These reactions are fundamental in chemistry for understanding how substances change.

🎯 Exam Tip: Clearly distinguish between the definitions of oxidation and reduction based on both oxygen/hydrogen transfer and electron transfer, and memorize common examples for each type.

 

Question 2.e. Explain the similarity and difference in two events, namely adding NaOH to water and adding CaO to water.
Answer: Similarity: Both NaOH and CaO, when dissolved separately in water, solid NaOH dissolves releasing heat, resulting in rise in temperature. This reaction is exothermic reaction. When solid CaO dissolves in water, \( Ca(OH)_2 \) is formed, large amount of heat is evolved. This reaction is also exothermic reaction. Both reactions are combination reactions and single product is obtained. \[ NaOH_{(s)} + H_2O \rightarrow NaOH_{(aq)} + \text{Heat} \] \[ CaO_{(s)} + H_2O \rightarrow Ca(OH)_{2(aq)} + \text{Heat} \] Difference:
1. Aqueous solution of NaOH is considered as a strong alkali.
2. Aqueous solution of \( Ca(OH)_2 \) is considered as a weak alkali.
In simple words: Both NaOH and CaO dissolving in water are exothermic combination reactions that release heat. The main difference is that NaOH forms a strong alkali, while CaO forms a weak alkali.

🎯 Exam Tip: Focus on identifying whether heat is released (exothermic) or absorbed (endothermic) and if a single product is formed (combination reaction) when analyzing such chemical events.

 

Question 3.Explain the following terms with examples.

 

Question 3.a. Endothermic reaction
Answer: Endothermic reaction: The reaction in which heat is absorbed is called an endothermic reaction. when \( KNO_{3(s)} \) dissolves in water, there is absorption of heat during the reaction and the temperature of the solution falls. \[ KNO_{2(s)} + H_2O_{(l)} + \text{Heat} \rightarrow KNO_{3(aq)} \]
In simple words: An endothermic reaction is a chemical process that absorbs heat from its surroundings, causing the temperature of the surroundings to drop.

🎯 Exam Tip: To identify an endothermic reaction, look for "Heat" or "Energy" written as a reactant in the chemical equation, or a decrease in temperature in the observation.

 

Question 3.b. Combination reaction
Answer: When two or more reactants combine in a reaction to form a single product, it is called a combination reaction. Examples:
1. The ammonia gas reacts with hydrogen chloride gas to form the salt in gaseous state, immediately it condenses at room temperature and gets transformed into the solid state. \[ NH_{3(g)} + HCl_{(g)} \rightarrow NH_4Cl_{(s)} \] Ammonia Hydrogen Ammonium chloride chloride (a single product)
2. Magnesium burns in air to form white powder of magnesium oxide as a single product. \[ 2Mg + O_2 \xrightarrow{\text{heat}} 2MgO \] Magnesium Magnesium oxide (a single product)
3. Iron reacts with sulphur to form iron sulphide. \[ Fe_{(s)} + S_{(s)} \text{ heat } \leftarrow FeS_{(s)} \] Iron Sulphur Iron sulphide
In simple words: A combination reaction occurs when two or more simple substances or compounds chemically join together to form a single, more complex product.

🎯 Exam Tip: The key characteristic of a combination reaction is that it always yields only one product from multiple reactants, often releasing energy.

 

Question 3.c. Balanced equation
Answer: In a chemical reaction, the number of atoms of the elements in the reactants is same as the number or atoms of those elements in the product, such an equation is called a balanced equation. Example: \( AgNO_3 + NaCl \rightarrow AgCl + NaNO_3 \) In the above reaction, the number of atoms of the elements in the reactants is same as the number of atoms of elements in the products.
In simple words: A balanced chemical equation shows that the number of atoms for each element is equal on both sides of the reaction arrow, adhering to the law of conservation of mass.

🎯 Exam Tip: Always double-check that the total number of atoms for each element is identical on both the reactant and product sides of the equation to ensure it is balanced.

 

Question 3.d. Displacement reaction
Answer: The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by the formation of its own ions is called displacement reaction. When zinc granules are added to the blue coloured copper sulphate solution, the zinc ions formed from zinc atoms take the place of \( Cu^{2+} \) ions in \( CuSO_4 \), and copper atoms, formed from \( Cu^{2+} \) ions comes out i.e. the more reactive zinc displaces the less reactive Cu from copper sulphate. \[ Zn_{(s)} + CuSO_{4(aq)} \rightarrow ZnSO_{4(aq)} + Cu_{(s)} + \text{Heat} \] zinc copper zinc sulphate copper sulphate (colourless)
In simple words: A displacement reaction happens when a more reactive element replaces a less reactive element from its compound.

🎯 Exam Tip: To predict displacement reactions, consult the reactivity series of metals; a metal higher in the series will displace one lower down.

 

Question 4. Give scientific reason:

 

Question 4.a. When the gas formed on heating lime stone is passed through freshly prepared lime water, the lime water turns milky.
Answer: when lime stone is heated, calcium oxide and carbon dioxide are formed. This carbon dioxide gas is passed through freshly prepared lime water, insoluble calcium carbonate and water are formed. In this reaction, lime water turns milky. \[ Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \] Calcium carbonate
In simple words: Limewater turns milky because carbon dioxide, released from heating limestone, reacts with calcium hydroxide in the limewater to form insoluble calcium carbonate.

🎯 Exam Tip: This reaction is a classic test for carbon dioxide gas, producing a visible precipitate that indicates its presence.

 

Question 4.b. It takes time for pieces of Shahabud tile to disappear in HCl, but its powder disappears rapidly.
Answer: The rate of a reaction depends upon the size of the particles of the reactants taking part in the reaction. The smaller the size of the reactants particles, the more is their total surface area and the faster is the rate of reaction. In the reaction of dil. HCl with pieces of Shahabad tile, \( CO_2 \) effervescence is formed amid the tile disappears slowly. On the other hand. \( CO_2 \) effervescence forms at faster rate with Shahabad tile powder and it disappears rapidly.
In simple words: Powdered Shahabad tile reacts faster with HCl than tile pieces because the smaller particle size of the powder increases the total surface area available for the reaction, speeding up the process.

🎯 Exam Tip: Remember that increasing the surface area of solid reactants by crushing them into powder significantly increases the rate of reaction.

 

Question 4.c. While preparing dilute sulphuric acid from concentrated sulphuric acid in the laboratory, the concentrated sulphuric acid is added slowly to water with constant stirring.
Answer: (1) The preparation of dilute sulphuric acid falls in the category of extreme exothermic process.
(2) During the preparation of dilute sulphuric acid. large amount of water is taken in a glass container which is surrounded by ice. Cool it for twenty minutes, Now small quantity of conc. \( H_2SO_4 \) is added slowly with stirring. Therefore, only a small amount of heat is liberated at a time. In this way dilute sulphuric acid is prepared.
(3) On the other hand, in the process of dilution or conc. sulphuric acid with water, very large amount of heat is liberated. As a result, water gets evaported instantaneously, if it is poured in to conc. \( H_2SO_4 \) which may cause an accident.
In simple words: Concentrated sulphuric acid is added slowly to water with constant stirring during dilution because the process is highly exothermic, and this method safely dissipates the heat generated, preventing dangerous boiling or splashing.

🎯 Exam Tip: Always add acid to water, never water to acid, to ensure safety during dilution of concentrated acids like sulphuric acid due to its highly exothermic nature.

 

Question 4.d. It is recommended to use air tight container for storing oil for long time.
Answer: 1. If edible oil is allowed to stand for a long time, it undergoes air oxidation, it becomes rancid and its smell and taste changes.
2. Rancidity in the food stuff cooked in oil or ghee is prevented by using antioxidants. The process of oxidation reaction of food stuff can also be slowed down by storing it in air tight container.
In simple words: Air-tight containers are recommended for storing oil to prevent rancidity, which is the oxidation of fats by air, causing changes in smell and taste.

🎯 Exam Tip: Storing food items containing fats and oils in airtight containers is a key method to slow down oxidative spoilage and extend their shelf life.

 

Question 5.Observe the following picture a write down the chemical reaction with explanation.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र लोहे पर जंग लगने की प्रक्रिया को दर्शाता है। एक पानी की बूंद लोहे की सतह पर है, जिसके नीचे एनोड और कैथोड क्षेत्र दिखाए गए हैं जहाँ क्रमशः Fe और O2 प्रतिक्रिया करते हैं। एकत्रित जंग (Fe2O3.H2O) को भी दर्शाया गया है।

 

Answer: The rusting of iron is an oxidation process. The rust on iron does not form by a simple reaction between oxygen and iron surface. The rust is formed by an electrochemical reaction. Fe oxidises to Fe2+ in the anode region while oxygen gets reduced to \( H_2O \) in the cathode region. Different regions on the surface of iron become anode and cathode.
(1) Fe is oxidised to \( Fe^{2+} \) in the anode region. \[ Fe_{(s)} \rightarrow Fe^{2+}_{(aq)} + 2e^- \]
(2) \( O_2 \) is reduced to form water in the cathode region. \[ O_{2(g)} + 4H^+_{(aq)} + 4e^- \rightarrow 2H_2O_{(l)} \] When \( Fe^{2+} \) ions migrate from the anode region they react with water and further get oxidised to form \( Fe^{3+} \) ions. A reddish coloured hydrated oxide is formed from \( Fe^{3+} \) ions. It is called rust. It collects on the surface. \[ 2Fe^{3+}_{(aq)} + 4H_2O_{(l)} \rightarrow Fe_2O_3.H_2O_{(s)} + 6H^+_{(aq)}... \] Because of various components in the atmosphere, oxidation of metals takes place, consequently resulting in their damage. This is called 'corrosion'. Iron rusts and a reddish coloured layer is formed on it. This is corrosion of iron.
In simple words: Rusting is an electrochemical oxidation process where iron reacts with oxygen and water to form hydrated iron (III) oxide (rust), occurring in distinct anode and cathode regions on the metal surface, leading to its degradation.

🎯 Exam Tip: Understand that rusting is a specific type of corrosion that requires both oxygen and moisture, and it involves both oxidation and reduction reactions occurring simultaneously on the iron surface.

 

Question 6.Identify from the following reactions the reactants that undergo oxidation and reduction.

 

Question 6.a. \( Fe + S \rightarrow FeS \)
Answer: \( Fe + S \rightarrow FeS \) In this reaction, Iron (Fe) undergoes oxidation and sulphur. (S) undergoes reduction.
In simple words: In this reaction, iron loses electrons and is oxidized, while sulfur gains electrons and is reduced.

🎯 Exam Tip: To identify oxidation and reduction, track changes in oxidation states: an increase means oxidation, a decrease means reduction.

 

Question 6.b. \( 2Ag_2O \rightarrow 4Ag + O_2\uparrow \)
Answer: \( 2Ag_2O \rightarrow 4Ag + O_2\uparrow \) In this reaction, reduction of \( Ag_2O \) takes place.
In simple words: In this reaction, silver oxide loses oxygen to form silver, indicating that \( Ag_2O \) undergoes reduction.

🎯 Exam Tip: Decomposition reactions, especially those involving metal oxides yielding metal and oxygen, often exemplify reduction of the metal compound.

 

Question 6.c. \( 2Mg + O_2 \rightarrow 2MgO \)
Answer: \( 2Mg + O_2 \rightarrow 2MgO \) In this reaction, oxidation of Mg takes place.
In simple words: Magnesium combines with oxygen to form magnesium oxide, meaning magnesium undergoes oxidation.

🎯 Exam Tip: The addition of oxygen to an element is a straightforward indicator of an oxidation reaction.

 

Question 6.d. \( NiO + H_2 \rightarrow Ni + H_2O \)
Answer: \( NiO + H_2 \rightarrow Ni + H_2O \) In this reaction, reduction of NiO takes place and oxidation of \( H_2 \) takes place.
In simple words: Nickel oxide loses oxygen (reduction) to become nickel, while hydrogen gains oxygen (oxidation) to become water.

🎯 Exam Tip: In reactions involving hydrogen and metal oxides, hydrogen often acts as the reducing agent, itself getting oxidized, while the metal oxide is reduced.

 

Question 7.Balance the following equation stepwise.

 

Question 7.a. \( H_2S_2O_{7(l)} + H_2O_{(l)} \rightarrow H_2SO_{4(l)} \)
Answer: Step 1: Rewrite the given equation as it is \( H_2S_2O_{7(l)} + H_2O_{(l)} \rightarrow H_2SO_{4(l)} \) Step 2: write the number or atoms of each element in the unbalanced equation on both sides of equations.

Element	Number of atoms in reactant (left side)	Number of atoms in products (right side)
H	4	2
S	2	1
O	8	4

Step 3: To equalise the number of hydrogen atoms, sulphur atoms and oxygen atoms we use 2 as the coemficient or factor in the product.

Element	Number of atoms in reactant (left side)	Number of atoms in products (right side)
H	4	2 × 2
S	2	1 x 2
O	8	4 x 2
Total	14	14

Now the equation becomes \( H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4 \) Now, count the atoms of each element on both sides of the equation. The number of atoms on both sides are equal. Hence, the balanced equation is \( H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4 \) Now indicate the physical states of the reactants and products. \[ H_2S_2O_{7(l)} + H_2O_{(l)} \rightarrow 2H_2SO_{4(l)} \]
In simple words: To balance the equation, we first count atoms on both sides, then adjust coefficients to ensure equal numbers of each element, finally verifying the physical states of reactants and products.

🎯 Exam Tip: Always balance the atoms one by one, usually starting with metals, then non-metals (other than O and H), then oxygen, and finally hydrogen. Recheck all atoms after each adjustment.

 

Question 7.b. \( SO_{2(g)} + H_2S_{(aq)} \rightarrow S_{(s)} + H_2O_{(l)} \)
Answer: Step 1: Rewrite the given equation as it is \( SO_{2(g)} + H_2S_{(aq)} \rightarrow S_{(s)} + H_2O_{(l)} \) Step 2: Write the number of atoms of each element in the unbalanced equation on both sides of equations.

Element	Number of atoms in reactants (left side)	Number of atoms in products (right side)
S	2	1
O	2	1
H	2	2

The number of hydrogen atoms on both sides of the equation is same, therefore, equalise the number of sulphur atoms and oxygen atoms. Step 3: To balance the number of sulphur atoms:

Number of atoms of sulphur	In reactants		In products
	\( S_2O \)	\( H_2S \)	(S)
Initially	1	1	1
To balance	1	1	1 x 2

To equalise the number of sulphur atoms, we use 2 as the factor in the product, now the equation becomes \( SO_2 + H_2S \rightarrow 2S + H_2O \) Step 4: To equalise the number of oxygen atoms in the unbalanced equation.

Number of atoms of oxygen	In reactants (\( SO_2 \))	In products \( H_2O \)
Initially	2	1
To balance	2	1 x 2

To equalise the number of oxygen atoms, we use 2 as the factor in the product i.e. \( H_2O \), now the unbalanced equation becomes \( SO_2 + H_2S \rightarrow 2S + 2H_2O \) Step 5: To equalise the number of hydrogen atoms in unbalanced equation:

Number of atoms of hydrogen	In reactants (\( H_2S \))	In products (\( H_2O \))
Initially	2	4
To balance	2 × 2	4

To equalise the number of hydrogen atoms we use 2 as the factor in the reactant i.e, \( H_2S \), now the unbalanced equation become \( SO_2 + 2H_2S \rightarrow 2S + 2H_2O \) Now, count the atoms of each element on both sides of the equation, there are less number of sulphur atoms in the product. Now equalise the sulphur atoms, the balanced equation becomes, \( SO_2 + 2H_2S \rightarrow 3S + 2H_2O \) Now indicate the physical states of reactants and products. \[ SO_{2(g)} + 2H_2S_{(aq)} \rightarrow 3S_{(s)} + 2H_2O_{(l)} \]
In simple words: Balancing this equation involves a multi-step process where we systematically adjust coefficients for sulfur, oxygen, and hydrogen atoms to ensure the same number of each element on both sides of the reaction.

🎯 Exam Tip: When balancing complex redox reactions, it's often helpful to balance elements other than oxygen and hydrogen first, then oxygen, and finally hydrogen, adjusting coefficients iteratively.

 

Question 7.c. \( Ag_{(s)} + HCl_{(l)} \rightarrow AgCl \downarrow + H_2 \uparrow \)
Answer: Step 1: Rewrite the given equation as it is \( Ag_{(s)} + HCl_{(l)} \rightarrow AgCl \downarrow + H_2 \uparrow \) Step 2: write the number of atoms or each element in the unbalanced equation on both sides of equations.

Element	Number of atoms in reactants (left side)	Number of atoms in products (right side)
Ag	1	1
H	1	2
Cl	1	1

The number of silver and chlorine atoms on both sides of the equation are same, therefore, equalise the number of hydrogen atoms. Step 3: To balance the number of hydrogen atoms.

Number of atoms of hydrogen	In reactants HCl	In products \( H_2 \)
Initially	1	2
To balance	1 × 2	2

To equalise the number of hydrogen atoms, we use 2 as the factor in the product HCl, now the unbalanced equation become \( Ag_{(s)} + 2HCl \rightarrow AgCl + H_2 \) Step 4: To balance the number of chlorine atoms:

Number of atoms of chlorine	In reactants (2HCl)	In products (AgCl)
Initially	2	1
To balance	2	2 x 1

To equalise the number of chlorine atoms, we use 2 as the factor in the product AgCl. now the unbalanced equation becomes \( Ag + 2HCl \rightarrow 2AgCl + H_2 \) Now count the atoms of each element on both sides of the equation, there are less number of silver atoms in the reactant. Now equalise the silver atoms, the balanced equation becomes \( 2Ag + 2HCl \rightarrow 2AgCl + H_2 \) Now indicate the physical states of the reactunts and products \[ 2Ag_{(s)} + 2HCl_{(l)} \rightarrow 2AgCl \downarrow + H_2 \uparrow \]
In simple words: To balance this reaction, we systematically adjusted coefficients for hydrogen and then chlorine, which consequently required balancing silver atoms, until all elements were equal on both sides.

🎯 Exam Tip: Balancing equations is a systematic process. Always re-evaluate all element counts after each coefficient adjustment to ensure overall balance is maintained.

 

To equalise the number of chlorine atoms, we use 2 as the factor in the product AgCl. now the unbalanced equation becomes
Ag + 2HCl → 2AgCl + H2
Now count the atoms of each element on both sides of the equation, there are less number of silver atoms in the reactant. Now equalise the silver atoms, the balanced equation becomes
2Ag + 2HCl → 2AgCl + H2
Now indicate the physical states of the reactunts and products
\[2Ag(s) + 2HCl(l) \rightarrow 2AgCl \downarrow + H_2 \uparrow\]
In simple words: Balancing a chemical equation involves ensuring the same number of atoms for each element on both reactant and product sides. For this specific reaction, adjusting coefficients for silver and hydrochloric acid helps achieve this balance, with silver chloride forming a precipitate.

🎯 Exam Tip: When balancing equations involving multiple elements, it's often strategic to balance elements that appear in fewer compounds first, and save hydrogen and oxygen for last, or elements that appear in a single reactant and a single product. Pay attention to the physical states for full marks.

 

Question 7.
d. H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)
Answer:
Step 1:
Rewrite the given equation as it is
H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)
Step 2:
write the number of atoms of each element in the unbalanced equation on both sides of the equation.

Element	Number of atoms in reactants	Number of atoms in products
Na	1	2
S	1	1
Ο	5	5
H	3	2

The number of oxygen atoms involved in different compounds on both sides (reactants and products) are equal. Therefore, balance the number of atoms of the second element, sodium.
Step 3:
To balance the number of sodium atoms:

Number of atoms of sodium	In reactants	In products
To begin with	1 (in NaOH)	2 (in Na2SO4)
To balance	1 × 2	2

To equalise the number of sodium atoms, we use 2 as the factor of NaOH in the reactants. Now, the partly balanced equation becomes
H2SO4 + 2NaOH → Na2SO4 + H2O
Step 4:
Now, balance the number of hydrogen atoms:

Number of atoms of hydrogen	In reactants	In products
To begin with	2 (in H2SO4) 2 (in NaOH)	2 (in H2O)
To balance	4	2 × 2

To equalise the number of hydrogen atoms, we use 2 as the factor or H2O in the products. The equation then becomes
H2SO4 + 2NaOH → Na2SO4 + 2H2O
Now, count the atoms of each element on both sides of the equation. The number of atoms on both sides are equal. Hence, the balanced equation is
H2SO4 + 2NaOH → Na2SO4 + 2H2O
Now indicate the physical states of the reactants and products.
\[H_2SO_4(aq) + 2NaOH(aq) \rightarrow Na_2SO_4(aq) + 2H_2O(l)\]
In simple words: This step-by-step process demonstrates how to balance the given chemical equation by ensuring the same number of atoms for sodium and hydrogen on both sides, resulting in a balanced neutralization reaction.

🎯 Exam Tip: When balancing a neutralization reaction, remember that balancing metal atoms (like Na) and non-metal atoms first, followed by hydrogen and oxygen, is often an effective strategy. Always re-verify the count for all atoms at the end.

 

Question 8.
Identify the endothermic and exothermic reaction.
a. HCl + NaOH → NaCl + H2O + heat
Answer:
Exothermic reaction.
In simple words: This reaction is exothermic because heat is released, indicating that energy is given out to the surroundings during the chemical change.

🎯 Exam Tip: Look for "heat" on the product side of a chemical equation (or a rise in temperature) to identify an exothermic reaction, where energy is released.

 

b. 2KClO3(s) \( \xrightarrow{\Delta} \) 2KCl(s) + 3O2↑
Answer:
Exothermic reaction.
In simple words: This reaction is exothermic as it involves the decomposition of potassium chlorate, releasing oxygen and heat in the process, similar to combustion reactions.

🎯 Exam Tip: The delta symbol (\( \Delta \)) above the arrow typically indicates that heat is supplied for the reaction to occur, but the decomposition itself can be exothermic once initiated, or the question implies a net release. In exam context, clearly stating the heat aspect is crucial.

 

c. CaO + H2O → Ca(OH)2 + heat
Answer:
Exothermic reaction.
In simple words: This is an exothermic reaction because the mixing of calcium oxide with water releases a significant amount of heat, indicating energy is given off.

🎯 Exam Tip: Reactions forming hydroxides from metal oxides and water are often highly exothermic. Recognizing "heat" on the product side is a clear indicator.

 

d. CaCO3(s) \( \xrightarrow{\Delta} \) CaO(s) + CO2↑
Answer:
Exothermic reaction.
In simple words: This decomposition of calcium carbonate is typically endothermic because it requires continuous heat input to break bonds, but given the preceding context, it might be miscategorized in the source. Based on standard chemistry, thermal decomposition is generally endothermic. Assuming the provided answer is from the source, it's categorized as exothermic, which is unusual for a thermal decomposition.

🎯 Exam Tip: Thermal decomposition reactions generally require heat absorption to break chemical bonds, making them endothermic. If "heat" is above the arrow, it signifies energy input, thus an endothermic process.

 

Question 9.
Match the column in the following table:

Reactants	products	Type of chemical reaction
BaCl2(aq) + ZnSO4(aq)	H2CO3(aq)	Displacement
2 AgCl(s)	FeSO4(aq) + Cu(s)	Combination
CuSO4(aq) + Fe(s)	BaSO4↓ + ZnCl2(aq)	Decomposition
H2O(l) + CO2(g)	2Ag(s) + Cl2(g)	Double displacement

Answer:

Reactants	products	Type of chemical reaction
BaCl2(aq) + ZnSO4(aq)	BaSO4↓ + ZnCl2(aq)	Double displacement
2 AgCl(s)	2Ag(s) + Cl2(g)	Decomposition
CuSO4(aq) + Fe(s)	FeSO4(aq) + Cu(s)	Displacement
H2O(l) + CO2(g)	H2CO3(aq)	Combination

In simple words: This matching exercise helps categorize different chemical reactions based on their reactants and products. Double displacement involves ion exchange, decomposition breaks a compound, displacement replaces one element with another, and combination forms a single product from multiple reactants.

🎯 Exam Tip: To accurately match reaction types, focus on the fundamental changes: combination (forming one product), decomposition (breaking one reactant), displacement (one element replaces another), and double displacement (ion exchange forming new compounds, often a precipitate).

 

Can you recall? (Text Book Page No.16)

 

Question 1.
what are the types of molecules of elements and compounds?
Answer:
Elements are divided into three classes i.e. metals, nonmetals and metalloids. When two or more elements combine chemically in a fixed proportion by weight, a compound is formed. The properties of a compound are altogether different from those of the constitutional elements.
In simple words: Elements are categorized as metals, nonmetals, or metalloids. Compounds form when multiple elements chemically combine in fixed ratios, creating substances with properties distinct from their original elements.

🎯 Exam Tip: Remember the key difference: elements are fundamental substances, while compounds are formed by the chemical combination of two or more elements in fixed proportions, leading to new properties.

 

Question 2.
what is meant by valency of element?
Answer:
The number of electrons that an atom of an element gives away or takes up while forming an ionic bond, is called the valency or that element.
In simple words: Valency is the combining capacity of an element, determined by how many electrons an atom gains, loses, or shares to form chemical bonds.

🎯 Exam Tip: Valency is a crucial concept for understanding how atoms bond and how to write chemical formulas. It's essentially the 'bonding power' of an element.

 

Question 3.
What is the requirement for writing molecular formulae of different compounds?
How are the molecular formulae of the compounds written?
Answer:
while writing the molecular formulae of different compounds, the symbol of the radicals and their valence should be known.
The number of the ions is written as subscript on the right of the symbol or the ion.
By cross multiplication of valenceies chemical formula is obtained.
In simple words: To write molecular formulas, you need to know the symbols and valencies of the constituent elements or radicals. The valency is then cross-multiplied and written as a subscript to represent the ratio of atoms in the compound.

🎯 Exam Tip: Mastering valencies and the cross-multiplication method is fundamental for correctly writing chemical formulas, which is a core skill in chemistry.

 

Find out (Text Book Page No. 44)

 

Question 1.
How are the blackened silver utensils and patinated (greenish) brass utensils cleaned?
Answer:
The blackened silver utensils and patinated (greenish) brass utensils are cleaned using baking soda, vinegar and lemon mix.
In simple words: Blackened silver and greenish brass utensils can be cleaned using a mixture of baking soda, vinegar, and lemon, which helps remove the tarnished layers.

🎯 Exam Tip: This question relates to practical applications of chemical reactions. Knowing common household remedies for cleaning tarnished metals demonstrates an understanding of basic chemistry in everyday life.

 

Use your brain power! (Text Book Page No. 35)

 

Question 1.
write down the physical states of reactants and products in the reaction
SO2 + 2H2S → 3S + 2H2O
Answer:
Reactants : SO2(g), 2H2S(g)
Products : 3S(s), 2H2O(l).
In simple words: In this reaction, sulfur dioxide and hydrogen sulfide are gases, while sulfur is a solid and water is a liquid.

🎯 Exam Tip: Always specify the physical states (solid (s), liquid (l), gas (g), aqueous (aq)) of reactants and products in chemical equations for completeness and accuracy, as this often carries marks.

 

Question 2.
write down the physical states of reactants and products in the reaction
2Ag + 2HCl → 2AgCl + H2
Answer:
Reactants: 2Ag(s), 2HCl(l)
Products: 2AgCl ↓, H2 ↑
In simple words: In this reaction, solid silver reacts with liquid hydrochloric acid to form solid silver chloride precipitate and hydrogen gas.

🎯 Exam Tip: Remember to use appropriate symbols for physical states (s, l, g, aq) and for precipitates (\( \downarrow \)) and gases (\( \uparrow \)) when writing chemical equations.

 

Question 3.
Identify the reactants and products of the following equation.
\[\text{Vegetable oil}(l) + H_2(g) \xrightarrow{60°C \text{ Ni Catalyst}} \text{Vanaspathi ghee}\]
Answer:
Reactants: vegetable oil, H2(g)
Product: Vanaspathi ghee
In simple words: In this hydrogenation reaction, vegetable oil and hydrogen gas are the substances that react, and vanaspathi ghee is the new substance formed.

🎯 Exam Tip: Clearly distinguishing between reactants (what goes in) and products (what comes out) is fundamental to understanding any chemical reaction. Remember that catalysts and conditions (like temperature) are not reactants or products.

 

Use your brain power! (Text Book Page No. 42)

 

Question 1.
Which is the oxidant used for purification of drinking water?
Answer:
The chlorine based oxidants are used in the purification of drinking water.
In simple words: Chlorine-based substances are commonly used as oxidants to purify drinking water by killing harmful microorganisms and oxidizing impurities.

🎯 Exam Tip: Oxidants play a vital role in water purification by chemically breaking down contaminants. Knowing common examples like chlorine is important for practical chemistry applications.

 

Question 2.
Why is potassium permanganate used during cleaning water tanks?
Answer:
Potassium permanganate is an oxidising agent. It oxidises dissolved iron, manganese and hydrogen sulphide into solid particles that are filtered out of the water tank. It is used to control iron bacteria growth in tank.
In simple words: Potassium permanganate is used because it's a strong oxidizing agent that converts dissolved impurities like iron, manganese, and hydrogen sulfide into solids, making them easy to filter out, and also helps control bacterial growth.

🎯 Exam Tip: Understand that oxidizing agents like potassium permanganate are effective in water treatment because they chemically transform soluble impurities into insoluble forms that can then be physically removed.

 

Can you tell? (Text Book Page No. 43)

 

Question 1.
what is the type of this reaction, in which Vanaspathi ghee is formed from vegetable oil?
Answer:
\[\text{Vegetable oil}(l) + H_2(g) \xrightarrow{60°C \text{ Ni Catalyst}} \text{Vanaspathi ghee}(s).\]
In the preparation of vanaspathi ghee from vegetable oil hydrogen gas is used. This process is known as hydrogenation. This is reduction reaction.
In simple words: The reaction where vanaspathi ghee is formed from vegetable oil by adding hydrogen is called hydrogenation, which is a type of reduction reaction.

🎯 Exam Tip: Hydrogenation is a key industrial process where unsaturated compounds are converted to saturated ones by adding hydrogen, always catalyzed (e.g., Ni) and typically an example of a reduction reaction.

 

Find out (Text Book page No. 33)

 

Question.
What are the other uses of silver nitrate in every day life?
Answer:
Silver nitrate is used in the voters-ink. It is used as reactant in the laboratory. Silver nitrate is used to prevent infection in wounds and skin burns.
In simple words: Beyond laboratory use, silver nitrate is known for its application in indelible voter's ink and for its antiseptic properties to prevent infections in wounds and burns.

🎯 Exam Tip: Questions about everyday uses of chemicals often appear in exams. Knowing the practical applications of compounds like silver nitrate (e.g., in ink, medicine) demonstrates broader chemical literacy.

 

Use your brain power! (Text Book Page No. 35)

 

Question 1.
N2(g) + H2(g) \( \rightleftharpoons \) NH3(g)
Answer:
Step 1:
Rewrite the given equation as it is
N2(g) + H2(g) \( \rightleftharpoons \) NH3(g)
Step 2:
Write the number of atoms of each element in the unbalanced equation on both sides of equations

Element	Number of atoms in reactants	Number of atoms in products
N	2	1
H	2	3

Step 3:
In the given equation. NH3 is a compound and it contains hydrogen element. On the left hand side there are two H atoms and on the right side 3H atoms. Equalise H atoms on both sides.

Hydrogen atoms	In reactants	In products
Initially	2	3
To balance	3 × 2	2 x 3

To equalise the number of hydrogen atoms, we use 3 as the factor in the reactant and 2 as the factor in the products. Now the equation becomes
N2 + 3H2 → 2NH3
Now, count the atoms of each element on both sides of the equation. The number of atoms on both sides are equal. Hence, the balanced equation is
N2 + 3H2 → 2NH3
Now indicate the physical states of the reactants and products
\[N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)\]
In simple words: To balance the formation of ammonia from nitrogen and hydrogen, we adjust the coefficients to ensure there are equal numbers of nitrogen and hydrogen atoms on both sides of the reversible reaction.

🎯 Exam Tip: When balancing equations, especially those with diatomic molecules (like N2, H2) and polyatomic products, it's often easiest to start by balancing the largest or most complex molecule, then proceed to individual elements. Always double-check atom counts on both sides.

 

Question 2.
Calcium chloride + Sulphuric acid → Calcium sulphate + Hydrogen chloride.
Answer:
Step 1:
Write the chemical equation from the given word equation.
CaCl2 + H2SO4 → CaSO4 + HCl
Step 2:
Write the number of atoms of each element in the unbalanced on both sides of equation.

Element	Number of atoms in reactants	Number of atoms in products
Ca	1	1
Cl	2	1
H	2	1
S	1	1
Ο	4	4

Step 3:
In the given equation H2SO4 is a compound and it contains hygrogen element. On the left hand side there are two hydrogen atoms and on the right side one hydrogen atom. Equalise H atoms on both sides.

Hydrogen atoms	In reactants (H2SO4)	In products (HCl)
Initially	2	1
To balance	2	2 x 1

To equalise the number of hydrogen atoms we use 2 as the factor in the product so that the number of H atoms on both sides are equal. Therefore, the equation becomes
CaCl2 + H2SO4 → CaSO4 + 2 HCl
Now, count the atoms of each element on both sides of the equation. The number of atoms on both sides are equal hence, the balanced equation is
CaCl2 + H2SO4 → CaSO4 + 2 HCl
Now, indicate the physical state of the reactants and products.
\[CaCl_2(s) + H_2SO_4(l) \rightarrow CaSO_4(s) + HCl(l)\]
In simple words: To balance this reaction, we first convert the word equation to a chemical formula, then count atoms on both sides. By adding a coefficient of 2 to HCl on the product side, we balance the hydrogen and chlorine atoms, achieving a complete balanced equation.

🎯 Exam Tip: For balancing equations derived from word descriptions, first write the correct chemical formulas for all reactants and products. Then, balance elements that are part of polyatomic ions or appear only once on each side before tackling hydrogen and oxygen.

 

Can you tell? (Text Book Page No. 39)

 

Question.
Take into account the time required for following processes. Classify them into two groups and give titles to the groups.
(1) Cooking gas starts burning on ignition.
(2) Iron article undergoes rusting.
(3) Erosion of rocks takes place to form soil.
(4) Alcohol is formed on mixing yeast in glucose solution under proper condition.
(5) Effervescence is formed on adding baking soda into a test tube containing dilute acid.
(6) A white precipitate is formed on adding dilute sulphuric acid to barium chloride solution.
Answer:
The above processes are classified into two groups (a) slow speed reactions (b) fast speed reactions.
Slow speed reactions: (2), (3) and (4).
Fast speed reactions: (1),(5) and (6).
In simple words: Chemical processes can be categorized by their speed: fast reactions like burning gas, effervescence, and precipitate formation occur quickly, while slow reactions like rusting, rock erosion, and alcohol fermentation take a longer time.

🎯 Exam Tip: Understanding reaction rates is crucial. Fast reactions often involve bond breaking or formation with low activation energy, while slow reactions usually have higher activation energy or involve complex multi-step processes.

 

Use your brain power! (Text Book Page No. 43)

 

Question 1.
Some more examples of redox reaction are as follows. Identify the reductants and oxidants from them.
(1) 2H2S + SO2 → 3S↓ + 2H2O
(2) MnO2 + 4HCl → MnCl2 + 2H2O + Cl2↑
Answer:
Oxidants: SO2, MnO2
Reductants: H2S, HCl
In simple words: In redox reactions, oxidants gain electrons and cause oxidation (e.g., SO2, MnO2), while reductants lose electrons and cause reduction (e.g., H2S, HCl).

🎯 Exam Tip: To identify oxidants and reductants, remember: the oxidant is reduced (gains electrons), and the reductant is oxidized (loses electrons). Look for changes in oxidation states of elements.

 

Question 2.
If oxidation means losing electrons, what is meant by reduction.
Answer:
Reduction means gaining one or more electrons.
In simple words: If oxidation is losing electrons, then reduction is the opposite-the process of gaining electrons.

🎯 Exam Tip: The mnemonic "OIL RIG" (Oxidation Is Loss, Reduction Is Gain) helps remember the electron definitions of oxidation and reduction, which are fundamental to redox chemistry.

 

Question 3.
Write the reaction of formation of Fe2+ by reduction Fe3+ by making use of the symbol (e¯).
Answer:
Fe3+ + e¯ → Fe2+ (reduction)
In simple words: The formation of an Fe2+ ion from an Fe3+ ion occurs through reduction, which means the Fe3+ ion gains one electron.

🎯 Exam Tip: For ionic reactions, remember that a decrease in positive charge (or an increase in negative charge) indicates a gain of electrons, thus a reduction process.

 

Think about it (Text Book Page No. 43)

 

Question 1.
The luster of the surface of the aluminium utensils in the house is lost after a few days. Why does this happen?
Answer:
The aluminium utensils when kept in the house for a few days, oxidation of aluminium takes place, a thin laver aluminium oxide (Al2O3) is deposited on the surface. Hence, aluminium utensils lose their lustre in a few days.
In simple words: Aluminum utensils lose their shine because aluminum reacts with oxygen in the air to form a thin, dull layer of aluminum oxide on the surface, a process called oxidation.

🎯 Exam Tip: This illustrates corrosion. Metals like aluminum quickly form a protective oxide layer (passivation) that, while dulling the surface, prevents further corrosion unlike iron rusting completely.

 

Question 3.
How many products are formed in each of the above reactions?
Answer:
A single product is formed in each of the above reaction.
In simple words: For the reactions mentioned above (referring to the context of "Think about it" from the textbook), only one single substance is produced as a result.

🎯 Exam Tip: Identifying the number of products helps classify reaction types. A single product often indicates a combination reaction, while multiple products can suggest decomposition or displacement.

 

Use your brain power! (Text Book Page No. 39)

 

Question 1.
What is the difference in the process of dissolution and a chemical reaction.
Answer:
In the process of dissolution, new substance is not necessarily formed. Whereas in a chemical reaction a new substance is definitely formed.
In simple words: Dissolution is simply mixing where no new substance is guaranteed, while a chemical reaction always creates new substances with different properties.

🎯 Exam Tip: A key distinction between physical changes (like dissolution) and chemical changes (reactions) is the formation of new chemical identities. Always look for evidence of new substance formation.

 

Question 2.
Does a new substance form when a solute dissolves in a solvent?
Answer:
It is not necessary that a new substance is always formed.
In simple words: When a solute dissolves, it usually just disperses in the solvent, so a new chemical substance isn't necessarily created; it's often a physical change.

🎯 Exam Tip: Dissolution is typically a physical change unless there's an obvious chemical interaction (e.g., acid-base reaction) that forms new compounds. Most simple dissolving processes do not form new substances.

 

Fill in the blanks:

 

Question 1.
Organic waste is decomposed by micro-organism and as a result manure and........are formed.
Answer:
Organic waste is decomposed by micro-organism and as a result manure and bio gas are formed.
In simple words: Microorganisms break down organic waste, producing both nutrient-rich manure and useful biogas.

🎯 Exam Tip: This question relates to biological decomposition and its products. Knowing the outputs of such processes (manure, biogas) is important for understanding environmental chemistry.

 

Question 2.
..........is formed on mixing yeast in glucose solution under proper condition.
Answer:
Alcohol is formed on mixing yeast in glucose solution under proper condition.
In simple words: When yeast is added to a glucose solution under specific conditions, alcohol is produced through fermentation.

🎯 Exam Tip: Fermentation is a biological process involving yeast, converting sugars into alcohol and carbon dioxide. It's a key example of a biochemical reaction.

 

Question 3.
The chemical reaction during which H2(g) is lost is termed as.........
Answer:
The chemical reaction during which H2(g) s lost is termed as oxidation.
In simple words: A chemical reaction where hydrogen gas is removed from a substance is defined as an oxidation reaction.

🎯 Exam Tip: Remember the multiple definitions of oxidation: addition of oxygen, removal of hydrogen, or loss of electrons. Each definition helps identify oxidation in different contexts.

 

Question 4.
Corrosion can be prevented by using.........
Answer:
Corrosion can be prevented by using antirust solution.
In simple words: To stop corrosion, a protective layer like an antirust solution can be applied to metals.

🎯 Exam Tip: Methods to prevent corrosion, such as painting, galvanizing, electroplating, or using antirust solutions, are important practical applications of chemistry.

 

Question 5.
The chemical reactions in which heat is liberated are called...........reactions.
Answer:
The chemical reactions in which heat is liberated are called exothermic reactions.
In simple words: Reactions that release heat into their surroundings are known as exothermic reactions.

🎯 Exam Tip: Exothermic reactions are characterized by a release of energy, often felt as heat, and have a negative change in enthalpy (\( \Delta H < 0 \)).

 

Question 6.
The chemical formula of rust is.........
Answer:
The chemical formula of rust is Fe2O3.H2O.
In simple words: Rust is chemically known as hydrated iron(III) oxide, represented by the formula Fe2O3.H2O.

🎯 Exam Tip: Know the chemical formula for rust (hydrated ferric oxide) as it's a common example of corrosion and frequently asked in exams.

 

Question 7.
A reaction in which heat is absorbed is called.........reaction.
Answer:
A reaction in which heat is absorbed is called endothermic reaction.
In simple words: Reactions that absorb heat from their surroundings are called endothermic reactions.

🎯 Exam Tip: Endothermic reactions require an input of energy (heat) to proceed, leading to a cooling effect on the surroundings and a positive change in enthalpy (\( \Delta H > 0 \)).

 

Question 8.
The process of rusting or iron is.........process.
Answer:
The process of rusting of iron is oxidation process.
In simple words: Rusting of iron is an oxidation process where iron reacts with oxygen in the presence of moisture.

🎯 Exam Tip: Rusting is a specific example of corrosion, which is fundamentally an oxidation reaction of a metal.

 

Question 9.
when oil and fats are oxidised or even allowed to stand in air for a long time, they become
Answer:
when oil and fats are oxidised or even allowed to stand in air for a long time, they become rancid.
In simple words: Over time, exposure to air causes oils and fats to oxidize, leading to a condition called rancidity, which changes their smell and taste.

🎯 Exam Tip: Rancidity is a common oxidation process affecting food, making it undesirable for consumption. This concept is important for food preservation.

 

Question 10.
..........are used to prevent oxidation of food.
Answer:
Antioxidants are used to prevent oxidation of food.
In simple words: Antioxidants are substances added to food to slow down or prevent the oxidation process, thus preserving its freshness and preventing spoilage.

🎯 Exam Tip: Antioxidants are critical in food chemistry for extending shelf life and preventing rancidity. They work by inhibiting oxidation reactions.

 

Question 11.
Carbon dioxide is passed through water. The reaction is a.........reaction.
Answer:
Carbon dioxide is passed through water. The reaction is a combination reaction.
In simple words: When carbon dioxide dissolves in water, they combine to form carbonic acid, which is an example of a combination reaction.

🎯 Exam Tip: A combination reaction always results in a single product formed from two or more reactants. Carbon dioxide dissolving in water to form carbonic acid is a classic example.

 

Question 12.
Calcium carbonate is heated. The reaction is a...........reaction.
Answer:
Calcium carbonate is heated. The reaction is a decomposition reaction.
In simple words: Heating calcium carbonate causes it to break down into simpler substances, making it a decomposition reaction.

🎯 Exam Tip: Decomposition reactions involve a single compound breaking down into two or more simpler substances, often requiring energy input (like heat).

 

Question 13.
Zinc strip is dipped in a CuSO4 solution. The reaction is a..........reaction.
Answer:
Zinc strip is dipped in a CuSO4 solution. The reaction is a displacement reaction.
In simple words: When zinc is put into copper sulfate solution, the more reactive zinc replaces copper, causing a displacement reaction.

🎯 Exam Tip: In a displacement reaction, a more reactive element displaces a less reactive element from its compound. Refer to the reactivity series to predict these reactions.

 

Question 14.
Silver nitrate solution is added to NaCl solution. The reaction is a..........reaction.
Answer:
Silver nitrate solution is added to NaCl solution. The reaction is a double displacement reaction.
In simple words: This reaction involves the exchange of ions between silver nitrate and sodium chloride solutions, forming two new compounds and typically a precipitate, which is characteristic of a double displacement reaction.

🎯 Exam Tip: Double displacement reactions are often identified by the formation of a precipitate, a gas, or water when two ionic compounds exchange their respective ions.

 

Question 15.
The slow process of decay or destruction of a metal due to effect of air, moisture and acids on it is known as..........
Answer:
The slow process of decay or destruction of a metal due to effect of air, moisture and acids on it is known as corrosion.
In simple words: Corrosion is the gradual degradation of a metal's surface caused by chemical reactions with environmental factors like air, moisture, or acids.

🎯 Exam Tip: Corrosion is a significant environmental and economic issue, referring to the oxidative decay of metals, with rusting of iron being its most common example.

 

Rewrite the following statements by selecting the correct options:

 

Question 1.
The reaction of iron nail with copper sulphate solution is.........reaction. (March 2019)
(a) double displacement
(b) displacement
(c) combination
(d) decomposition
Answer:
(b) displacement
In simple words: When an iron nail is placed in copper sulphate solution, iron displaces copper because iron is more reactive, classifying it as a displacement reaction.

🎯 Exam Tip: Always refer to the reactivity series of metals to determine if one metal can displace another from its salt solution. Iron being above copper means it will displace it.

 

Question 2.
Reddish brown deposit formed on iron nails kept in a solution of copper sulphate is
(a) Cu2O
(b) Cu
(c) CuO
(d) CuS
Answer:
(b) Cu
In simple words: The reddish-brown substance that coats iron nails in copper sulfate solution is elemental copper, formed when iron displaces copper from the solution.

🎯 Exam Tip: Identify the products of a displacement reaction: the more reactive metal displaces the less reactive metal, which then deposits as a pure element (e.g., copper).

 

Question 3.
The reaction CuSO4(aq) + Zn(s) → ZnSO4(aq) + Cu(s) is a........reaction.
(a) displacement
(b) double displacement
(c) decomposition
(d) combination
Answer:
(a) displacement
In simple words: This reaction is a displacement reaction because zinc, a more reactive metal, replaces copper from its sulfate solution.

🎯 Exam Tip: In a displacement reaction, one element takes the place of another in a compound. Zinc replacing copper is a classic example to remember.

 

Question 4.
.........is a combination reaction.
(a) Cu + H2SO4 → CuSO4 + H2
(b) H2 + Cl2 → 2HCl
(c) 2HgO \( \xrightarrow{\Delta} \) 2Hg + O2
(d) CaCO3 \( \xrightarrow{\Delta} \) CaO + CO2
Answer:
(b) H2 + Cl2 → 2HCl
In simple words: The reaction where hydrogen and chlorine combine to form hydrochloric acid is a combination reaction because two reactants form a single product.

🎯 Exam Tip: A combination reaction is easily identified by having two or more reactants forming a single, more complex product.

 

Question 5.
...........a decomposition reaction.
(a) CaCO3 \( \xrightarrow{\Delta} \) CaO + CO2
(b) H2O + CO2 → H2CO3
(c) CaS + 2HCl → CaCl2 + H2S
(d) 2H2 + O2 → 2H2O
Answer:
(a) CaCO3 \( \xrightarrow{\Delta} \) CaO + CO2
In simple words: The breakdown of calcium carbonate into calcium oxide and carbon dioxide upon heating is a decomposition reaction because one compound splits into two simpler ones.

🎯 Exam Tip: Decomposition reactions are characterized by a single reactant breaking down into multiple products, often requiring heat, light, or electricity.

 

Question 6.In a chemical equation the----------are written on the left hand side.
(a) products
(b) reactants
(c) catalysts
(d) elements
Answer: (b) reactants
In simple words: Reactants are the starting substances in a chemical reaction and are always placed on the left side of the chemical equation.

🎯 Exam Tip: Understanding the basic layout of a chemical equation, specifically the placement of reactants and products, is fundamental for balancing equations and interpreting reactions.

 

Question 7.The \( \Delta \) sign written above the arrow indicates----------of the reaction.
(a) reactant
(b) product
(c) heat
(d) direction of the reaction
Answer: (c) heat
In simple words: The triangle symbol, \( \Delta \), placed above the reaction arrow indicates that heat is supplied to the reaction to make it proceed.

🎯 Exam Tip: Recognizing symbols like \( \Delta \) in chemical equations helps interpret reaction conditions and energy changes, which are crucial for higher-level chemistry problems.

 

Question 8.The reaction \( \text{KNO}_3(\text{S}) + \text{H}_2\text{O}(\text{l}) + \text{Heat} \rightarrow \text{KNO}_3(\text{aq}) \) is a/an----------reaction.
(a) exothermic
(b) endothermic
(c) oxidation
(d) reduction
Answer: (b) endothermic
In simple words: Since "Heat" is added as a reactant (on the left side) in the equation, it signifies that heat is absorbed during the reaction, characteristic of an endothermic process.

🎯 Exam Tip: Always look for "Heat" or "Energy" on the reactant side for endothermic reactions and on the product side for exothermic reactions to correctly identify the type of thermal change.

 

Question 9.The reaction \( \text{NaOH}(\text{S}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{NaOH}(\text{aq}) \) is a/an----------reaction.
(a) exothermic
(b) endothermic
(c) oxidation
(d) reduction
Answer: (a) exothermic
In simple words: When sodium hydroxide dissolves in water, it releases heat, which is a characteristic of an exothermic reaction.

🎯 Exam Tip: Dissolution processes can be exothermic or endothermic; strong bases like NaOH often dissolve exothermically, which is an important concept to remember for practical applications.

 

Question 10.A solution of \( \text{Al}_2(\text{SO}_4)_3 \) in water is----------
(a) blue
(b) pink
(c) green
(d) colourless
Answer: (d) colourless
In simple words: Aluminum sulfate, when dissolved in water, forms a solution that does not exhibit any distinct color.

🎯 Exam Tip: Knowing the typical colors of common ionic solutions (e.g., copper salts are blue, iron salts can be green/yellow/brown, aluminum salts are generally colorless) is helpful in qualitative analysis.

 

Question 11.Carbon dioxide----------
(a) turns lime water milky
(b) is odourless
(c) is colourless
(d) All the three (a), (b) and (c) are correct
Answer: (d) All the three (a), (b) and (c) are correct
In simple words: Carbon dioxide is a colorless and odorless gas that reacts with lime water (calcium hydroxide solution) to form insoluble calcium carbonate, causing the solution to turn milky.

🎯 Exam Tip: The lime water test is a classic method to detect carbon dioxide, and understanding its physical properties (colorless, odorless) is essential for identifying gases.

 

Question 12.----------is the correct set up to pass \( \text{CO}_2 \) through lime water.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र चूने के पानी के माध्यम से कार्बन डाइऑक्साइड गैस को प्रवाहित करने के लिए चार अलग-अलग प्रयोगात्मक सेटअप (A, B, C, D) दिखाता है। प्रत्येक सेटअप में एक टेस्ट-ट्यूब है जिसमें चूने का पानी होता है और एक CO2 स्रोत से गैस प्रवाहित करने के लिए एक ट्यूब होती है, लेकिन गैस के निकलने और टेस्ट-ट्यूब के अंदर ट्यूब की गहराई अलग-अलग है। सेटअप D में गैस-प्रवाह ट्यूब चूने के पानी में गहराई तक डूबी हुई है, जिससे गैस का अधिकतम संपर्क सुनिश्चित होता है और एक निकास नली भी दिखाई गई है।
Answer: Correct set up D.
In simple words: Setup D correctly shows the gas delivery tube extended deep into the lime water for efficient reaction and a proper exit for excess gas, which is ideal for testing carbon dioxide.

🎯 Exam Tip: Proper experimental setup is critical in chemistry. For gas reactions in liquid, the delivery tube should be immersed deep to ensure maximum contact and reaction, and an outlet should be provided if needed.

 

Question 13.when----------is passed through fresh lime water, it turns milky.
(a) H2
(b) CO
(c) CO2
(d) SO2
Answer: (c) CO2
In simple words: Carbon dioxide is the gas that reacts with calcium hydroxide in lime water to form insoluble calcium carbonate, causing the solution to become milky.

🎯 Exam Tip: Remember that the turning of lime water milky is a specific test for carbon dioxide gas due to the formation of a white precipitate of calcium carbonate.

 

Question 14.Magnesium reacts with con. HCl to form----------salt.
(a) copper chloride
(b) ferrous chloride
(c) calcium chloride
(d) magnesium chloride
Answer: (d) magnesium chloride
In simple words: When magnesium reacts with hydrochloric acid, it displaces hydrogen to form magnesium chloride salt and hydrogen gas.

🎯 Exam Tip: Metals above hydrogen in the reactivity series react with acids to form salt and hydrogen gas. Identify the metal in the reactant to predict the metal component of the salt formed.

 

Question 15.Zinc reacts with hydrochloric acid. The reaction is a reaction.
(a) combination
(b) decomposition
(c) displacement
(d) double decomposition
Answer: (c) displacement
In simple words: In this reaction, zinc, a more reactive metal, replaces hydrogen from hydrochloric acid, which is a classic example of a displacement reaction.

🎯 Exam Tip: Displacement reactions involve a more reactive element displacing a less reactive element from its compound. Zinc is more reactive than hydrogen.

 

Question 16.In a double displacement reaction,---------- (Practice Activity Sheet - 1)
(a) ions remain at rest
(b) ions get liberated
(c) ions are exchanged
(d) ions are not created
Answer: (c) ions are exchanged
In simple words: A double displacement reaction is characterized by the mutual exchange of ions between two reactant compounds, usually leading to the formation of a precipitate, gas, or water.

🎯 Exam Tip: The key feature of a double displacement reaction is the exchange of cationic and anionic partners between two compounds, often in aqueous solutions, forming new compounds.

 

State whether the following statements are True or False:

Question 1.Rusting of iron is a fast reaction.
Answer: False. (Rusting of iron is a slow reaction.)
In simple words: Rusting is a slow oxidation process that occurs over an extended period.

🎯 Exam Tip: Classifying reactions by speed (fast or slow) is important for understanding reaction kinetics and practical implications, like corrosion prevention.

 

Question 2.Milk is set into curd is a chemical change.
Answer: True.
In simple words: The conversion of milk into curd involves the formation of new substances with different chemical properties, hence it is a chemical change.

🎯 Exam Tip: Differentiating between physical and chemical changes is fundamental; chemical changes result in new substances, while physical changes only alter form or state.

 

Question 3.The reaction between salt and water is an example of exothermic reaction.
Answer: False. (The reaction between salt and water is an example of endothermic reaction.)
In simple words: Many salts, when dissolved in water, absorb heat from the surroundings, leading to a decrease in temperature, which is an endothermic process.

🎯 Exam Tip: The dissolution of salts can be either exothermic or endothermic; it's not universally exothermic. KNO3 dissolving in water is a common endothermic example.

 

Question 4.The speed of a chemical reaction depends on the catalyst used in the chemical reaction.
Answer: True.
In simple words: A catalyst can alter the rate of a chemical reaction by providing an alternative reaction pathway, either speeding it up or slowing it down, without being consumed itself.

🎯 Exam Tip: Catalysts are crucial in industrial chemistry for optimizing reaction rates and increasing efficiency without changing the final products.

 

Question 5.The simple form of representation of a chemical reaction in words is known as word reaction.
Answer: True.
In simple words: A word equation describes a chemical reaction using the names of the reactants and products in a simple, verbal form.

🎯 Exam Tip: Word equations are a basic step towards writing balanced chemical equations, helping to identify the substances involved before using chemical formulas.

 

Question 6.Nascent oxygen is always denoted by showing the symbol of oxygen.
Answer: False. (Nascent oxygen is always denoted by showing symbol of oxygen [O] in square brackets.)
In simple words: Nascent oxygen, being a highly reactive atomic form, is specifically represented as [O] to distinguish it from molecular oxygen \( \text{O}_2 \).

🎯 Exam Tip: Correctly representing nascent species (like [O] or [H]) is vital in organic and redox reactions where highly reactive intermediates are involved.

 

Question 7.Antioxidants are used to prevent oxidation or food containing fats and oils.
Answer: True.
In simple words: Antioxidants are substances that inhibit oxidation, thereby protecting fats and oils in food from becoming rancid.

🎯 Exam Tip: Understanding the role of antioxidants in food preservation is a practical application of redox chemistry, relevant to everyday life and food science.

 

Question 8.When oils and fats are allowed to stand for a long time, they become rancid.
Answer: True.
In simple words: Oils and fats undergo oxidation when exposed to air over time, leading to unpleasant odors and tastes, a process known as rancidity.

🎯 Exam Tip: Rancidity is a significant food spoilage mechanism, and knowing its cause (oxidation) helps in understanding food storage methods.

 

Question 9.The chemical formula of rust is \( \text{Fe}_3\text{O}_4 \).xH2O.
Answer: False. (The chemical formula or rust is \( \text{Fe}_2\text{O}_3 \).xH2O.)
In simple words: Rust is chemically hydrated iron (III) oxide, with the formula \( \text{Fe}_2\text{O}_3 \).xH2O, not \( \text{Fe}_3\text{O}_4 \).xH2O.

🎯 Exam Tip: Precise chemical formulas are crucial. Magnetite, \( \text{Fe}_3\text{O}_4 \), is also an iron oxide but not typically referred to as common rust.

 

Question 10.Glucose combines with oxygen in our body and provides energy. The reaction is an endothermic reaction.
Answer: False. (Glucose combines with oxygen in our body and provides energy. The reaction is an exothermic reaction.)
In simple words: The cellular respiration of glucose is an exothermic process, releasing energy for bodily functions, rather than absorbing it.

🎯 Exam Tip: Biological processes like respiration are key examples of exothermic reactions, providing energy for life. Understanding energy release/absorption is central to thermochemistry.

 

Question 11.Chemical reactions in which reactants gain oxygen are reduction reactions.
Answer: False. (Chemical reactions in which reactants gain oxygen are oxidation reactions.)
In simple words: The gain of oxygen by a substance is defined as oxidation, while the loss of oxygen is reduction.

🎯 Exam Tip: Remember the basic definitions of oxidation and reduction in terms of oxygen (gain for oxidation, loss for reduction) and hydrogen (loss for oxidation, gain for reduction).

 

Question 12.\( \text{CuSO}_4(\text{aq}) + \text{Znl}(\text{s}) \rightarrow \text{ZnISO}_4(\text{aq}) + \text{Cu}(\text{s}) \) is an example of decomposition reaction.
Answer: False. (It is an example of displacement reaction.)
In simple words: In this reaction, zinc iodide is displacing copper from copper sulfate, indicating a single displacement reaction, not decomposition.

🎯 Exam Tip: Carefully examine the reactants and products to identify the type of reaction. A single element reacting with a compound to form a new compound and a different element usually indicates displacement.

 

Question 13.The chemical reactions in which heat is liberated are called endothermic reactions.
Answer: False. (The chemical reactions in which heat is liberated are called exothermic reactions.)
In simple words: Reactions that release heat into the surroundings are termed exothermic, while those that absorb heat are endothermic.

🎯 Exam Tip: Understanding the difference between exothermic (heat released) and endothermic (heat absorbed) reactions is fundamental to energy changes in chemistry.

 

Question 14.The product or insoluble solid in chemical reaction is indicated by an arrow pointing upwards.
Answer: False. (The product or insoluble solid in chemical reaction is indicated by an arrow \( \downarrow \) pointing downwards.)
In simple words: An upward arrow \( \uparrow \) indicates a gaseous product, while a downward arrow \( \downarrow \) indicates a precipitate (insoluble solid).

🎯 Exam Tip: Correct use of symbols like \( \uparrow \) for gas and \( \downarrow \) for precipitate in chemical equations helps convey the physical states and observations of the products.

 

Question 15.The rate of a reaction increases on increasing the temperature.
Answer: True.
In simple words: Increasing the temperature provides more kinetic energy to reactant molecules, leading to more frequent and energetic collisions, thus increasing the reaction rate.

🎯 Exam Tip: Temperature is a key factor affecting reaction rates. For most reactions, a higher temperature accelerates the reaction by increasing molecular motion and collision energy.

 

Question 16.The digestion of food is a chemical decomposition process.
Answer: True.
In simple words: Digestion breaks down complex food molecules into simpler substances through chemical reactions, which is a form of decomposition.

🎯 Exam Tip: Biological processes like digestion involve complex chemical reactions, often decomposition, which are essential for nutrient absorption.

 

Question 17.The reaction between sodium hydroxide and hydrochloric acid is a slow reaction.
Answer: False (The reaction between sodium hydroxide and hydrochloric acid is a fast reaction.)
In simple words: The neutralization reaction between a strong acid (HCl) and a strong base (NaOH) is typically very rapid.

🎯 Exam Tip: Neutralization reactions involving strong acids and bases are classic examples of fast reactions due to the high reactivity of \( \text{H}^+ \) and \( \text{OH}^- \) ions.

 

Question 18.When calcium carbonate is heated, it decomposes into calcium oxide and oxygen gas.
Answer: False (when calcium carbonate is heated. it decomposes into calcium oxide and carbon dioxide gas.)
In simple words: Thermal decomposition of calcium carbonate produces calcium oxide (quicklime) and carbon dioxide gas, not oxygen gas.

🎯 Exam Tip: Knowing common decomposition products of carbonates is crucial. Calcium carbonate decomposition is an industrial process for lime production, releasing CO2.

 

Question 19.The rate of a chemical reaction changes in presence of catalyst.
Answer: True.
In simple words: Catalysts modify the rate of a chemical reaction by changing the activation energy, making the reaction faster or sometimes slower.

🎯 Exam Tip: Catalysts are substances that alter reaction rates without being consumed, either by speeding up (positive catalysts) or slowing down (negative catalysts or inhibitors) the reaction.

 

Question 20.Chlorines is an oxidant.
Answer: True.
In simple words: Chlorine readily accepts electrons in chemical reactions, making it an effective oxidizing agent.

🎯 Exam Tip: Halogens like chlorine are strong oxidizing agents due to their high electronegativity and tendency to gain electrons.

 

Taking into consideration the relationship in the first pair, complete the second pair. (OR) Complete the following:

Question 1.\( \text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\text{O} \) Combination reaction :: \( \text{2HgO} \rightarrow \text{2Hg} + \text{O}_2 \):----------
Answer: Decomposition reaction
In simple words: The first reaction shows two elements combining to form a single compound, while the second shows a single compound breaking down into two simpler substances, which is decomposition.

🎯 Exam Tip: Identify the number of reactants and products. Combination reactions have multiple reactants and one product, while decomposition reactions have one reactant and multiple products.

 

Question 2.\( \text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl} \): Combination reaction :: \( \text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu} \):----------
Answer: Displacement reaction
In simple words: The first reaction combines two substances into one, while the second involves iron replacing copper from copper sulfate, indicating a displacement reaction.

🎯 Exam Tip: Recognize that a more reactive metal (Fe) displaces a less reactive metal (Cu) from its salt solution, which is the hallmark of a displacement reaction.

 

Question 3.\( \text{2C}_2\text{H}_5\text{OH} + \text{2Na} \rightarrow \text{2C}_2\text{H}_5\text{ONa} + \text{H}_2 \): Oxidation :: \( \text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O} \):----------
Answer: Reduction
In simple words: The first reaction involves the removal of hydrogen (oxidation), while the second shows copper oxide losing oxygen to hydrogen, which is a reduction process for copper oxide.

🎯 Exam Tip: Define oxidation as gain of oxygen or loss of hydrogen/electrons, and reduction as loss of oxygen or gain of hydrogen/electrons. Apply these rules carefully.

 

Question 4.\( \text{CuCl}_2 + \text{2KI} \rightarrow \text{CuI}_2 + \text{2KCl} \): Double displacement :: \( \text{Zn} + \text{2HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \):----------
Answer: Displacement reaction
In simple words: The first reaction shows an exchange of ions between two compounds, whereas the second depicts zinc replacing hydrogen from hydrochloric acid.

🎯 Exam Tip: Double displacement involves swapping partners between two compounds, while single displacement involves an element reacting with a compound.

 

Question 5.\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \xrightarrow{\text{heat } \Delta} \text{12C} + \text{11H}_2\text{O} \): Decomposition reaction:: \( \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{H}_2\text{CO}_3 \):----------
Answer: Combination reaction
In simple words: The first reaction breaks down a complex substance (sugar) into simpler ones (carbon and water), which is decomposition. The second combines two substances (water and carbon dioxide) into one (carbonic acid), which is combination.

🎯 Exam Tip: Distinguishing between combination and decomposition reactions depends on whether substances combine to form a single product or a single reactant breaks down into multiple products.

 

Question 6.\( \text{CuI}_2 \): Brown :: AgCl :----------
Answer: White.
In simple words: Copper(II) iodide is known to be brown, while silver chloride is a white precipitate.

🎯 Exam Tip: Knowing the colors of common chemical compounds, especially precipitates, is very useful for observational chemistry and lab experiments.

 

Match the column in the following table:

Question 1.

Reactants	products	Type of chemical reaction
Fe + S	NaCl + H2O	Oxidation
CuSO4 + Zn	2CuO	Neutralization
2Cu + O2	ZnSO4 + Cu	Displacement
HCl + NaOH	FeS	Combination

Answer:

Reactants	products	Type of chemical reaction
Fe + S	FeS	Combination
CuSO4 + Zn	ZnSO4 + Cu	Displacement
2Cu + O2	2CuO	Oxidation
HCl + NaOH	NaCl + H2O	Neutralization

In simple words: Matching reactants, products, and reaction types involves recognizing fundamental reaction patterns: combination (Fe+S -> FeS), displacement (CuSO4+Zn -> ZnSO4+Cu), oxidation (2Cu+O2 -> 2CuO), and neutralization (HCl+NaOH -> NaCl+H2O).

🎯 Exam Tip: To successfully match columns, focus on the defining characteristics of each reaction type (e.g., number of reactants/products for combination/decomposition, element replacing element for displacement, acid-base for neutralization, oxygen gain/loss for oxidation/reduction).

 

Rewrite the second column so as to match the item from first column or Match the following:

Question 1.

Column I	Column II
1. Reduction	(a) Type of a chemical reaction
2. Oxidation	(b) Combination with hydrogen
3. Double displacement	(c) Losing hydrogen
4. Displacement	(d) Exchange of ions

Answer:
(1) Reduction - Combination with hydrogen
(2) Oxidation - Losing hydrogen
(3) Double displacement - Exchange of ions
(4) Displacement - Type of chemical reaction.
In simple words: This matching exercise connects chemical terms with their definitions or characteristic features. Reduction often involves gaining hydrogen, oxidation losing hydrogen, double displacement involves ion exchange, and displacement is a general type of reaction.

🎯 Exam Tip: Thoroughly understand the definitions and key features of core chemical concepts like reduction, oxidation, and different reaction types to match them correctly.

 

Question 2.

Column I	Column II
1. Oils and fats are allowed to stand in air for a long time	(a) Slow reaction
2. NaOH dissolves in water	(b) Rancid
3. Zinc is added to CuSO4 solution	(c) Exothermic reaction
4. Rusting of iron	(d) Colourless Solution

Answer:
(1) Oils and fats are allowed to stand in air for a long time - Rancid
(2) NaOH dissolves in water - Exothermic reaction
(3) Zinc is added to CuSO2 solution - Colourless solution
(4) Rusting of iron - Slow reaction.
In simple words: This exercise links observations and processes to their scientific explanations. Oils and fats become rancid due to oxidation. NaOH dissolving is exothermic. Zinc displacing copper forms a colorless solution. Rusting is a slow process.

🎯 Exam Tip: For observation-based matching questions, relate each observation to the underlying chemical principle or characteristic (e.g., rancidity to oxidation, heat release to exothermic, color changes to product formation, and reaction speed).

 

Question 3.

Column I	Column II
(1) Combination reaction	(a) \( \text{Zn} + \text{2HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\uparrow \)
(2) Double displacement reaction	(b) \( \text{C}_{12}\text{H}_{22}\text{O}_{11}(\text{s}) \xrightarrow{\Delta} \text{12C}(\text{s}) + \text{11H}_2\text{O}(\text{g}) \)
(3) Decomposition reaction	(c) \( \text{2Cu} + \text{O}_2 \rightarrow \text{2CuO} \)
(4) Displacement reaction	(d) \( \text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} \downarrow + \text{NaNO}_3 \)

Answer:
(1) Combination reaction - \( \text{2Cu} + \text{O}_2 \rightarrow \text{2CuO} \)
(2) Double displacement reaction - \( \text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} \downarrow + \text{NaNO}_3 \)
(3) Decomposition reaction - \( \text{C}_{12}\text{H}_{22}\text{O}_{11}(\text{s}) \xrightarrow{\Delta} \text{12C}(\text{s}) + \text{11H}_2\text{O}(\text{g}) \)
(4) Displacement reaction - \( \text{Zn}+ \text{2HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \uparrow \)
In simple words: This matching task involves identifying the type of chemical reaction based on its balanced equation. Combination combines elements into a compound, double displacement exchanges ions, decomposition breaks down a compound, and displacement replaces one element with another.

🎯 Exam Tip: To match reactions, analyze the structure of reactants and products: multiple reactants to one product is combination; one reactant to multiple products is decomposition; an element and a compound changing places is displacement; two compounds exchanging ions is double displacement.

 

Classify each of the following reactions as combination, decomposition, displacement or double displacement reactions:

Question 1.(1) \( \text{3CaO.Al}_2\text{O}_3(\text{s}) + \text{6H}_2\text{O}(\text{l}) \rightarrow \text{3CaO.Al}_2\text{O}_3.\text{6H}_2\text{O}(\text{S}) + \text{Heat} \) Tricalcium aluminate Water Concrete
Answer: Combination reaction
In simple words: Two simpler substances, tricalcium aluminate and water, combine to form a single, more complex product (hydrated tricalcium aluminate), which is characteristic of a combination reaction.

🎯 Exam Tip: Look for multiple reactants forming a single, more complex product to identify a combination reaction.

 

Question 2.(2) \( \text{C}_{12}\text{H}_{22}\text{O}_{11}(\text{s}) \xrightarrow{\text{heat } \Delta} \text{12C}(\text{s}) + \text{11H}_2\text{O}(\text{g}) \) Sugar Carbon
Answer: Decomposition reaction
In simple words: A single complex reactant, sugar, breaks down into two simpler products, carbon and water vapor, under heating, thus it's a decomposition reaction.

🎯 Exam Tip: A reaction where one compound breaks into two or more simpler substances, often with heat, is a decomposition reaction.

 

Question 3.(3) \( \text{CuSO}_4(\text{aq}) + \text{Fe}(\text{s}) \rightarrow \text{FeSO}_4(\text{aq}) + \text{Cu}(\text{s}) \) Copper sulphate Ferrous sulphate
Answer: Displacement reaction
In simple words: Iron, a more reactive metal, displaces copper from copper sulfate, forming iron sulfate and elemental copper, which defines a displacement reaction.

🎯 Exam Tip: In a displacement reaction, a more reactive element replaces a less reactive element from its compound. Observing an element reacting with a compound to form a new compound and a new element is a key indicator.

 

Question 4.(4) \( \text{CuCl}_2(\text{aq}) + \text{2KI}(\text{aq}) \rightarrow \text{CuI}_2 \downarrow + \text{2KCl}(\text{aq}) \) Copper chloride Potassium iodide Copper iodide Potassium chloride
Answer: double displacement reaction
In simple words: The copper chloride and potassium iodide exchange their ionic partners to form copper iodide precipitate and potassium chloride, demonstrating a double displacement reaction.

🎯 Exam Tip: Double displacement reactions are characterized by the exchange of ions between two compounds, often leading to the formation of a precipitate, gas, or water.

 

Question 5.(5) \( \text{CaO}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{Ca(OH)}_2(\text{aq}) \) Calcium oxide Calcium hydroxide
Answer: Combination reaction
In simple words: Calcium oxide and water combine to produce a single product, calcium hydroxide, which is a clear sign of a combination reaction.

🎯 Exam Tip: When two or more reactants yield only one product, it's a combination reaction, often exothermic if a strong bond is formed.

 

Question 6.(6) \( \text{BaCl}_2(\text{aq}) + \text{2AgNO}_3(\text{aq}) \rightarrow \text{Ba(NO}_3)_2(\text{aq}) + \text{2AgCl}(\text{s}) \downarrow \) Barium chloride Silver nitrate Barium nitrate Silver chloride
Answer: double displacement reaction
In simple words: Barium chloride and silver nitrate exchange their ions to form barium nitrate and a precipitate of silver chloride, fitting the definition of a double displacement reaction.

🎯 Exam Tip: The formation of a precipitate from two soluble reactants is a common indicator of a double displacement reaction.

 

Name the following:

Question 1.The product formed in the thermal decomposition of sugar.
Answer: Carbon is formed in the thermal decomposition of sugar.
In simple words: When sugar is heated and decomposes, it primarily yields carbon and water.

🎯 Exam Tip: Thermal decomposition reactions often simplify complex organic molecules into their elemental carbon and simpler compounds like water.

 

Question 2.The gas evolved when sorghum metal reacts with ethanol.
Answer: Hydrogen (H2) gas is evolved when sodium metal reacts with ethanol.
In simple words: When a reactive metal like sodium reacts with an alcohol such as ethanol, hydrogen gas is produced.

🎯 Exam Tip: Alkali metals react vigorously with alcohols, similar to water, to produce hydrogen gas and an alkoxide.

 

Question 3.The precipitate formed when barium sulphide reacts with zinc sulphate.
Answer: When barium sulphide reacts with zinc sulphide, a precipitate of barium sulphate is formed.
\( \text{BaS} + \text{ZnSO}_4 \rightarrow \text{BaSO}_4 \downarrow + \text{ZnS} \)
precipitate
In simple words: The reaction between barium sulfide and zinc sulfate leads to the formation of barium sulfate, which is an insoluble precipitate.

🎯 Exam Tip: In double displacement reactions, identifying the insoluble product (precipitate) is key. Barium sulfate (\( \text{BaSO}_4 \)) is a common white precipitate.

 

Question 4.The reducing agent used for the reduction of copper oxide.
Answer: Hydrogen is used for the reduction of copper oxide.
In simple words: Hydrogen acts as a reducing agent by removing oxygen from copper oxide, thereby reducing it to copper.

🎯 Exam Tip: A reducing agent is a substance that causes another substance to be reduced (gain electrons/hydrogen, lose oxygen) and is itself oxidized (loses electrons/hydrogen, gains oxygen).

 

Question 5.The catalyst used to accelerate the rate of decomposition of hydrogen peroxide.
Answer: Manganese dioxide (MnO2) is used as a catalyst to accelerate the rate of decomposition of hydrogen peroxide.
In simple words: Manganese dioxide speeds up the breakdown of hydrogen peroxide into water and oxygen without being consumed in the process.

🎯 Exam Tip: Catalysts are essential for many industrial and laboratory reactions, and specific catalysts like \( \text{MnO}_2 \) for hydrogen peroxide decomposition are common examples.

 

Question 6.which oxidising agent is used to oxidise ferrous sulphate.
Answer: Potassium permanganate (KMnO4) is used as an oxidising agent to oxidise ferrous sulphate.
In simple words: Potassium permanganate is a strong oxidizing agent capable of oxidizing ferrous sulfate to ferric sulfate.

🎯 Exam Tip: Potassium permanganate (\( \text{KMnO}_4 \)) is a versatile and commonly used strong oxidizing agent in titrations and various chemical reactions.

 

Question 7.The product formed in the oxidation of ethyl alcohol.
Answer: Acetic acid is formed in the oxidation of ethyl alcohol.
In simple words: The oxidation of ethyl alcohol, typically with strong oxidizing agents, yields acetic acid.

🎯 Exam Tip: Primary alcohols can be oxidized to aldehydes, and further to carboxylic acids. Ethyl alcohol (a primary alcohol) oxidizes to acetaldehyde, then to acetic acid.

 

Answer the following questions in one sentence each:

Question 1.what is meant by a chemical equation?
Answer: The simple representation or a chemical reaction in a condensed form with the help of chemical formulae is called a chemical equation.
In simple words: A chemical equation is a shorthand way to show what happens in a chemical reaction using chemical symbols and formulas.

🎯 Exam Tip: Understanding the definition of a chemical equation is the first step to mastering stoichiometry and reaction interpretation.

 

Question 2.what is meant by a word equation?
Answer: The simple form or representation or a chemical reaction in words is known as word equation.
In simple words: A word equation expresses a chemical reaction using the full names of the reactants and products.

🎯 Exam Tip: Word equations are useful for initial representation before converting to chemical formulas and balancing equations.

 

Question 3.what happens in a combination reaction?
Answer: A single compound (product) is formed from two or more substances during a combination reaction.
In simple words: In a combination reaction, two or more simpler substances join together to create one more complex substance.

🎯 Exam Tip: The hallmark of a combination reaction is a single product formed from multiple reactants.

 

Question 4.what happens in a displacement reaction?
Answer: In a displacement reaction. a more reactive element displaces another element, having less reactivity, from its compound.
In simple words: A displacement reaction occurs when a more reactive element takes the place of a less reactive element in a compound.

🎯 Exam Tip: Reactivity series is crucial for predicting whether a displacement reaction will occur. A more reactive element will always displace a less reactive one.

 

Question 5.what happens in a decomposition reaction?
Answer: A single substance is broken down and two or more substances are formed during a decomposition reaction.
In simple words: In a decomposition reaction, one complex compound breaks apart into two or more simpler substances.

🎯 Exam Tip: Decomposition reactions often require energy input (heat, light, electricity) to break bonds.

 

Question 6.what happens in a double displacement reaction?
Answer: A precipitate is formed by exchange of ions between the reactants during a double displacement reaction.
In simple words: In a double displacement reaction, the positive and negative ions of two compounds switch places, usually forming a precipitate, water, or gas.

🎯 Exam Tip: The formation of an insoluble product (precipitate) is a very common outcome and diagnostic feature of double displacement reactions.

 

Question 7.Identlry the type of following reaction:
\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \xrightarrow{\Delta} \text{12C} + \text{11H}_2\text{O} \) (Practice Activity Sheet - 2)
Answer: The above reaction is a decomposition reaction.
In simple words: A single complex molecule (sugar) breaks down into multiple simpler substances (carbon and water) when heated.

🎯 Exam Tip: Thermal decomposition is indicated by the \( \Delta \) symbol over the arrow and the breakdown of one reactant into multiple products.

 

Question 8.what happens in an endothermic reaction?
Answer: In an endothermic reaction, the reactants absorb heat to form products.
In simple words: An endothermic reaction takes in energy from its surroundings, causing the temperature of the surroundings to drop.

🎯 Exam Tip: Look for a decrease in temperature of the surroundings or "heat" written on the reactant side to identify an endothermic reaction.

 

Question 9.State the use of antioxidants in food containing fats and oils.
Answer: Antioxidants are used to prevent oxidation of food containing fats and oils.
In simple words: Antioxidants protect fats and oils in food from reacting with oxygen, which prevents spoilage and rancidity.

🎯 Exam Tip: Antioxidants play a vital role in food preservation by delaying or preventing oxidation, thereby extending shelf life and maintaining quality.

 

Question 10.What are edible oils?
Answer: Edible oils are compounds of alcohols and organic acids (carboxylic acids). The compounds formed are known as esters of carboxylic acids.
In simple words: Edible oils are fats that are liquid at room temperature and are primarily composed of triglycerides, which are esters formed from glycerol and fatty acids.

🎯 Exam Tip: Understanding the chemical composition of edible oils as esters formed from glycerol and fatty acids helps grasp their properties and reactions like rancidity.

 

Question 11.Is rancidity a phenomenon of oxidation or reduction?
Answer: Rancidity is a phenomenon of oxidation.
In simple words: Rancidity occurs when fats and oils react with oxygen from the air, causing them to break down and develop unpleasant tastes and smells.

🎯 Exam Tip: Link rancidity directly to oxidation, as this is the primary chemical process causing the spoilage of fats and oils.

 

Answer the following questions:

Question 1.What do you understand by a physical change? OR Define physical change.
Answer: The change in which only the physical state of a substance is changed; no new substance is formed. This change is temporary. During this change the composition of the substance does not change.
In simple words: A physical change alters a substance's appearance or state but not its chemical identity, and it is usually reversible.

🎯 Exam Tip: Key identifiers of a physical change are that no new chemical substance is formed, and the change is typically reversible, such as melting or boiling.

 

Question 2.Explain giving two examples or physical change.
Answer:
(1) Conversion of ice into water is a physical change. On heating, ice melts into water. when the water is cooled, it freezes into ice. Thus, we get ice from water by a simple method and no new substance is formed. Hence, conversion of ice into water is a physical change.
(2) Magnetization of iron nail is a physical change. An iron nail magnetized by induction loses its magnetism as soon as it is detached from the magnet which induces magnetism in it. An iron nail magnetized by some other methods can also be demagnetized by simple means such as hammering or heating it. Thus, the magnetization of an iron nail can be easily reversed to get original nail. Hence, it is a physical change.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक चुंबक के पास एक लोहे की कील को दिखाता है, जो कील में चुंबकत्व को प्रेरित करने की प्रक्रिया को दर्शाता है। कील के सिरे पर एक चुंबक लगाया गया है, जिससे कील अस्थायी रूप से चुंबकित हो जाती है और चुंबकत्व को खो देती है जब कील को हटा दिया जाता है।
In simple words: Two examples of physical change are melting ice into water and magnetizing an iron nail; in both cases, the substance's chemical composition remains unchanged, and the process is reversible.

🎯 Exam Tip: When explaining physical changes, emphasize the retention of chemical identity and the reversibility of the process. Examples like phase changes and magnetism are excellent for illustration.

 

Question 1. Define physical change. OR Define physical change.
Answer: The change in which only the physical state of a substance is changed; no new substance is formed. This change is temporary. During this change the composition of the substance does not change.
In simple words: A physical change alters a substance's appearance but not its chemical composition, and no new substance is created; it's usually temporary.

🎯 Exam Tip: Focus on the reversibility and lack of new substance formation as key indicators for physical changes.

 

Question 2. Explain giving two examples or physical change.
Answer:
(1) Conversion of ice into water is a physical change. On heating, ice melts into water. when the water is cooled, it freezes into ice. Thus, we get ice from water by a simple method and no new substance is formed. Hence, conversion of ice into water is a physical change.
(2) Magnetization of iron nail is a physical change. An iron nail magnetized by induction loses its magnetism as soon as it is detached from the magnet which induces magnetism in it. An iron nail magnetized by some other methods can also be demagnetized by simple means such as hammering or heating it. Thus, the magnetization of an iron nail can be easily reversed to get original nail. Hence, it is a physical change.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक चुंबक और एक कील को दिखाता है, जहाँ चुंबक कील को अपनी ओर आकर्षित कर रहा है। यह प्रक्रिया लोहे की कील के अस्थायी चुंबकीकरण को दर्शाती है, जो एक भौतिक परिवर्तन का उदाहरण है।
In simple words: Melting ice into water and magnetizing an iron nail are physical changes because they don't form new substances and can be easily reversed.

🎯 Exam Tip: Illustrating concepts with clear, relatable examples like water changing states or temporary magnetization can earn full marks.

 

Question 3. what do you understand by a chemical change? OR Define Chemical change.
Answer: The change in which a substance or substances are converted into a new substance or substances, possessing properties altogether different from the original ones, is called a chemcial change. During this change, the original substance cannot be recovered by any simple means. This change is permanent.
In simple words: A chemical change is a permanent alteration where a new substance with different properties is formed, and the original substance cannot be easily recovered.

🎯 Exam Tip: The key difference to highlight for chemical changes is the formation of entirely new substances with distinct properties and their irreversible nature.

 

Question 4. Explain giving two examples of chemical change.
Answer:
(1) When carbon is burnt, carbon dioxide is formed. In this process carbon combines with oxygen, therefore carbon and oxygen are reactants, while curbon dioxide is a product. This change is permanent. \[C_{(s)} + O_{2(g)} \xrightarrow{Heat} CO_{2(g)}\]
(2) When a magnesium wire is burnt in air, a white powder of magnesium oxide is formed. We cannot obtain magnesium from magnesium oxide by simple methods. Properties of magnesium oxide are altogether different from those of magnesium. A new substance MgO is formed in the reaction. Hence, this change is a chemical change. \[2Mg + O_2 \xrightarrow{\Delta} 2MgO\]
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक मैग्नीशियम रिबन को जलते हुए दिखाता है, जिससे मैग्नीशियम ऑक्साइड का सफेद पाउडर बनता है, जो एक रासायनिक परिवर्तन का उदाहरण है। यह दर्शाता है कि एक पदार्थ जलकर पूरी तरह से एक नए पदार्थ में बदल गया है।
In simple words: Burning carbon to form carbon dioxide and burning magnesium to form magnesium oxide are chemical changes because they produce new substances with different properties permanently.

🎯 Exam Tip: Ensure your examples clearly demonstrate the formation of new substances and the irreversibility typical of chemical changes, including balanced equations.

 

Question 5. What is meant by a chemical reaction?
Answer: A process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds is called a chemical reaction.
In simple words: A chemical reaction is a process where old chemical bonds break and new ones form, transforming reactants into new products.

🎯 Exam Tip: Emphasize bond breaking and bond formation as the fundamental aspects defining a chemical reaction.

 

Question 6. What is the importance of a chemical equation?
Answer:
1. Reactants are converted into products.
2. Mass is conserved.
3. Atoms are conserved.
4. The properties and compositions of the products of a chemical reaction are different from those of its reactants.
5. Generally, energy is either absorbed or evolved.
In simple words: Chemical equations are important because they show what reactants become products, confirm mass and atom conservation, highlight property changes, and indicate energy changes in a reaction.

🎯 Exam Tip: When discussing importance, focus on both qualitative (properties, new substances) and quantitative (conservation laws) aspects represented by equations.

 

Question 7. What are the conventions used in writing a chemical equation?
Answer: Conventions used in writing a chemical equation:
(1) The reactants are written on the left hand side (LHS), while the products are written on the right hand side (RHS).
(2) Whenever there are two or more reactants, a plus sign (+) is written between each two of them. Similarly, if there are two or more products, a plus sign is written between each two of them.
(3) Reactant side and product side are connected with an arrow (→) pointing from reactants to products. The arrow represents the direction of the reaction. Heat is to be given from outside to the reaction, it is indicated by the sign \( \Delta \) written above the arrow.
(4) The conditions like temperature, pressure, catalyst, etc., are mentioned above the arrow (→) pointing towards the product side.
(5) The physical states of the reactants and products are also mentioned in a chemical equation. The notations g, l, s, and aq are written in brackets as a subscript along with the symbols / formulae of reactants and products. The symbols g, l, s, and aq stand for gaseous, liquid, solid and aqueous respectively.
If the product is gaseous, instead of (g) it can be indicated by an arrow \( \uparrow \) pointing upwards. If the product formed is insoluble solid, then instead of (s) it can be indicated by an arrow \( \downarrow \) pointing downwards.
(6) Special information or names of reactants/products are written below their formulae.
In simple words: Chemical equations use conventions like reactants on the left, products on the right, plus signs for multiple substances, an arrow showing reaction direction, and symbols for physical states, catalysts, and energy changes.

🎯 Exam Tip: Memorize the specific symbols and their meanings (\( \Delta \), \( \uparrow \), \( \downarrow \), (g), (l), (s), (aq)) for accurately representing chemical reactions.

 

Write the balanced equations for the following reactions:

 

Question 1. Ba(OH)2 + HBr → BaBr2 + H2O
Answer:
Step 1: Rewrite the given equation as it is
\(Ba(OH)_2 + HBr \rightarrow BaBr_2 + H_2O\)
Step 2: write the number of atoms of each element in the unbalanced equation on both sides of the equation.

Element	Number of atoms in reactants (left side)	Number of atoms in products (right side)
Ba	1	1
Br	1	2
O	2	1
H	3	2

Step 3: To balance the number of oxygen atoms:

Number of atoms of oxygen	In reactants	In products
To begin with	2 [in Ba(OH)2]	1 (in H2O)
To balance	2	1 \( \times \) 2

To equalise the number of oxygen atoms, we use 2 as the coefficient of \(H_2O\) in the product.
Now, the partly balanced equation become as follows
\(Ba(OH)_2 + HBr \rightarrow BaBr_2 + 2H_2O\)
Step 4: Now, balance the number of hydrogen atoms.
In the partly balanced equation:

Number of atoms of hydrogen	In reactants	In products
To begin with	2 [in Ba(OH)2] 1 (in HBr)	4 (in 2H2O)
To balance	1 \( \times \) 2 + 2	4

To equalise the number of hydrogen atoms, we use 2 as the coefficient of HBr in the reactants. Now, the equation becomes
\(Ba(OH)_2 + 2HBr \rightarrow BaBr_2 + 2H_2O\)
Now, count the atoms or each element on both sides of the equation. The number of atoms on both
sides are equal. Hence, the balanced equation is
\(Ba(OH)_2 + 2HBr \rightarrow BaBr_2 + 2H_2O\)
Now indicate the physical states of the reactants and products.
\(Ba(OH)_{2(aq)} + 2HBr_{(aq)} \rightarrow BaBr_{2(aq)} + 2H_2O_{(l)}\)
In simple words: To balance the equation \(Ba(OH)_2 + HBr \rightarrow BaBr_2 + H_2O\), we need two HBr molecules and two H2O molecules to ensure all atoms are equal on both sides. The balanced equation is \(Ba(OH)_{2(aq)} + 2HBr_{(aq)} \rightarrow BaBr_{2(aq)} + 2H_2O_{(l)}\).

🎯 Exam Tip: Always list out the number of atoms for each element on both sides of the equation in a table format before and after balancing to ensure accuracy.

 

Question 2. KCN + H2SO4 → K2SO4 + HCN
Answer:
Step 1: Rewrite the given equation as it is
\(KCN + H_2SO_4 \rightarrow K_2SO_4 + HCN\)
Step 2: Write the number of atoms of each element or group in the unbalanced equation on both sides of the equation.

Element	Number of atoms in reactants	Number of atoms in products
K	1	2
CN (group)	1	1
O	4	4
H	2	1
S	1	1

The number of oxygen atoms involved in different compounds on both sides (reactants and products) are equal. Therefore, balance the number of atoms of the second element, potassium.
Step 3: To balance K atoms:

Number of atoms of Potassium	In reactants	In products
To begin with	1 (KCN)	2 (in K2SO4)
To balance	1 \( \times \) 2	2

To equalise the number of potassium atoms, we use 2 as the coefficient of KCN in the reactants.
Now, the partly balanced equation becomes
\(2KCN + H_2SO_4 \rightarrow K_2SO_4 + 2HCN\)
Now, count the atoms of each element on both sides of the equation. The number of atoms on both sides are equal. Hence, the balanced equation is
\(2KCN + H_2SO_4 \rightarrow KgSO_4 + 2HCN\)
Now indicate the physical states of the reactants and products.
\(2KCN_{(aq)} + H_2SO_{4(aq)} \rightarrow K_2SO_{4(aq)} + 2HCN_{(g)}\)
In simple words: To balance \(KCN + H_2SO_4 \rightarrow K_2SO_4 + HCN\), we adjust the coefficients to 2 for KCN and HCN to balance potassium and hydrogen, resulting in \(2KCN_{(aq)} + H_2SO_{4(aq)} \rightarrow K_2SO_{4(aq)} + 2HCN_{(g)}\).

🎯 Exam Tip: When balancing equations with polyatomic ions like CN, treat them as a single unit if they remain intact on both sides; otherwise, balance individual atoms.

 

Question 3. CH4 + O2 → 4CO2 + H2O
Answer:
Step 1: Rewrite the given equation as it is
\(CH_4 + O_2 \rightarrow 4CO_2 + H_2O\)
Step 2: Write the number of atoms of each element in the unbalanced equation on both sides of the equation.

Element	Number of atoms in reactants	Number of atoms in products
C	1	1
O	2	3
H	4	2

Step 3: To balance the number of oxygen atoms:

Number of atoms of oxygen	In reactants	In products
To begin with	2 (in O2)	1 (in H2O) 2 (in CO2)
To balance	2 \( \times \) 2	1 \( \times \) 2 + 2

To equalise the number of oxygen atoms, we use 2 as the coefficient of \(O_2\) in the reactants and 2 as the coefficient of \(H_2O\) in the product.
Now, the partly balanced equation becomes
\(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)
Now, count the atoms of each element on both sides of the equation. The number of atoms on both sides are equal. Hence, the balanced equation is,
\(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)
Now, indicate the physical states of the reactants and products.
\(CH_{4(g)} + 2O_{2(g)} \rightarrow CO_{2(g)} + 2H_2O_{(l)}\)
In simple words: To balance the combustion of methane, \(CH_4 + O_2 \rightarrow CO_2 + H_2O\), we add a coefficient of 2 to \(O_2\) and 2 to \(H_2O\) to balance oxygen and hydrogen, resulting in \(CH_{4(g)} + 2O_{2(g)} \rightarrow CO_{2(g)} + 2H_2O_{(l)}\).

🎯 Exam Tip: For combustion reactions, balance carbon first, then hydrogen, and finally oxygen. This systematic approach usually leads to a correct balance more efficiently.

 

Answer the following questions:

 

Question 1. what are the different types of chemical reaction?
Answer: Types of chemical reaction
1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
4. Double displacement reaction.
In simple words: The main types of chemical reactions include combination, decomposition, displacement, and double displacement reactions.

🎯 Exam Tip: Being able to quickly list the main reaction types is fundamental; be prepared to also define and provide examples for each.

 

Question 2. What is meant by a combination reaction? OR Define: combination reaction.
Answer: When two or more reactants combine in a reaction to form a single product, it is called a combination reaction.
In simple words: A combination reaction is when two or more substances merge to create a single, new product.

🎯 Exam Tip: The defining characteristic of a combination reaction is the formation of *only one* product from multiple reactants.

 

Question 3. Give two examples of combination reaction.
Answer: Examples of combination reaction :
(1) The ammonia gas reacts with hydrogen chloride gas to form the salt in gaseous state, immediately it condenses at room temperature and gets transformed into the solid state. \[NH_{3(g)} + HCl_{(g)} \rightarrow NH_4Cl_{(s)}\]
(2) Magnesium burns in air to form white powder of magnesium oxide as a single product. \[2Mg + O_2 \xrightarrow{heat} 2MgO\]
(3) Iron reacts with sulphur to form iron sulphide. \[Fe_{(s)} + S_{(s)} \xrightarrow{heat} FeS_{(s)}\]
In simple words: Ammonia reacting with hydrogen chloride to form ammonium chloride, and magnesium burning in air to form magnesium oxide, are examples of combination reactions.

🎯 Exam Tip: When providing examples, ensure the products are clearly stated as single substances, reinforcing the definition of a combination reaction.

 

Question 4. What Is meant by a decomposition reaction?
Answer: The chemical reaction in which two or more products are formed from a single reactant is called decomposition reaction.
In simple words: A decomposition reaction is when a single compound breaks down into two or more simpler substances.

🎯 Exam Tip: The key identifier for a decomposition reaction is *one* reactant forming *multiple* products.

 

Question 5. what is meant by a thermal decomposition?
Answer: The reaction in which a compound is decomposed by heating it to a high temperature is called thermal decomposition.
In simple words: Thermal decomposition is a type of decomposition reaction where heat energy is used to break down a single compound into multiple products.

🎯 Exam Tip: Remember that "thermal" specifically refers to the use of heat as the energy source for decomposition.

 

Question 6. What is meant by a electrolytic decomposition?
Answer: The reaction in which a compound is decomposed by passing an electric current through its solution or molten mass is called an electrolytic decomposition.
In simple words: Electrolytic decomposition is when a compound breaks down into simpler substances due to the passage of electric current through its molten state or solution.

🎯 Exam Tip: Distinguish electrolytic decomposition from thermal decomposition by the energy source: electricity for electrolytic, heat for thermal.

 

Question 7. Give two examples of thermal decomposition.
Answer:
(1) At high temperature, calcium carbonate decomposes into calcium oxide and carbon dioxide. \[CaCO_{3(s)} \xrightarrow{heat} CaO_{(s)} + CO_{2(g)}\]
(2) At high temperature sugar decomposes into black mass of carbon and water vapour. \[C_{12}H_{22}O_{11(s)} \xrightarrow{heat \ \Delta} 12C_{(s)} + 11H_2O_{(g)} \uparrow\]
In simple words: Calcium carbonate breaking down into calcium oxide and carbon dioxide, and sugar decomposing into carbon and water upon heating, are examples of thermal decomposition.

🎯 Exam Tip: Ensure that your examples clearly show a single reactant breaking into multiple products specifically due to heat, often indicated by a \( \Delta \) symbol.

 

Question 8. Give an example of electrolytic decomposition.
Answer: When an electric current is passed through acidified water, it is electrolysed giving hydrogen and oxygen. \[2H_2O_{(l)} \xrightarrow{electric \ current} 2H_{2(g)} \uparrow + O_{2(g)} \uparrow\]
In simple words: Electrolysis of water, where electric current breaks water into hydrogen and oxygen gases, is a prime example of electrolytic decomposition.

🎯 Exam Tip: The electrolysis of water is a classic example; remember to indicate the "electric current" condition above the arrow and the gaseous products with upward arrows.

 

Question 9. Study the following reaction and answer the questions asked. \(2H_2O_{(l)} \xrightarrow{Electrical \ energy} 2H_2 \uparrow + O_2 \uparrow\)
Answer:
(a) The type of reaction is electrolytic decomposition reaction.
(b) The reaction in which a compound is decomposed by passing an electric current through its solution or molten mass is called an electrolytic decomposition.
In simple words: The given reaction is an electrolytic decomposition where water is broken down into hydrogen and oxygen gas using electrical energy, a process defined by decomposing a compound via electric current.

🎯 Exam Tip: For reactions involving energy input like electricity, explicitly mention the energy type and its role in the decomposition process.

 

Question 10. what is meant by a displacement reaction?
Answer: The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.
In simple words: A displacement reaction occurs when a more reactive element replaces a less reactive element from its compound.

🎯 Exam Tip: Emphasize the concept of "more reactive replacing less reactive" as the core principle of displacement reactions.

 

Question 11. Give an example of displacement reaction.
Answer: when zinc granules are added to the blue coloured copper sulphate solution, the zinc ions formed from zinc atoms take the place or \(Cu^{2+}\) ions in \(CuSO_4\), and copper atoms, formed from \(Cu^{2+}\) ions comes out i.e. the more reactive zinc displaces the less reactive Cu from copper sulphate. \[Zn_{(s)} + CuSO_{4(aq)} \rightarrow ZnSO_{4(aq)} + Cu_{(s)} + Heat\]
In simple words: When zinc metal is added to copper sulphate solution, zinc, being more reactive, displaces copper to form zinc sulphate and solid copper, demonstrating a displacement reaction.

🎯 Exam Tip: For displacement reactions, clearly identify the more reactive element and the less reactive element it displaces, and mention observable changes like color change or precipitate formation.

 

Question 12. Observe the reaction and answer the following questions. \(CuSO_{4(aq)} + Fe_{(s)} \rightarrow FeSO_{4(aq)} + Cu_{(s)}\)
Answer:
(a) When iron powder is added to the blue coloured copper sulphate solution, the iron ions formed from iron atoms take the place or \(Cu^{2+}\) ions in \(CuSO_4\), and copper atoms, formed from \(Cu^{2+}\) ions comes out i.e. the more reactive iron displaces the less reactive Cu from copper sulphate. Therefore this reaction is a displacement reaction.
(b) The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.
In simple words: This reaction is a displacement reaction where iron, being more reactive than copper, replaces copper from copper sulphate solution, forming ferrous sulphate and solid copper.

🎯 Exam Tip: To fully explain a displacement reaction, describe the reactivity difference between the elements and how this leads to one replacing the other in the compound.

 

Question 13. what is meant by a double displacement reaction?
Answer: The reaction in which the ions in the reactants are exchanged to form a precipitate is called double displacement reaction.
In simple words: A double displacement reaction involves the exchange of ions between two reactant compounds, typically forming a precipitate or a gas or water.

🎯 Exam Tip: The critical feature of a double displacement reaction is the mutual exchange of ions, often resulting in an insoluble product (precipitate).

 

Question 14. Give two examples of double displacement reaction.
Answer:
(1) Solutions of sodium chloride and silver nitrate react with each other forming a precipitate of silver chloride and a solution of sodium nitrate. \[AgNO_3 + NaCl \rightarrow AgCl_{(s)} \downarrow + NaNO_3\]
White precipitate of AgCl is formed by exchange of ions \(Ag^+\) and \(Cl^-\) between the reactants.
(2) Barium suiphide reacts with zinc sulphate to form zinc sulphide and a white precipitate of barium sulphate. \[BaS + ZnSO_4 \rightarrow BaSO_{4(s)} \downarrow + ZnS\]
white precipitate is formed by exchange of ions \(Ba^{++}\) and \(SO_4^-\) between the reactants.
In simple words: Examples include silver nitrate reacting with sodium chloride to form silver chloride precipitate, and barium sulphide reacting with zinc sulphate to form barium sulphate precipitate, both involving ion exchange.

🎯 Exam Tip: When providing examples, clearly identify the precipitate formed and explain how the exchange of ions leads to its formation.

 

Question 15. Write down what you understand from the following chemical reaction: \(AgNO_{3(aq)} + NaCl_{(aq)} \rightarrow AgCl \downarrow + NaNO_{3(aq)}\)
Answer:
(i) The above reaction is a double displacement reaction.
(ii) \(AgNO_3\) and \(NaCl\) are the reactants while \(AgCl\) and \(NaNO_3\) are the products.
(iii) The reactants and the product \(NaNO_3\) are in aqueous state. The product \(AgCl\) is formed in the form of precipitate.
In simple words: This is a double displacement reaction where aqueous silver nitrate and sodium chloride react to exchange ions, forming a solid precipitate of silver chloride and aqueous sodium nitrate.

🎯 Exam Tip: For explaining a given reaction, identify the type, reactants, products, and their physical states, paying special attention to precipitates.

 

Question 16. Study the following chemical reaction and answer the questions given below: \(AgNO_{3(aq)} + NaCl_{(aq)} \rightarrow AgCl_{(s)} \downarrow + NaNO_{3(aq)}\)
Answer:
(i) The type of chemical reaction: Double displacement reaction.
(ii) The reaction in which the ions in the reactants are exchanged to form a precipitate is called double displacement reaction.
(iii)
1. The above reaction is a double displacement reaction.
2. \(AgNO_3\) and \(NaCl\) are the reactants while \(AgCl\) and \(NaNO_3\) are the products.
3. The reactants and the product \(NaNO_3\) are in aqueous state. The product \(AgCl\) is formed in the form of precipitate.
In simple words: This is a double displacement reaction where silver nitrate and sodium chloride exchange ions, forming silver chloride precipitate and aqueous sodium nitrate.

🎯 Exam Tip: When asked to analyze a reaction, systematically identify its type, define that type, and then list all reactants and products with their states.

 

Question 17. When sodium chromate solution is mixed with barium sulphate solution, a precipitate is formed.
Answer:
(i) The colour of the precipitate is yellow.
(ii) The yellow precipitate formed is barium chromate.
(iii) The type of chemical reaction is double displacement.
In simple words: Mixing sodium chromate and barium sulphate solutions results in a yellow precipitate of barium chromate, indicating a double displacement reaction.

🎯 Exam Tip: For reactions involving precipitation, knowing the color of common precipitates can be a bonus point, alongside identifying the reaction type.

 

Question 18. Explain the term Exothermic reaction.
Answer: Exothermic reaction : The process in which heat is given out is called an exothermic reaction.
In simple words: An exothermic reaction is a chemical process that releases heat energy into its surroundings, often causing a rise in temperature.

🎯 Exam Tip: The key characteristic of an exothermic reaction is the *release* of heat, which can be noted by a temperature increase in the surroundings.

 

Question 19. State whether the following reactions are exothermic or endothermic:
Answer:
(i) \(3CaO \cdot Al_2O_{3(s)} + 6H_2O_{(l)} \rightarrow 3CaO \cdot Al_2O_3 \cdot 6H_{2}O_{(s)} + Heat\)
Answer: Exothermic reaction
In simple words: This reaction is exothermic because "Heat" is listed as a product, indicating energy release.

🎯 Exam Tip: Look for "Heat" or "energy released" on the product side to identify exothermic reactions.

Answer:
(ii) \(2CaSO_4 \cdot H_2O + 3H_2O \rightarrow 2CaSO_4 \cdot 2H_2O + Heat\)
Answer: Exothermic reaction
In simple words: This reaction is exothermic because "Heat" is shown as a product, signifying that energy is released.

🎯 Exam Tip: The presence of "+ Heat" on the product side is a direct indicator of an exothermic process.

Answer:
(iii) \(KNO_{3(aq)} + H_2O_{(l)} + Heat \rightarrow KNO_{3(aq)}\)
Answer: Endothermic reaction
In simple words: This reaction is endothermic because "Heat" is on the reactant side, meaning energy is absorbed from the surroundings.

🎯 Exam Tip: If "Heat" or "energy absorbed" is on the reactant side, the reaction is endothermic.

Answer:
(iv) \(NaOH_{(s)} + H_2O_{(l)} \rightarrow NaOH_{(aq)} + Heat\)
Answer: Exothermic reaction
In simple words: This reaction is exothermic as "Heat" is produced, indicating energy release during the dissolution of sodium hydroxide.

🎯 Exam Tip: Dissolution processes that release heat are exothermic; always check for the "Heat" term on the product side.

Answer:
(v) Transformation of ice into water.
Answer: Endothermic reaction
In simple words: Melting ice into water is an endothermic process because it absorbs heat from the surroundings to change state.

🎯 Exam Tip: Phase changes like melting require energy input (endothermic), while freezing releases energy (exothermic).

Answer:
(vi) Water turns into ice.
Answer: Exothermic reaction
In simple words: Water turning into ice is an exothermic reaction because it releases heat energy to the surroundings as it freezes.

🎯 Exam Tip: Freezing is an exothermic process as latent heat is released when water changes to a solid state.

Answer:
(vii) Cooking of food.
Answer: Endothermic reaction
In simple words: Cooking food is an endothermic process as it requires continuous heat absorption for the chemical changes to occur.

🎯 Exam Tip: Many everyday processes that require continuous heating, like cooking, are endothermic reactions.

Question 20. What do you mean by slow speed reaction?
OR
Define: Slow speed reaction.
Answer: The reaction which requires long time for completion i.e. occurs slowly is called slow speed reaction.
In simple words: A slow speed reaction is one that takes a long time to complete, like the rusting of iron.

🎯 Exam Tip: Defining reaction types clearly with examples is key for scoring well.

Question 21. What do you mean by fast speed reaction?
OR
Define: Fast speed reaction.
Answer: The reaction which is completed in short time i.e. occurs rapidly is called fast speed reaction.
In simple words: A fast speed reaction is one that completes quickly, often almost instantaneously, such as an acid-base neutralization.

🎯 Exam Tip: Differentiate clearly between slow and fast reactions, often requiring time as a key parameter.

Question 22. Give two examples of slow speed reactions.
Answer:
(1) On heating potassium chlorate (KClO3) it decomposes slowly into potassium chloride and oxygen gas.
\[ 2KClO_3 \xrightarrow{} 2KCl + 3O_2 \uparrow \] Potassium chlorate Oxygen
This reaction requires long time for completion, therefore it is slow speed reaction.
(2) Rusting of iron is a slow speed reaction. In this reaction iron reacts with oxygen from air to form iron oxide.
\[ 4Fe_{(s)} + 3O_{2(g)} \xrightarrow{} 2Fe_2O_{3(s)} \] Iron Iron oxide
In simple words: Two examples of slow reactions are the decomposition of potassium chlorate upon heating and the rusting of iron. These reactions proceed gradually over an extended period.

🎯 Exam Tip: Always provide balanced chemical equations when asked for examples of reactions.

Question 23. Give two examples of fast speed reactions.
Answer:
(1) The reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) is a neutralization reaction and it is fast speed reaction.
(2) Aqueous solution of sodium chloride reacts with silver nitrate solution to form white precipitate of silver chloride (NaCl) and sodium nitrate.
In simple words: Fast reactions occur quickly; examples include the neutralization of an acid and a base, and the formation of silver chloride precipitate from sodium chloride and silver nitrate solutions.

🎯 Exam Tip: Note how different reactions have varying speeds, which is an important characteristic in chemistry.

Question 24. Write a short note on slow speed and fast speed reactions.
Answer:
Slow speed reaction:
The reaction which requires long time for completion i.e. occurs slowly is called slow speed reaction.
Examples:
(1) On heating potassium chlorate (KClO3) it decomposes slowly into potassium chloride and oxygen gas.
\[ 2KClO_3 \xrightarrow{} 2KCl + 3O_2 \uparrow \] Potassium chlorate Oxygen
This reaction requires long time for completion, therefore it is slow speed reaction.
(2) Rusting of iron is a slow speed reaction. In this reaction iron reacts with oxygen from air to form iron oxide.
\[ 4Fe_{(s)} + 3O_{2(g)} \xrightarrow{} 2Fe_2O_{3(s)} \] Iron Iron oxide
This reaction requires long time for completion.
Fast speed reaction:
The reaction which is completed in short time i.e., occurs rapidly is called fast speed reaction.
Examples:
(1) The reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) is a neutralization reaction and it is fast speed reaction.
\[ NaOH_{(aq)} + HCl_{(aq)} \xrightarrow{} NaCl_{(aq)} + H_2O_{(l)} \] This neutralization reaction is completed in short time, therefore it is fast speed reaction.
(2) Aqueous solution of sodium chloride reacts with silver nitrate solution to form white precipitate of silver chloride (NaCl) and sodium nitrate.
\[ NaCl_{(aq)} + AgNO_{3(aq)} \xrightarrow{} NaNO_{3(aq)} + AgCl_{(white)} \] This reaction is completed in short time, therefore it is fast reaction.
In simple words: Slow reactions take a long time to complete, like rusting and decomposition of potassium chlorate. Fast reactions happen quickly, such as acid-base neutralization or precipitate formation, completing in a short duration.

🎯 Exam Tip: When asked for a short note, ensure you define both types of reactions and provide at least two distinct examples for each, including balanced equations where appropriate.

Question 25. State the factors which affect the speed (or rate) of a reaction.
Answer: The factors which affect the rate of a reaction are
1. Nature of the reactants.
2. Size of the particles of the reactants.
3. Concentration of the reactants.
4. Temperature of the reaction.
5. Catalyst.
In simple words: The rate of a chemical reaction is influenced by the inherent properties of the substances involved, the size of their particles, how concentrated they are, the temperature, and the presence of catalysts.

🎯 Exam Tip: Memorize these five key factors as they are frequently tested in exams regarding reaction kinetics.

Question 26. How does the rate of reaction depend on the nature of the reactants? Illustrate with suitable example.
Answer:
(1) when the reactant combines with two or more other reactants then the rate of a chemical reaction depends on the nature of the reactants.
(2) Both Al and Zn reacts with dilute hydrochloric acid, H2 gas is liberated and water soluble salts of these metals are formed. However, aluminium metal reacts faster with dil. HCl as compared to zinc metal.
In simple words: The rate of a reaction varies based on the chemical nature of the reactants; for instance, more reactive metals like aluminum react faster with acids than less reactive ones like zinc, even under similar conditions.

🎯 Exam Tip: When explaining the "nature of reactants," link it to their reactivity or inherent properties that influence bond breaking and formation.

Question 27. How does the rate of a reaction depend on the size of the particles of reactants?
Answer:
(1) In the reaction of dil. HCl and Shahabad tile, CO2 effervescence is formed slowly. On the other hand, CO2 effervescence forms at faster speed with the powder of Shahabad tile.
(2) The above observation indicates that the rate of a reaction depends upon the size of the particles of the reactants taking part in the reaction. Smaller the size of the reactant particles taking part in a reaction faster will be the rate of reaction.
In simple words: The rate of a chemical reaction increases as the surface area of solid reactants increases, meaning smaller particles react faster due to more exposed contact points.

🎯 Exam Tip: Always relate particle size to surface area when discussing its effect on reaction rate.

Question 28. How does the rate of a reaction depend upon the concentration of the reactants? Give suitable example.
Answer:
(1) A chemical reaction takes place due to collisions of the reactant molecules. Higher the concentrations of the reactants more will be the frequency of collisions and faster will be the rate of the reaction.
(2) In the reaction of dil. HCl and CaCO3, CaCO3 disappears slowly and CO2 also liberates slowly. On the other hand the reaction with concentrated HCl takes place rapidly and CaCO3 disappears fast.
(3) Concentrated acid reacts faster than dilute acid, that means the rate of a reaction is proportional to the concentration of reactants.
Slow reaction:
\[ CaCO_3 + dli.2HCl \xrightarrow{} CaCl_2 + CO_2 + H_2O \]In simple words: Increasing the concentration of reactants typically speeds up a reaction because there are more reactant particles available to collide and react, as seen when concentrated HCl reacts faster with calcium carbonate than dilute HCl.

🎯 Exam Tip: Explain the effect of concentration in terms of collision theory - higher concentration leads to more frequent effective collisions.

Question 29. How does the rate of a reaction depend upon the temperature of reactants? Give suitable example.
Answer:
(i) (1) When the temperature of the reactants is increased, the reactant molecules start moving with more velocity and their kinetic energy increases. As a result, the number collisions increases. Hence, the rate of chemical reaction increases.
(2) Lime stone on heating decomposes to give CO2, which turns lime water milky. On the other hand, the lime water does not turn milky before heating the lime stone: because of the zero rate of reaction. The above observation indicates that the rate of a reaction increases on increasing the temperature.
(ii) Solid CaCO3 does not decompose at room temperature when heated, it decomposes to give CaO and CO2 that means the rise in temperature increases the rate of reaction. CaCO3 room temperature No chemical reaction.
\[ CaCO_3 \xrightarrow{\text{Room temperature, No chemical reaction.}} \] \[ CaCO_3 \xrightarrow{} CaO + CO_2 \uparrow \]
In simple words: Increasing the temperature of reactants boosts the reaction rate because molecules gain more kinetic energy, leading to more frequent and energetic collisions. For example, limestone (CaCO3) only decomposes to produce CO2 when heated, not at room temperature.

🎯 Exam Tip: Always connect temperature to kinetic energy and collision frequency when explaining its effect on reaction rate.

Question 30. How does the rate if a reaction depend upon the catalyst? Give suitable example.
Answer:
(1) The substance in whose presence the rate of a chemical reaction changes, without causing any chemical change to it is called a catalyst.
(2) On heating potassium chlorate (KClO3) decomposes into potassium chloride and oxygen slowly.
\[ 2KClO_3 \xrightarrow{} 2KCl + 3O_2 \] The rate of the above reaction neither increases by reducing the particle size nor by increasing the reaction temperature. However in the presence of manganese dioxide, KClO3 decomposes at a comparatively lower temperature and oxygen is produced more briskly. No chemical change takes place in MnO2 in this reaction. It acts as catalyst.
In simple words: A catalyst changes the speed of a chemical reaction without being consumed itself. For example, manganese dioxide (MnO2) speeds up the decomposition of potassium chlorate, allowing it to happen faster and at a lower temperature.

🎯 Exam Tip: Clearly state that catalysts participate in the reaction but are recovered unchanged at the end, and illustrate with a common example like the decomposition of KClO3.

Question 31. State the Importance of rate in a chemical reaction.
Answer:
1. The use of strong acid and strong base in a chemical reaction increases the rate of reaction.
2. In a chemical reaction, if the smaller size of the reactant particles, the concentrated solution, high temperature and use of catalyst increases the rate of chemical reaction.
3. The rate of chemical reaction is important with respect to environment.
4. If the rate of chemical reaction is fast it is profitable for the chemical factories.
5. The ozone layer in the earth's atmosphere protects the life of earth from the ultraviolet radiation of the sun. The process of depletion or maintenance of this layer depends upon the rate of production or destruction of ozone molecules.
In simple words: Understanding reaction rates is crucial for various reasons: it helps control industrial processes for efficiency, explains environmental phenomena like ozone depletion, and highlights how factors like acid strength, particle size, concentration, temperature, and catalysts influence reaction speeds.

🎯 Exam Tip: Focus on real-world applications and environmental relevance to emphasize the importance of reaction rates.

Question 32. Define Oxidation reaction.
Answer: Oxidation: The chemical reaction in which a reactant combines with oxygen or loses hydrogen to form the product is called oxidation reaction.
In simple words: Oxidation is a chemical reaction where a substance either gains oxygen or loses hydrogen.

🎯 Exam Tip: Remember both definitions of oxidation: gain of oxygen and loss of hydrogen. For advanced levels, also consider loss of electrons.

Question 33. Give examples of oxidation.
Answer:
(1) when carbon burns in air, it forms carbon dioxide. In this reaction carbon accepts oxygen, therefore, this is an oxidation reaction.
\[ C_{(s)} + O_{2(g)} \xrightarrow{} CO_{2(g)} \] (2) when sodium reacts with ethyl alcohol, sodium ethoxide and hydrogen gas is formed. In this reaction, hydrogen is removed from ethyl alcohol, therefore this is an oxidation reaction.
\[ 2C_2H_5OH_{(l)} + 2Na \xrightarrow{} 2C_2H_5ONa + H_2 \uparrow \] Ethyl alcohol Sodium Sodium ethoxide Hydrogen
(3) Acidified potassium dichromate (K2Cr2O7 / H2SO4) oxidises ethyl alcohol to acetic acid.
\[ CH_3-CH_2-OH \xrightarrow{\text{2[O]}} CH_3-COOH + H_2O \] K2Cr2O7/H2SO4
In simple words: Oxidation examples include carbon burning to form carbon dioxide (gaining oxygen), sodium reacting with ethyl alcohol to remove hydrogen, and potassium dichromate oxidizing ethyl alcohol to acetic acid.

🎯 Exam Tip: Provide diverse examples showcasing both gain of oxygen and loss of hydrogen to demonstrate a complete understanding of oxidation.

Question 34. What do you mean by oxidant? Explain with suitable example.
Answer: The chemical substances which bring about an oxidation reaction by making oxygen available are called oxidants or oxidizing agents.
1. In the combustion of carbon, oxygen is an oxidant.
2. In the oxidation of ethyl alcohol, potassium dichromate is used as oxidant.
In simple words: An oxidant, or oxidizing agent, is a substance that causes another substance to be oxidized by providing oxygen or taking electrons. For example, oxygen itself acts as an oxidant during combustion, and potassium dichromate oxidizes ethyl alcohol.

🎯 Exam Tip: Always distinguish between the process of oxidation and the substance that causes it (oxidant).

Question 35. Name the various oxidants. How nascent oxygen is liberated from these oxidants?
Answer: K2Cr2O7/H2SO4, KMnO4/H2SO4 are the commonly used chemical oxidants. Hydrogen peroxide (H2O2) is used as a mild oxidant. Ozone (O3) is also a chemical oxidant. Nascent oxygen is generated by chemical oxidants and it is used for the oxidation reaction.
\[ O_3 \xrightarrow{} O_2 + [O] \] \[ H_2O_2 \xrightarrow{} H_2O + [O] \] \[ K_2Cr_2O_7 + 4H_2SO_4 \xrightarrow{} K_2SO_4 + Cr_2(SO_4)_3 + 4H_2O + 3[O] \] \[ 2KMnO_4 + 3H_2SO_4 \xrightarrow{} K_2SO_4 + 2MnSO_4 + 3H_2O + 5[O] \] Nascent oxygen is a state prior to the formation of the O2 molecule. It is the reactive form of oxygen and is represented by the symbol as [O]
In simple words: Common oxidants include potassium dichromate/sulfuric acid, potassium permanganate/sulfuric acid, hydrogen peroxide, and ozone. These substances liberate nascent oxygen (represented as [O]), which is a highly reactive form of oxygen responsible for carrying out oxidation reactions.

🎯 Exam Tip: When writing reactions involving nascent oxygen, clearly show its production and represent it with [O] to signify its highly reactive atomic state.

Question 36. Acidified potassium permanganate (KMnO4) is a chemical oxidant and explain, how acidified potassium permanganate oxidise ferrous sulphate (FeSO4). Accordingly write a new definition of oxidation and reduction.
Answer: Acidified KMnO4 oxidises ferrous sulphate (FeSO4) to ferric sulphate Fe2(SO4)3 and in addition to above K2SO4 and MnSO4 by-products are formed.
\[ 2KMnO_4 + 10 FeSO_4 \xrightarrow{} 8H_2SO_4 \xrightarrow{} K_2SO_4 + 2MnSO_4 + 5Fe_2(SO_4)_3 + 8H_2O \] \[ 2FeSO_4 \xrightarrow{} Fe_2(SO_4)_3 \] Ionic reaction :
\[ 2Fe^{2+} + 2SO_4^{2+} \xrightarrow{} 2Fe^{3+} + 3SO_2^{2-} \] Net Ionic equation
\[ Fe^{2+} \xrightarrow{} Fe^{3+} + e^- \] When ferric ion is formed from ferrous ion the positive charge is increased by one unit. While this happens the ferrous ion loses one electron.
When metal or its ion loses electron, it is called an oxidation and gain of electron is called reduction.
\[ Fe^{2+} \xrightarrow{\text{Oxidation}} Fe^{3+} + e^- \] Ferrous Ferric
\[ Fe^{3+} + e^- \xrightarrow{\text{Reduction}} Fe^{2+} \]In simple words: Acidified potassium permanganate is a strong oxidant that converts ferrous sulfate (Fe2+) to ferric sulfate (Fe3+). This involves the ferrous ion losing an electron, which is defined as oxidation. Conversely, gaining an electron is defined as reduction.

🎯 Exam Tip: For redox reactions, mastering the definitions based on electron transfer (loss for oxidation, gain for reduction) is crucial, especially for ionic equations.

Question 37. Define reduction reaction.
Answer: The chemical reaction in which a reactant gains hydrogen and loses oxygen to form the product is called the reduction reaction.
In simple words: Reduction is a chemical reaction where a substance either gains hydrogen or loses oxygen.

🎯 Exam Tip: Similar to oxidation, remember both definitions of reduction: gain of hydrogen and loss of oxygen. For advanced levels, also consider gain of electrons.

Question 38. Give two examples of reduction.
Answer:
(1) When hydrogen gas is passed over black copper oxide a reddish coloured layer of copper is formed.
In this reaction an oxygen atom removed from CuO to form copper, hence, this is reduction.
\[ CuO_{(s)} + H_{2(g)} \xrightarrow{} Cu_{(s)} + H_2O_{(l)} \] Copper oxide Copper
(2) When hydrogen gas is passed over red hot coke, methane is obtained.
Here, hydrogen is added to coke (carbon). Hence, this is reduction.
\[ C_{(s)} + 2H_{2(g)} \xrightarrow{} CH_{4(g)} \] Carbon Hydrogen gas Methane
In simple words: Two examples of reduction are hydrogen gas reacting with copper oxide to remove oxygen and form copper, and hydrogen gas reacting with red-hot carbon (coke) to add hydrogen and form methane.

🎯 Exam Tip: Provide balanced chemical equations for each example to fully illustrate the reduction process.

Question 39. What do you mean by reductant? Explain with suitable example.
Answer: The chemical substances which bring about reduction by making hydrogen available are called reductant. In the preparation of methane from carbon, hydrogen is a reductant.
In simple words: A reductant, or reducing agent, is a substance that causes another substance to be reduced, typically by providing hydrogen or electrons. In the formation of methane from carbon, hydrogen acts as the reductant.

🎯 Exam Tip: Clearly differentiate between the process of reduction and the substance that causes it (reductant).

Question 40. What are redox reactions? Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(1) 2H2S2(g) + SO2(g) → 3S(s) + 2H2O(l)
(2) CuO(s) + H2(g) → CU(s) + H2O(l)
(3) MnO2 + 4HCl → MnCl2 + 2H2O + Cl2↑
Answer: When oxidation and reduction take place simultaneously in a given chemical reaction, it is known as a redox reaction.
(1)
\[ 2H_2S + SO_2 \xrightarrow{} 3S + 2H_2O \]
\( \uparrow \) oxidation

\( \downarrow \) reduction
H2S is oxidised and SO2 is reduced.
(2)
\[ CuO + H_2 \xrightarrow{} Cu + H_2O \]
\( \downarrow \) reduction

\( \uparrow \) oxidation
CuO is reduced and H2 is oxidised.
(3)
\[ MnO_2 + 4HCl \xrightarrow{} MnCl_2 + 2H_2O + Cl_2 \uparrow \]
\( \downarrow \) reduction

\( \uparrow \) oxidation
(1) Oxidation: H2S, H2O and HCl.
(2) Reduction: SO2, CuO and MnO2
In simple words: Redox reactions involve both oxidation (loss of electrons/gain of oxygen/loss of hydrogen) and reduction (gain of electrons/loss of oxygen/gain of hydrogen) happening at the same time. In the given reactions, H2S and H2 are oxidized while SO2 and CuO are reduced. HCl is oxidized in the third reaction.

🎯 Exam Tip: For redox reactions, clearly indicate which substance is oxidized (loses electrons/gains oxygen/loses hydrogen) and which is reduced (gains electrons/loses oxygen/gains hydrogen).

Question 41. Observe the following reaction and answer the questions given below:
BaSO4 + 4C → BaS + 4CO
(1) what type of reaction is it? Justify.
(2) Give one more example.
Answer:
(1) This is a redox reaction. In this reaction the reduction of BaSO4 and oxidation of carbon take place simultaneously.
\[ BaSO_4 + 4C \xrightarrow{} BaS + 4CO \]
\( \uparrow \) oxidation

\( \downarrow \) reduction
(2) Example
\[ CuO+H_2 \xrightarrow{} Cu + H_2O \] \[ 2H_2S + SO_2 \xrightarrow{} 3S + 2H_2O \]In simple words: The reaction BaSO4 + 4C → BaS + 4CO is a redox reaction because BaSO4 loses oxygen (reduction) while carbon gains oxygen (oxidation) simultaneously. Another example is copper oxide reacting with hydrogen to form copper and water, where copper oxide is reduced and hydrogen is oxidized.

🎯 Exam Tip: For justification, clearly state which species is oxidized and which is reduced based on changes in oxygen content or oxidation states.

Question 42. What is corrosion?
Answer: The slow process of decay or oxidation of metals due to various components of atmosphere is known as corrosion.
Iron rusts and a reddish coloured layer is collected on it. This is corrosion of iron.
In simple words: Corrosion is the gradual degradation of metals, typically through oxidation, caused by their reaction with atmospheric components like moisture and oxygen, such as the formation of reddish rust on iron.

🎯 Exam Tip: Emphasize that corrosion is a slow, oxidative process and provide a common example like rusting.

Question 43. How does rusting of iron occur?
Answer: Iron when exposed to moist air forms a reddish layer of hydrated ferric oxide.
\[ 2Fe + 3O_2 + xH_2O \xrightarrow{} 2Fe_2O_3.xH_2O \] Ferric oxide (rust)
In simple words: Rusting of iron happens when iron reacts with oxygen and moisture in the air over time, forming a reddish-brown substance called hydrated ferric oxide (rust).

🎯 Exam Tip: Include the chemical formula for rust and mention the necessary conditions (oxygen and moisture) for rusting to occur.

Question 44. How can corrosion be prevented?
Answer:
1. Corrosion damages buildings, bridges, automobiles, ships, iron railings and other articles made of iron.
2. It can be prevented by using an anti-rust solution, coating the surface by a paint, processes like galvanising and electroplating with other metals.
In simple words: Corrosion can be prevented by applying protective coatings like paint, using anti-rust solutions, or employing techniques like galvanizing (coating with zinc) and electroplating with other metals to shield the metal surface from air and moisture.

🎯 Exam Tip: List practical methods of corrosion prevention, focusing on those that create a barrier or use more reactive metals for sacrificial protection.

Question 45. What is corrosion? Do gold ornaments corrode? Justify.
Answer: The slow process of decay or oxidation or metal due to the effect of air, moisture and acids on it is known as corrosion.
(1) Gold is a noble metal. There is no effect or moist air or action of acid on it at any temperature.
(2) Pure gold is a very soft metal. it breaks and gets bent easily. Hence, in gold ornaments, gold is alloyed with other metals like copper or silver in appropriate proportion to make it hard and resistant to corrosion. Hence gold ornaments do not get corroded.
In simple words: Corrosion is the gradual degradation of metals through oxidation. Gold ornaments do not corrode because gold is a noble metal that does not react with air, moisture, or acids; it is often alloyed with other metals to enhance its hardness without losing its corrosion resistance.

🎯 Exam Tip: When justifying why gold doesn't corrode, highlight its 'noble' nature and explain the role of alloying in making ornaments durable without losing corrosion resistance.

Question 46. Complete the process of iron rusting by filling the blanks. Suggest a way to prohibit the process.
The iron rust is formed due to reaction. Different regions on iron surface become anode and cathode.
Reaction on anode region:
Fe(s) → Fe2+ (aq) + 2e-
Reaction on cathode region:
O2(g) + 4H+ (aq) +.........→ 2H2O(l)
when Fe2+ ions migrate from anode region they react with.........to fomm Fe3+ ions.
Answer: The iron rust is formed due to electrochemical reaction. Different regions on iron surface become anode and cathode.
Reaction on anode region:
\[ Fe_{(s)} \xrightarrow{} Fe^{2+}_{(aq)} + 2e^- \] Reaction on cathode region:
\[ O_{2(g)} + 4H^+_{(aq)} + 4e^- \xrightarrow{} 2H_2O_{(l)} \] When Fe2+ ions migrate from anode region they react with water to form Fe3+ ions.
A reddish coloured hydrated oxide is formed from Fe3+ ions. It is called rust.
\[ 2Fe^{3+}_{(aq)} + 4H_2O_{(l)} \xrightarrow{} Fe_2O_3.H_2O_{(s)} + 6H^+_{(aq)} \] A way to prevent rusting by colouring with acrylic paints, Zn plating, galvanizing, anodizing, alloying, etc.
In simple words: Iron rusting is an electrochemical process where iron acts as an anode, losing electrons to form Fe2+ ions, while oxygen and H+ ions act as a cathode, forming water. These Fe2+ ions further react with water to form Fe3+ ions, which then form hydrated ferric oxide (rust). Rusting can be prevented by painting, galvanizing, or alloying.

🎯 Exam Tip: For detailed questions on rusting, explain the electrochemical mechanism, including anode/cathode reactions, and provide multiple practical prevention methods.

Question 17. Deifne: Rancidity.
Answer: When oil or fat or left over cooking oil for making food stuff undergoes oxidation if stored for a long time and it is found to have foul odour called rancidity.
In simple words: Rancidity is the unpleasant smell and taste that develops in oils and fats or foods containing them when they undergo oxidation upon prolonged exposure to air.

🎯 Exam Tip: The key aspects of rancidity are oxidation, foul odor/taste, and its occurrence in fats/oils over time.

Distinguish between the following:

Question 1. Combination reaction and Decomposition reaction.
Answer:

Combination reaction	Decomposition reaction
1. In a combination reaction, two or more reactants take part in the chemical reaction.	1. In a decomposition reaction there is only one reactant in the chemical reaction.
1. In the combination reaction, only one product is formed.	2. In a decomposition reaction, two or more products are formed.

In simple words: A combination reaction involves two or more substances combining to form a single product, whereas a decomposition reaction involves a single reactant breaking down into two or more products.

🎯 Exam Tip: When distinguishing, provide clear, contrasting points for each characteristic, such as the number of reactants and products.

Question 2. Oxidation and reduction
Answer:

Oxidation	Reduction
1. The chemical reaction in which reactants gain oxygen or lose hydrogen is called oxidation.	1. The chemical reaction in which reactants gain hydrogen or lose oxygen is called reduction.
2. A reducing agent undergoes oxidation.	2. An oxidising agent undergoes reduction.

In simple words: Oxidation is the process of gaining oxygen or losing hydrogen, while reduction is the process of gaining hydrogen or losing oxygen. A reducing agent itself gets oxidized, and an oxidizing agent itself gets reduced.

🎯 Exam Tip: Ensure you define both processes in terms of oxygen/hydrogen transfer and how the agents (oxidizing/reducing) behave. For a complete answer, consider electron transfer as well if appropriate for the syllabus.

Question 3. Exothermic and Endothermic reaction.
Answer:

Exothermic reaction	Endothermic reaction
1. The reaction in which heat is evolved is called an exothermic reaction.	1. The reaction in which heat is absorbed is called an endothermic reaction.
2. The evolution of heat leads to a rise in the temperature of the solution.	2. The absorption of heat leads to a fall in the temperature of the solution.

In simple words: Exothermic reactions release heat, causing the surrounding temperature to rise, while endothermic reactions absorb heat, leading to a decrease in the surrounding temperature.

🎯 Exam Tip: Clearly define each type of reaction and explain the observable effect on temperature in the surroundings.

Give scientific reasons:

Question 1. Grills of doors and windows are always painted before they are used.
Answer:
• Grills of doors and windows are made from iron. Iron has a tendency to undergo corrosion.
• Paint does not allow air or moisture to come in contact with iron surface. Therefore, to prevent rusting of iron. grills of doors and windows are always painted before they are used.
In simple words: Grills are painted because they are usually made of iron, which corrodes (rusts) when exposed to air and moisture. The paint acts as a protective barrier, preventing these elements from reaching the iron surface and thus preventing rusting.

🎯 Exam Tip: Connect the material (iron) to its tendency (corrosion/rusting) and then explain how the preventive measure (painting) works by creating a barrier.

Question 2. Physical states of reactants and products are mentioned while writing a chemical equation.
Answer:
(1) while writing a chemical equation, gaseous, liquid and solid states are symbolised as (g), (l) and (s) respectively.
(2) This is done to make it more informative and to emphasise that those reactions occur in that manner only under those conditions. Hence, physical states of reactants and products are mentioned while writing a chemical equation.
In simple words: The physical states (solid, liquid, gas, aqueous) of reactants and products are noted in chemical equations to provide crucial information about the reaction conditions and the nature of the substances involved, making the equation more descriptive and accurate.

🎯 Exam Tip: Highlight that indicating physical states makes the chemical equation more informative and specifies the conditions under which the reaction occurs.

Question 3. Iron articles rust readily whereas steel which is also mainly made of iron does not undergo corrosion.
Answer:
(1) Iron articles rust readily as iron reacts with oxygen and moisture of air to convert into its hydroxide and oxide (Fe2O3. x H2O), while steel is an alloy of iron, carbon and chromium.
(2) The properties of an alloy are different from the properties of its constituents. The added metals increase its resistance to corrosion. It is more durable and clean.
In simple words: Iron rusts easily due to its direct reaction with air and moisture, forming rust. Steel, being an alloy of iron with elements like carbon and chromium, has altered properties, making it more resistant to corrosion and hence does not rust as readily as pure iron.

🎯 Exam Tip: Emphasize the difference between a pure metal (iron) and an alloy (steel) and how alloying modifies properties, particularly corrosion resistance.

Question 4. Concentrated hydrochloric acid reacts more vigorously with calcium carbonate than dilute hydrochloric acid.
Answer:
1. The rate of a reaction increases with the concentration of the reactant.
2. As concentrated hydrochloric acid contains more number of HCl molecules than those in an equal volume of dilute HCl, concentrated HCl reacts more vigorously with calcium carbonate.
In simple words: Concentrated hydrochloric acid reacts more vigorously with calcium carbonate because it has a higher concentration of HCl molecules compared to dilute acid. This higher concentration leads to more frequent collisions between reactant particles, thereby increasing the reaction rate.

🎯 Exam Tip: Directly link higher concentration to an increased number of reactant molecules, which in turn leads to a higher frequency of collisions and thus a faster reaction rate.

Question 5.
Zinc powder reacts much faster with dil. H2SO4 than does granulated zinc of the
Same mass.
Answer:
(1) In a reaction. the rate of the reaction depends upon the particle size of the solid
reactant as the reaction takes place on the surface only. Smaller the particles are, the
more will be their total surface area and faster will be the rate of the reaction.
(2) Hence, zinc powder reacts much faster with dil. H2SO4 than does granulated zinc.
In simple words: Smaller particles have a larger surface area, allowing more reactant molecules to interact and thus speeding up the reaction compared to larger particles of the same mass.

🎯 Exam Tip: Understanding the effect of surface area on reaction rate is crucial for explaining chemical kinetics. Use a clear, comparative example.

Question 6.
When copper articles exposed to air for a long time, gets corroded.
Answer:
Copper oxidises to form black coloured laver of copper oxide. when copper oxide
combines with carbon dioxide from air, copper loses its lustre due to formation of
greenish layer of copper carbonate on its surface. Thus, copper articles exposed to air
for a long time get corroded.
In simple words: Copper corrodes over time when exposed to air, forming a black layer of copper oxide and then a greenish layer of copper carbonate, which causes it to lose its shine.

🎯 Exam Tip: Remember to specify the color changes and the compounds formed during copper corrosion (oxidation) in your explanation.

Question 7.
When silver vessels exposed to air turns blackish after sometime.
Answer:
On exposure to air, silver vessels turns blackish after sometime. This is because of the
layer of silver sulphide (\( \text{Ag}_2\text{S} \)) formed by the reaction or silver with hydrogen suphide
in air.
In simple words: Silver items turn black over time because silver reacts with hydrogen sulfide in the air to form a layer of black silver sulfide on their surface.

🎯 Exam Tip: Identify the specific compound (silver sulfide) and the gas (hydrogen sulfide) responsible for tarnishing silver.

Explain The Following Reactions Giving Their Balanced Chemical Equations:

Question 1.
Calcium carbonate (Lime stone) is heated.
Answer:
When calcium carbonate (Lime stone) is heated at high temperature it decomposes
to form quicklime and carbon dioxide gas.
\( \text{CaCO}_3\text{(s)} \stackrel{\text{heat}}{\longrightarrow} \text{CaO (s)} + \text{CO}_2\text{(g)} \)
Calcium carbonate Quick lime Carbon dioxide
In simple words: When limestone (calcium carbonate) is heated, it breaks down into quicklime (calcium oxide) and carbon dioxide gas.

🎯 Exam Tip: This is a classic example of a thermal decomposition reaction. Ensure the states of matter for reactants and products are correctly indicated.

Question 2.
Copper reacts with dil. nitric acid.
Answer:
When copper reacts with dil. nitric acid, nitric oxide gas is formed.
\( \text{Cu(s)} + \text{8HNO}_3\text{(aq)} \longrightarrow \text{3Cu(NO}_3\text{)}_2\text{(aq)} + \text{2NO(g)} + \text{2H}_2\text{O(l)} \)
(dil.) Nitric oxide
In simple words: Copper reacts with dilute nitric acid to produce copper nitrate, nitric oxide gas, and water.

🎯 Exam Tip: Pay close attention to the stoichiometry and products when copper reacts with dilute versus concentrated nitric acid, as they differ.

Question 3.
Copper reacts with conc. nitric acid.
Answer:
When copper reacts with conc. nitric acid, reddish coloured poisonous nitrogen
dioxide gas is formed.
\( \text{Cu(s)} + \text{4HNO}_3\text{(aq)} \longrightarrow \text{Cu(NO}_3\text{)}_2\text{(aq)} + \text{2NO}_2 + \text{2H}_2\text{O(l)} \)
(conc.) Nitrogen dioxide
In simple words: Copper reacts with concentrated nitric acid to yield copper nitrate, reddish-brown nitrogen dioxide gas, and water.

🎯 Exam Tip: Distinguish the products, especially the nitrogen oxide, when copper reacts with concentrated versus dilute nitric acid.

Question 4.
Ammonia gas reacts with hydrogen chloride gas.
Answer:
When ammonia gas reacts with hydrogen chloride gas, it forms the salt ammonium
chloride in gaseous state, but immediately it got transformed into the solid state.
\( \text{NH}_3\text{(g)} + \text{HCl(g)} \longrightarrow \text{NH}_4\text{Cl(s)} \)
Ammonia hydrogen chloride Ammonium chloride
In simple words: Ammonia gas combines with hydrogen chloride gas to form solid ammonium chloride, appearing as white fumes.

🎯 Exam Tip: This is a combination reaction forming a single solid product from two gases. Note the change in state from gaseous reactants to a solid product.

Question 5.
Magnesium strip is burnt in air.
Answer:
When magnesium strip is burnt in air, a white powder of magnesium oxide is formed.
\( \text{2Mg} + \text{O}_2 \stackrel{\Delta}{\longrightarrow} \text{2MgO} \)
Magnesium Magnesium oxide
In simple words: Burning a magnesium strip in air results in a bright flame and the formation of a white powdery substance, which is magnesium oxide.

🎯 Exam Tip: This is a classic combination reaction and an oxidation process, often demonstrated for its intense light emission.

Question 6.
Calcium oxide is mixed with water.
Answer:
When calcium oxide (slaked lime) is mixed with water, calcium hydroxide is formed
with evolution of large amount of heat.
\( \text{CaO} + \text{H}_2\text{O} \longrightarrow \text{Ca(OH)}_2 + \text{Heat} \)
Calcium oxide Calcium hydroxide
In simple words: When calcium oxide (quicklime) is mixed with water, it reacts vigorously to form calcium hydroxide (slaked lime) and releases a significant amount of heat.

🎯 Exam Tip: This is an exothermic combination reaction, often referred to as slaking of lime. Mentioning heat evolution is key.

Question 7.
Sugar is heated.
Answer:
When sugar is heated, it decomposes to form carbon (black substance).
\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \stackrel{\Delta}{\longrightarrow} \text{12C} + \text{11H}_2\text{O(g)} \)
Sugar Carbon
In simple words: When sugar is heated, it undergoes thermal decomposition, breaking down into black carbon and releasing water vapor.

🎯 Exam Tip: This is an example of thermal decomposition, specifically dehydration. Note the characteristic black carbon residue.

Question 8.
Electric current is passed through acidulated water.
Answer:
When an electric current is passed through acidulated water, it decomposes into
hydrogen and
oxygen gas.
\( \text{2H}_2\text{O(l)} \stackrel{\text{electric current}}{\longrightarrow} \text{2H}_2\uparrow + \text{O}_2\uparrow \)
Acidulated water hydrogen oxygen
In simple words: Passing an electric current through acidulated water causes it to break down into hydrogen gas and oxygen gas, a process known as electrolysis.

🎯 Exam Tip: This is an electrolytic decomposition reaction. Remember to show the upward arrows for the gaseous products and specify "electric current" as the condition.

Question 9.
Zinc powder is added to copper sulphate solution.
Answer:
When zinc powder is added to copper sulphate solution, more reactive zinc displaces
less reactive copper from copper sulphate solution. The colourless zinc sulphate is
formed with evolution of heat.
\( \text{CuSO}_4\text{(aq)} + \text{Zn(s)} \longrightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)} + \text{Heat} \)
Copper sulphate Zinc sulphate Copper
In simple words: Zinc powder reacts with blue copper sulfate solution, displacing copper to form colorless zinc sulfate and reddish-brown copper metal, while also releasing heat.

🎯 Exam Tip: This is a single displacement reaction. Note the color change from blue (copper sulfate) to colorless (zinc sulfate) and the formation of copper metal.

Question 10.
Iron powder is added to copper sulphate solution.
Answer:
When iron powder is added to copper sulphate solution, more reactive iron displaces
less reactive copper from copper sulphate. The colourless ferrous sulphate solution is
formed with evolution of heat.
\( \text{Fe(s)} + \text{CuSO}_4\text{(aq)} \longrightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)} + \text{Heat} \)
Iron copper sulphate ferrous sulphate copper
In simple words: Iron powder reacts with blue copper sulfate solution, displacing copper to form light green ferrous sulfate and reddish-brown copper metal, releasing heat.

🎯 Exam Tip: Another example of a single displacement reaction. The key observations are the color change from blue to light green and the deposition of copper on the iron.

Question 11.
Lead is added to copper sulphate solution.
Answer:
When lead is added to copper sulphate solution, more reactive lead displaces less
reactive copper from copper sulphate. The colourless lead sulphate solution is formed
with evolution of heat.
\( \text{CuSO}_4\text{(aq)} + \text{Pb(s)} \longrightarrow \text{PbSO}_4\text{(aq)} + \text{Cu(s)} + \text{Heat} \)
Lead sulphate Lead sulphate Copper
In simple words: Lead, being more reactive than copper, displaces copper from copper sulfate solution, forming lead sulfate and copper, and releasing heat.

🎯 Exam Tip: This displacement reaction shows how a more reactive metal can replace a less reactive one from its salt solution.

Question 12.
Potassium chromate solution is added to barium sulphate solution.
Answer:
When potassium chromate solution is added to barium sulphate solution, yellow
precipitate of barium chromate is formed.
\( \text{K}_2\text{CrO}_4\text{(aq)} + \text{BaSO}_4\text{(aq)} \longrightarrow \text{BaCrO}_4\downarrow + \text{K}_2\text{SO}_4 \)
Potassium chromate Barium sulphate Precipitate (yellow) Potassium sulphate
In simple words: Mixing potassium chromate and barium sulfate solutions results in a double displacement reaction, forming a yellow precipitate of barium chromate and potassium sulfate.

🎯 Exam Tip: This is a double displacement (precipitation) reaction. Remember that barium chromate is the yellow precipitate formed.

Question 13.
Calcium chloride solution is added to sodium carbonate solution.
Answer:
When calcium chloride solution is added to sodium carbonate solution, white
precipitate of calcium carbonate is formed.
\( \text{CaCl}_2\text{(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \longrightarrow \text{CaCO}_3\downarrow + \text{2NaCl(aq)} \)
Calcium chloride Sodium carbonate Calcium carbonate Sodium chloride
In simple words: When calcium chloride solution is mixed with sodium carbonate solution, a double displacement reaction occurs, producing a white precipitate of calcium carbonate and sodium chloride.

🎯 Exam Tip: This is a precipitation reaction. Identifying calcium carbonate as the white precipitate is essential.

Question 14.
Sodium chloride solution is mixed with silver nitrate solution.
Answer:
When sodium chloride solution is mixed with silver nitrate solution, white precipitate
or silver chloride is formed.
\( \text{NaCl(aq)} + \text{Ag (NO}_3\text{) (aq)} \longrightarrow \text{AgCl} \downarrow + \text{NaNO}_3 \)
Sodium chloride Silver nitrate Silver chloride Sodium nitrate
In simple words: Mixing sodium chloride and silver nitrate solutions causes a double displacement reaction, forming a white precipitate of silver chloride and sodium nitrate.

🎯 Exam Tip: This is a classic precipitation reaction often used to demonstrate double displacement. Silver chloride is a distinct white precipitate.

Question 15.
Dilute sulphuric acid is added to barium chloride solution.
Answer:
When dilute sulphuric acid is added to barium chloride solution, white precipitate of
barium sulphate is rormed.
\( \text{BaCl}_2\text{(g)} + \text{H}_2\text{SO}_4 \longrightarrow \text{BaSO}_4\downarrow + \text{2HCl} \)
Barium chloride dil. Barium sulphate
In simple words: Adding dilute sulfuric acid to barium chloride solution leads to a double displacement reaction, producing a white precipitate of barium sulfate and hydrochloric acid.

🎯 Exam Tip: Barium sulfate is a highly insoluble white precipitate, characteristic of this double displacement reaction.

Question 16.
Calcium carbonate (Lime stone) is treated with dil. hydrochloric acid.
Answer:
When calcium carbonate (lime stone) is treated with dil. hydrochloric acid, carbon
dioxide gas is formed.
\( \text{CaCO}_3 + \text{2HCl} \longrightarrow \text{CaCl}_2\text{(aq)} + \text{CO}_2\uparrow + \text{H}_2\text{O} \)
Calcium carbonate dil. Calcium chloride Carbon dioxide
In simple words: Treating calcium carbonate (limestone) with dilute hydrochloric acid produces calcium chloride, carbon dioxide gas, and water.

🎯 Exam Tip: This is an acid-carbonate reaction that releases carbon dioxide gas, often observed as effervescence.

Question 17.
Aluminium is treated with dil. hydrochloric acid.
Answer:
When aluminium is treated with dilute hydrochloric acid, hydrogen gas is liberated.
\( \text{2Al(s)} + \text{6HCl(aq)} \longrightarrow \text{2AlCl}_3 + \text{3H}_2\uparrow \)
Aluminium Aluminium chloride Hydrogen
In simple words: When aluminum reacts with dilute hydrochloric acid, it produces aluminum chloride and releases hydrogen gas.

🎯 Exam Tip: This is a displacement reaction where a more reactive metal (Aluminum) displaces hydrogen from an acid.

Question 18.
Magnesium is treated with hydrochloric acid.
Answer:
When magnesium is treated with hydrochloric acid, hydrogen gas is liberated.
\( \text{Mg(s)} + \text{2HCl} \longrightarrow \text{MgCl}_2 + \text{H}_2\uparrow \)
Magnesium (dil.) Magnesium chloride Hydrogen
In simple words: Magnesium metal reacts with hydrochloric acid to form magnesium chloride and hydrogen gas.

🎯 Exam Tip: This is a common displacement reaction where a metal reacts with an acid to produce a salt and hydrogen gas.

Question 19.
Hydrogen peroxide is decomposed in the presence of manganese dioxide (MnO2).
Answer:
When hvdrogen peroxide is decomposed in the presence of manganese dioxide
(MnO2), water and oxygen are formed.
\( \text{2H}_2\text{O}_2\text{(g)} \stackrel{\text{MnO}_2}{\Delta} \text{2H}_2\text{O(l)} + \text{O}_2 \)
Hydrogen peroxide water oxygen
In simple words: Hydrogen peroxide decomposes into water and oxygen gas, a reaction that is accelerated by the presence of manganese dioxide as a catalyst.

🎯 Exam Tip: Manganese dioxide acts as a catalyst, speeding up the decomposition of hydrogen peroxide without being consumed itself.

Question 20.
Ethyl alcohol is treated with acidified potassium dlchromate.
Answer:
When ethly alcohol is treated with acidified potassium dichromate, acetic acid is
formed. This is oxidation reaction.
\( \text{CH}_3-\text{CH}_2-\text{OH} \stackrel{\text{2[O]}}{\text{K}_2\text{Cr}_2\text{O}_7 + \text{H}_2\text{SO}_4} \text{CH}_3 - \text{COOH} + \text{H}_2\text{O} \)
Ethyl alcohol Acetic acid
In simple words: Ethyl alcohol undergoes oxidation when treated with acidified potassium dichromate, converting it into acetic acid.

🎯 Exam Tip: Potassium dichromate acts as a strong oxidizing agent in this reaction, converting an alcohol to a carboxylic acid.

Question 21.
Hydrogen gas is passed over black copper oxide.
Answer:
When hydrogen gas is passed over black copper oxide, a reddish coloured layer of
copper is formed.
\( \text{CuO(s)} + \text{H}_2\text{(g)} \longrightarrow \text{Cu(s)} + \text{H}_2\text{O(l)} \)
copper oxide copper
In simple words: Passing hydrogen gas over black copper oxide reduces the copper oxide to reddish copper metal, with water being formed.

🎯 Exam Tip: This is a redox reaction where copper oxide is reduced and hydrogen is oxidized. Observe the color change from black to reddish-brown.

10th Std Science Part 1 Questions And Answers: