GSEB Class 10 Science Solutions Chapter 1 Chemical Reactions and Equations

Question 1. Why should a magnesium ribbon be cleaned before burning in air?
Answer: You should clean a magnesium ribbon before burning it in air. This is because magnesium slowly combines with oxygen to create magnesium oxide. This oxide layer then stops the magnesium from burning well. So, we must remove this layer using sand paper.
In simple words: Clean magnesium ribbons before burning because an oxide layer forms, stopping them from burning. This layer needs to be taken off with sandpaper.

Exam Tip: Always mention the formation of magnesium oxide layer and its obstructive effect on burning, along with the cleaning method.

Question 2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride.
Answer:
(i) \( H_2 + Cl_2 \rightarrow 2HCl \)
(ii) \( 3BaCl_2 + Al_2(SO_4)_3 \rightarrow 3BaSO_4 + 2AlCl_3 \)
In simple words: You need to write down the chemical formulas and numbers for hydrogen, chlorine, and hydrogen chloride, making sure the atoms on both sides are equal. Do the same for barium chloride, aluminium sulphate, barium sulphate, and aluminium chloride.

Exam Tip: For balancing equations, always ensure the number of atoms of each element is identical on both sides of the arrow.

Question 3. Write a balanced chemical equation with state symbols for the following reaction.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer:
(i) \( BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq) \)
(ii) \( NaOH(aq) + HCl(l) \rightarrow NaCl(aq) + H_2O(l) \)
In simple words: For the first reaction, barium chloride and sodium sulphate dissolved in water mix to make solid barium sulphate and dissolved sodium chloride. For the second, sodium hydroxide solution combines with hydrochloric acid solution to form dissolved sodium chloride and liquid water.

Exam Tip: Remember to include state symbols (s, l, g, aq) as they provide important information about the physical state of reactants and products.

Question 4. A solution of a substance 'X' is used for white washing.
(i) Name the substance 'X' and write its formula.
(ii) Write the reaction of the substance 'X' named in (i) above with water.
Answer:
(i) Substance 'X' is called quick lime, and its formula is CaO. People use this substance for whitewashing walls.
(ii) When quick lime (CaO) reacts with water (H2O), it forms calcium hydroxide (Ca(OH)2).
In simple words: The substance 'X' for whitewashing is quick lime (CaO). When quick lime mixes with water, it makes calcium hydroxide.

Exam Tip: Identify common names for compounds like quick lime (calcium oxide) and slaked lime (calcium hydroxide) to avoid confusion.

Question 5. Why is the amount of gas collected in Activity 1.7 double of the amount collected in the other gas? Name the gas.
Answer: This occurs because water is created when hydrogen and oxygen join together in a 2:1 volume ratio. The gas with twice the volume is hydrogen. Therefore, during the breakdown of water, the amount of hydrogen produced is double that of oxygen gas.
In simple words: Hydrogen gas collects as double the amount because water is made from two parts hydrogen and one part oxygen. So, when water breaks apart, more hydrogen is released. The gas is hydrogen.

Exam Tip: When dealing with electrolysis of water, always remember the 2:1 volume ratio of hydrogen to oxygen, and identify the gas based on its properties.

Question 6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer: The blue colour of copper sulphate solution changes when an iron nail is put into it. This occurs because of a displacement reaction. Iron is more reactive than copper, so it replaces copper, forming a green-coloured iron sulphate solution and copper metal.
\( Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s) \)
(blue) (green)
In simple words: The blue copper sulphate solution changes colour when an iron nail is added. This is because iron is stronger than copper and pushes it out, making a green solution and copper metal.

Exam Tip: Explain the colour change using the concept of displacement reaction and compare the reactivity of the metals involved.

Question 7. Give an example of a double displacement reaction other than the one given in Activity
Answer:
\( Pb(NO_3)_2 + 2HCl \rightarrow PbCl_2 (s) + 2HNO_3 \)
(Lead nitrate) (Hydrochloric acid) (Lead chloride) (Nitric acid)
In simple words: An example of a double displacement reaction is when lead nitrate reacts with hydrochloric acid. They swap their partners, forming solid lead chloride and nitric acid.

Exam Tip: For double displacement reactions, ensure that two compounds exchange ions to form two new compounds, often with one being a precipitate.

Question 8. Identify the substance that are oxidised and the substances that are reduced in the following reactions.
(i) \( 4Na(s) + O_2(g) \rightarrow 2Na_2O(s) \)
(ii) \( CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l) \)
Answer:
(i) In this reaction, sodium metal undergoes oxidation, while oxygen is reduced to form sodium oxide.
(ii) Copper oxide is reduced to copper, and hydrogen is oxidised to create water.
In simple words: For the first reaction, sodium gets oxidized and oxygen gets reduced. For the second, copper oxide gets reduced, and hydrogen gets oxidized.

Exam Tip: Clearly identify which substance gains oxygen/loses hydrogen (oxidized) and which loses oxygen/gains hydrogen (reduced).

In-Text Activities Solved

Activity 1.1
Answer: A magnesium ribbon burns with a very bright, dazzling light. The substance that is created from this burning is magnesium oxide.
\( 2Mg(s) + O_2(g) \rightarrow 2MgO(s) \)
In simple words: When a magnesium ribbon burns, it makes a super bright light, and magnesium oxide is formed.

Exam Tip: Describe the visual observation (dazzling light) and the chemical product (magnesium oxide).

Activity 1.2
Answer: Lead nitrate combines with potassium iodide to produce lead iodide. This lead iodide does not dissolve in water and has a yellow colour.
\( Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq) \)
(yellow ppt.)
In simple words: When lead nitrate and potassium iodide mix, they create lead iodide, a yellow substance that doesn't dissolve in water.

Exam Tip: Note the formation of a precipitate and its specific colour (yellow) when describing reactions that produce insoluble products.

Activity 1.3
Answer: When zinc metal combines with dilute hydrochloric acid, zinc chloride is produced. You can also see bubbles of hydrogen gas forming. The conical flask feels warm, which indicates that this reaction releases heat.
\( Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g) + \Delta \)
In simple words: Zinc metal and dilute hydrochloric acid create zinc chloride and hydrogen gas bubbles, making the flask warm because it's an exothermic reaction.

Exam Tip: Mention observable changes like gas bubbles and temperature change, and correctly identify the gas (hydrogen, pop sound) and reaction type (exothermic).

Activity 1.4
Answer: Calcium oxide energetically combines with water, forming slaked lime, which is calcium hydroxide. This process also releases a lot of heat. This reaction is a combination type because calcium oxide mixes with water to make just one product, calcium hydroxide.
\( CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \Delta \)
In simple words: Calcium oxide vigorously reacts with water, making slaked lime (calcium hydroxide) and a lot of heat. This is a combination reaction as two things form one new thing.

Exam Tip: Highlight the vigorous nature of the reaction, the product formed (slaked lime), and its classification as a combination reaction with heat release.

Activity 1.5
Answer: The green color of ferrous sulphate crystals turns into a brownish-black ferric oxide. Additionally, you can notice the distinct smell of burning sulphur dioxide.
\( 2FeSO_4(s) \xrightarrow{\text{Heat}} Fe_2O_3(s) + SO_2(g) + SO_3(g) \)
In simple words: Green ferrous sulphate crystals change to brownish-black ferric oxide when heated, and you can smell burning sulphur dioxide.

Exam Tip: Describe the specific colour change from green to brownish-black and the distinctive smell of sulfur dioxide.

Activity 1.6
Answer: Strong-smelling, brown fumes are given off because of NO2 gas. A brownish solid, lead oxide (PbO), remains behind.
\( 2Pb(NO_3)_2(s) \rightarrow 2PbO(s) + 4NO_2(g) + O_2(g) \)
In simple words: When lead nitrate is heated, sharp-smelling brown fumes of NO2 gas come out, and a brown solid called lead oxide is left.

Exam Tip: Focus on the observation of pungent brown fumes (NO2) and the remaining brownish residue (PbO).

Activity 1.7
Answer: The amount of one gas, hydrogen, is double the amount of the other gas, oxygen. Hydrogen gas catches fire and burns with a 'pop' sound. The other gas, oxygen, makes a candle burn very brightly. Oxygen is collected at the anode, while hydrogen is gathered at the cathode.
In simple words: During water decomposition, hydrogen gas volume is double that of oxygen. Hydrogen burns with a 'pop' sound, and oxygen makes a candle burn brighter. Oxygen is at the anode, hydrogen at the cathode.

Exam Tip: Remember the key features: hydrogen's pop sound, oxygen's ability to relight a splint, and their collection at cathode and anode respectively.

Activity 1.8
Answer: White silver chloride becomes grey when exposed to sunlight. This occurs because silver metal is created through a photolytic reaction, and chlorine gas is also released.
\( 2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g) \)
In simple words: White silver chloride turns grey in sunlight because light breaks it down into silver metal and chlorine gas.

Exam Tip: Connect the colour change (white to grey) directly to the photolytic decomposition of silver chloride into silver metal and chlorine gas.

Activity 1.9
Answer: Copper sulphate solution, which is originally blue, changes its color when an iron nail is submerged in it. This occurs because of a displacement reaction. Iron is more reactive than copper, so it replaces copper, forming a new product: iron sulphate and copper metal. The dark blue color of the copper sulphate in one test tube turns greenish as iron sulphate forms. Additionally, the iron nail itself turns brownish as copper metal gets deposited on its surface.
\( Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s) \)
In simple words: When an iron nail is put into blue copper sulphate solution, the solution's color changes to green and the nail turns brownish. This happens because iron is more reactive and pushes out the copper.

Exam Tip: Reinforce the concept of reactivity series and how a more reactive metal displaces a less reactive one, leading to observable changes.

Activity 1.10
Answer: When sodium sulphate combines with barium chloride, a white solid substance, barium sulphate, is created. This solid appears as a precipitate.
\( Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq) \)
(white ppt.)
In simple words: When sodium sulphate and barium chloride react, they form a white solid called barium sulphate, which appears as a precipitate.

Exam Tip: The key here is the formation of a white precipitate (barium sulphate) in a double displacement reaction.

Activity 1.11
Answer: When copper powder is heated, its surface turns black. This happens because copper reacts with oxygen in the air, creating copper oxide.
\( 2Cu + O_2 \xrightarrow{\Delta} 2CuO \)
In simple words: Heating copper powder in air makes its surface black because copper mixes with oxygen to form copper oxide.

Exam Tip: Emphasize the colour change of copper powder to black due to the formation of copper oxide via oxidation.

Gujarat Board Class 10 Science Chemical Reactions and Equations and Answers

Question 1. Which of the following statements about the reaction below are incorrect?
\( 2PbO(s) + C(s) \rightarrow 2Pb(s) + CO_2(g) \)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Answer: (i) (a) and (b)
In simple words: The wrong statements are that lead is reduced and carbon dioxide is oxidized. In the reaction, lead oxide becomes lead, and carbon becomes carbon dioxide.

Exam Tip: Carefully analyze each statement to determine if it accurately describes oxidation or reduction based on gain/loss of oxygen or hydrogen.

Question 2. Fe2O3 + 2AI → Al2O3 + 2Fe
The above reaction is an example of
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Answer: (d) displacement reaction.
In simple words: This reaction is a displacement reaction because aluminum takes the place of iron in the compound.

Exam Tip: Recognize that an element replacing another element from its compound is characteristic of a displacement reaction.

Question 3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer: (a) Hydrogen gas and iron chloride are produced.
In simple words: When dilute hydrochloric acid is mixed with iron filings, hydrogen gas and iron chloride are formed.

Exam Tip: Recall that metals above hydrogen in the reactivity series react with acids to produce hydrogen gas and a metal salt.

Question 4. What is balanced chemical equation? Why should chemical equations be balanced?
Answer: A balanced chemical equation means that the total count of atoms for every element must be the same on both the reactant and product sides of the reaction. We must balance equations because matter cannot be made or destroyed. The overall mass of the substances that react should always be equal to the overall mass of the substances that are produced.
In simple words: A balanced chemical equation has the same number of atoms for each element on both sides. Equations must be balanced because matter cannot be created or destroyed, so the total mass before and after a reaction stays the same.

Exam Tip: Explain balancing equations based on the Law of Conservation of Mass, stating that atoms are conserved in a chemical reaction.

Question 5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) \( 3H_2(g) + N_2(g) \rightarrow 2NH_3(g) \)
(b) \( 2H_2S(g) + 3O_2(g) \rightarrow 2H_2O(l) + 2SO_2(g) \)
(c) \( 3BaCl_2(aq) + Al_2(SO_4)_3(aq) \rightarrow 2AlCl_3(aq) + 3BaSO_4(s) \)
(d) \( 2K(s) + 2H_2O(l) \rightarrow 2KOH(aq) + H_2(g) \)
In simple words: These are the balanced chemical equations for the given statements, showing hydrogen with nitrogen, hydrogen sulphide burning, barium chloride with aluminum sulphate, and potassium with water.

Exam Tip: Practice converting word equations into chemical formulas and then systematically balancing each element's atoms.

Question 6. Balance the following chemical equations.
(a) \( HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O \)
(b) \( NaOH + H_2SO_4 \rightarrow Na_2SO_4 + H_2O \)
(c) \( NaCl + AgNO_3 \rightarrow AgCl + NaNO_3 \)
(d) \( BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + HCl \)
Answer:
(a) \( 2HNO_3(aq) + Ca(OH)_2 (aq) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l) \)
(b) \( 2NaOH(aq) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + 2H_2O(l) \)
(c) \( NaCl(aq) + AgNO_3(aq) \rightarrow AgCl(s) + NaNO_3(aq) \)
(d) \( BaCl_2(aq) + H_2SO_4(aq) \rightarrow BaSO_4(s) + 2HCl(aq) \)
In simple words: These are the balanced forms of the given chemical equations, ensuring that the number of atoms for each element is the same on both sides of the reaction.

Exam Tip: Apply balancing techniques, starting with elements that appear only once on each side, and checking polyatomic ions as a single unit where possible.

Question 7. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer:
(a) \( Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l) \)
(b) \( Zn(s) + 2AgNO_3(aq) \rightarrow Zn(NO_3)_2(aq) + 2Ag(s) \)
(c) \( 2Al(s) + 3CuCl_2(aq) \rightarrow 2AlCl_3(aq) + 3Cu(s) \)
(d) \( BaCl_2(aq) + K_2SO_4(aq) \rightarrow BaSO_4(s) + 2KCl(aq) \)
In simple words: These are the balanced chemical equations for the reactions involving calcium hydroxide and carbon dioxide, zinc and silver nitrate, aluminum and copper chloride, and barium chloride and potassium sulphate.

Exam Tip: Pay attention to the correct chemical formulas and stoichiometry when writing balanced equations for different reaction types.

Question 8. Write the balanced chemical equation for the following and identify the type of reaction
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium bromide (s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride (aq) + Hydrogen (g)
Answer:
(a) \( 2KBr(aq) + BaI_2(aq) \rightarrow 2KI(aq) + BaBr_2(s) \) (Double displacement reaction)
(b) \( ZnCO_3(s) \rightarrow ZnO(s) + CO_2(g) \) (Decomposition reaction)
(c) \( H_2(g) + Cl_2(g) \rightarrow 2HCl(g) \) (Combination reaction)
(d) \( Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g) \) (Displacement reaction)
In simple words: Here are the balanced equations for the reactions. For (a), it's a double displacement. For (b), it's a decomposition. For (c), it's a combination. And for (d), it's a displacement reaction.

Exam Tip: After balancing, classify the reaction as combination, decomposition, displacement, or double displacement based on the change in reactants and products.

Question 9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer: Exothermic reactions are those processes where heat is released. The heat symbol (Δ) appears on the product side of the equation. For instance, carbon combining with oxygen to form carbon dioxide also releases heat.
Example:
\( C(s) + O_2(g) \rightarrow CO_2(g) + \Delta \)
Endothermic reactions, conversely, are processes where heat is taken in. The heat symbol (Δ) is shown on the reactant side. An example is calcium carbonate breaking down into calcium oxide and carbon dioxide, which requires heat.
Example:
\( CaCO_3(s) + \Delta \rightarrow CaO(s) + CO_2(g) \)
In simple words: Exothermic reactions release heat, like burning carbon. Endothermic reactions absorb heat, like heating calcium carbonate to break it down.

Exam Tip: Distinguish exothermic (heat released) from endothermic (heat absorbed) reactions by remembering the position of 'heat' or 'Δ' in the equation.

Question 10. Why is respiration considered as exothermic reaction? Explain.
Answer:
1. During respiration, glucose undergoes oxidation to produce carbon dioxide, water, and heat.
2. Since heat energy is given out during this process, respiration is therefore considered an exothermic reaction.
In simple words: Respiration is exothermic because glucose is oxidized, releasing carbon dioxide, water, and heat energy.

Exam Tip: Link respiration to the breakdown of glucose and the subsequent release of energy in the form of heat, making it an exothermic process.

Question 11. Why are the decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: In a decomposition reaction, one complex compound breaks apart into simpler compounds or elements. For example, calcium carbonate decomposes into calcium oxide and carbon dioxide.
Example:
\( CaCO_3(s) \rightarrow CaO(s) + CO_2(g) \)
In contrast, a combination reaction involves two or more elements or compounds joining together to form just one new, single compound. Examples include carbon dioxide combining with calcium oxide to form calcium carbonate, or sulphur dioxide reacting with oxygen to form sulphur trioxide.
Example:
\( CO_2(g) + CaO(s) \rightarrow CaCO_3(s) \)
\( 2SO_2(g) + O_2(g) \rightarrow 2SO_3(g) \)
Therefore, decomposition and combination reactions are considered to be opposites.
In simple words: Decomposition reactions break one compound into simpler ones, like calcium carbonate splitting. Combination reactions join simpler substances to form one new compound, like carbon dioxide and calcium oxide forming calcium carbonate. They are opposite processes.

Exam Tip: Clearly define both reaction types, emphasizing that they are inverse processes, and provide a balanced chemical equation for each.

Question 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.
Answer: During a decomposition reaction, a single compound breaks down into simpler compounds or elements. Here are examples for each energy type:
Heat: Calcium carbonate, when heated, breaks down into calcium oxide and carbon dioxide.
\( CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g) \)
Light: Silver bromide, exposed to sunlight, decomposes into silver and bromine gas.
\( 2AgBr(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Br_2(g) \)
Electricity: Water, with the help of electric current, splits into hydrogen and oxygen gas.
\( 2H_2O(l) \xrightarrow{\text{electric current}} 2H_2(g) + O_2(g) \)
In simple words: Decomposition means breaking a compound into simpler parts using energy. Heat can break calcium carbonate. Light can break silver bromide. Electricity can break water.

Exam Tip: Provide specific examples for each type of decomposition, associating the energy source (heat, light, electricity) with the correct reaction.

Question 13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: In a displacement reaction, a more reactive metal takes the place of a less reactive metal from its salt solution. For example, zinc metal displaces copper from copper chloride to form zinc chloride and copper metal.
Example:
\( Zn(s) + CuCl_2(aq) \rightarrow ZnCl_2(aq) + Cu(s) \)
In contrast, a double displacement reaction involves two different compounds exchanging their ions with each other to create two new compounds. An example is sodium hydroxide reacting with hydrochloric acid to produce sodium chloride and water.
Example:
\( NaOH + HCl \rightarrow NaCl + H_2O \)
In simple words: In displacement, a stronger metal replaces a weaker one in a solution. In double displacement, two compounds swap parts to make two new compounds.

Exam Tip: Differentiate based on what is exchanged: one element (displacement) versus a pair of ions (double displacement), and provide clear examples.

Question 14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer: \( Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s) \)
In simple words: When refining silver, copper metal takes silver's place in silver nitrate solution, creating copper nitrate solution and solid silver.

Exam Tip: Understand that a more reactive metal (copper) will displace a less reactive metal (silver) from its salt solution in a single displacement reaction.

Question 15. What do you mean by precipitation reaction? Explain it by giving examples.
Answer: Precipitation reactions are chemical processes where two or more reactants combine to create a solid, insoluble compound. This solid compound is known as a precipitate.
Example:
\( AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq) \)
(white ppt.)
In simple words: A precipitation reaction is when two liquids mix and form a solid substance that doesn't dissolve, called a precipitate.

Exam Tip: Define a precipitation reaction as one that forms an insoluble solid (precipitate) and give a clear example.

Question 16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Answer:
(a) Oxidation: This is a chemical process where a substance either gains oxygen or loses hydrogen.
Example:
\( 2Mg(s) + O_2(g) \xrightarrow{\text{Burning}} 2MgO(s) \) (Mg is oxidized to MgO).
\( 2Cu(s) + O_2(g) \xrightarrow{\text{Heat}} 2CuO(s) \) (Cu is oxidised to CuO)
(b) Reduction: This is a chemical process where a substance either loses oxygen or gains hydrogen.
Example:
\( CuO(s) + H_2(g) \xrightarrow{\text{Heat}} Cu(s) + H_2O(g) \) (CuO is reduced to form Cu).
\( ZnO(s) + C(s) \xrightarrow{\text{Heat}} Zn(s) + CO(g) \) (ZnO is reduced to Zn)
In simple words: Oxidation means gaining oxygen or losing hydrogen. Reduction means losing oxygen or gaining hydrogen.

Exam Tip: Remember oxidation as gain of oxygen/loss of hydrogen, and reduction as loss of oxygen/gain of hydrogen, with specific examples.

Question 17. An element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.
Answer: The element 'X' is copper metal. When copper is heated in air, it reacts with oxygen and changes into copper oxide (CuO), which is black in color. This process is called oxidation.
\( 2Cu(s) + O_2(g) \xrightarrow{\text{Heat}} 2CuO(s) \)
In simple words: Element 'X' is copper. When copper is heated in air, it turns black because it forms copper oxide.

Exam Tip: Connect the colour change to the oxidation of common metals, like copper forming black copper oxide.

Question 18. Why do we apply paint on iron articles?
Answer: We put paint on iron objects to stop them from rusting. When an iron surface has a coat of paint, it can't touch oxygen or moisture. Because of this, rusting does not occur.
In simple words: Painting iron prevents rust because the paint stops oxygen and moisture from touching the iron surface.

Exam Tip: Explain that painting creates a barrier, preventing contact between the iron surface and the rust-causing agents (oxygen and moisture).

Question 19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer: Foods that contain oil and fat are filled with nitrogen gas. This is because oils and fats spoil and become rancid when they react with oxygen, causing a bad taste and smell. Filling the packaging with nitrogen helps to stop the food from oxidizing, ensuring it doesn't turn rancid.
In simple words: Foods with oil and fat are packed with nitrogen to stop them from spoiling. Nitrogen prevents oxygen from reacting with the fats, which would make them taste and smell bad.

Exam Tip: Relate nitrogen flushing to preventing oxidation and thus rancidity, preserving the taste and smell of fatty foods.

Question 20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer:
(a) Corrosion: This is a process where a metal reacts with substances such as moisture and gases in the air, creating new compounds on its surface. For instance, iron combines with oxygen and moisture to create rust. Silver becomes black due to silver sulphide formation, and copper turns green from copper carbonate.
(b) Rancidity: This occurs when fatty or oily food spoils after being exposed to oxygen. It causes changes in the taste and smell of these foods. For example, butter kept at room temperature for too long will start to smell and taste bad, becoming sour. The spoiled product from food oxidation is called rancid, and the process is known as rancidity.
In simple words: (a) Corrosion is when metals react with air and moisture, like iron rusting or silver turning black. (b) Rancidity is when fatty foods spoil and get a bad taste or smell from exposure to oxygen.

Exam Tip: Provide comprehensive definitions for both terms, distinguishing between metal degradation (corrosion) and food spoilage (rancidity), with clear examples.

Gujarat Board Class 10 Science Chemical Reactions and Equations Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1. Give the formula for lime.
Answer: The chemical formula for lime is CaO.
In simple words: Lime's formula is CaO.

Exam Tip: Memorize common chemical names and their corresponding formulas, such as lime (CaO).

Question 2. Give the formula for iron (II) oxide and iron (III) oxide.
Answer: The formula for iron (II) oxide is FeO. The formula for iron (III) oxide is Fe2O3.
In simple words: Iron (II) oxide is FeO, and iron (III) oxide is Fe2O3.

Exam Tip: Differentiate between iron (II) oxide (FeO) and iron (III) oxide (Fe2O3) based on the oxidation state of iron.

Question 3. Give the formula for rust.
Answer: The chemical formula for rust is \( Fe_2O_3.xH_2O \), where 'x' indicates a variable amount of water molecules.
In simple words: The formula for rust is \( Fe_2O_3.xH_2O \).

Exam Tip: Remember that rust is hydrated iron (III) oxide, typically represented as Fe2O3.xH2O, indicating variable water content.

Question 4. Name the conditions necessary for rusting.
Answer: The two essential conditions needed for rusting to occur are the presence of oxygen and moisture.
In simple words: Rusting needs both oxygen and moisture to happen.

Exam Tip: Always state both oxygen and moisture as the two essential conditions required for iron to rust.

Question 5. Name the reaction seen during rancidity of food.
Answer: The chemical reaction observed during the rancidity of food is oxidation.
In simple words: When food becomes rancid, it's due to an oxidation reaction.

Exam Tip: Recall that rancidity in food is caused by the oxidation of fats and oils.

Question 6. Name the forms of energy required for breaking down reactants in a decomposition reaction.
Answer: The types of energy needed to break down reactants in a decomposition reaction are heat, light, and electricity.
In simple words: Heat, light, and electricity are the forms of energy needed for decomposition reactions.

Exam Tip: List the three main forms of energy-heat, light, and electricity-that can drive decomposition reactions.

Question 7. What is the insoluble substance formed in a chemical reaction called?
Answer: The insoluble solid substance created during a chemical reaction is called a precipitate.
In simple words: The solid that doesn't dissolve in a chemical reaction is called a precipitate.

Exam Tip: Define a precipitate as the insoluble solid that forms and separates from a liquid solution during a chemical reaction.

Question 8. What does (II) in lead (II) nitrate indicate?
Answer: The (II) in lead (II) nitrate shows that lead's valency, or oxidation state, is 2, meaning it forms \( +2 \) ions in this compound.
In simple words: The (II) in lead (II) nitrate means lead has a valency of 2 and forms \( +2 \) ions.

Exam Tip: Understand that Roman numerals in chemical names indicate the oxidation state or valency of the metal in that specific compound.

Question 9. What are noble metals?
Answer: Noble metals are those metals that do not easily react, even at various temperatures. Gold is a common example.
In simple words: Noble metals, like gold, are metals that don't react easily with other substances, even when heated.

Exam Tip: Define noble metals as those that are unreactive and resistant to corrosion or oxidation, like gold and platinum.

Question 10. Name two metals which do not corrode?
Answer: Two metals that do not corrode are gold and platinum.
In simple words: Gold and platinum are two metals that do not corrode.

Exam Tip: Provide examples of noble metals like gold and platinum, which are known for their resistance to corrosion.

Question 11. Name the products obtained when silver bromide is exposed to sunlight.
Answer: When silver bromide is exposed to sunlight, it breaks down to form silver and bromine.
In simple words: Silver and bromine are made when silver bromide is put in sunlight.

Exam Tip: Recall the photolytic decomposition of silver bromide, yielding silver metal and bromine gas.

Question 12. Name the gas which burns with pop sound.
Answer: The gas that burns with a 'pop' sound is hydrogen gas.
In simple words: Hydrogen gas burns with a popping sound.

Exam Tip: Associate the characteristic "pop" sound with the presence and combustion of hydrogen gas.

Question 13. Name the compound used to test the evolution of carbon dioxide gas.
Answer: Freshly prepared calcium hydroxide solution, also known as lime water, is employed to check for the presence of carbon dioxide gas.
In simple words: Freshly made calcium hydroxide solution, or lime water, is used to test if carbon dioxide gas is present.

Exam Tip: Remember that lime water (calcium hydroxide solution) is the standard reagent used to test for carbon dioxide, which turns it milky.

Question 14. Name the gas evolved when lead nitrate is heated.
Answer: When lead nitrate is heated, it releases two gases: nitrogen dioxide and oxygen.
In simple words: Heating lead nitrate produces nitrogen dioxide and oxygen gases.

Exam Tip: Recall that heating lead nitrate produces nitrogen dioxide (brown fumes) and oxygen gas.

Question 15. Write the formula for two oxides of sulphur.
Answer: The formulas for two oxides of sulphur are SO2 and SO3.
In simple words: The formulas for sulphur oxides are SO2 and SO3.

Exam Tip: List the common oxides of sulphur, sulfur dioxide (SO2) and sulfur trioxide (SO3), remembering their formulas.

Question 16. Give the examples of exothermic reaction.
Answer: Examples of exothermic reactions include respiration and the process of adding water to lime.
In simple words: Respiration and mixing water with lime are examples of exothermic reactions.

Exam Tip: Provide common examples of exothermic reactions such as respiration or the reaction of quicklime with water.

Question 17. Give one example of decomposition reaction that occurs in nature.
Answer: The natural process of fruits and vegetables rotting is an example of a decomposition reaction.
In simple words: Fruits and vegetables rotting naturally is an example of a decomposition reaction.

Exam Tip: Think of everyday examples of natural decomposition, like the rotting of organic matter, which breaks down complex substances.

Question 18. Name the type of reaction in which two or more than two reactants form a single compound.
Answer: The type of reaction where two or more reactants combine to form a single product is called a combination reaction.
In simple words: When two or more things join to make one new thing, it's a combination reaction.

Exam Tip: Identify the definition as a combination reaction, where multiple reactants unite to form one product.

Question 19. What is breaking and making of bonds in chemicals called?
Answer: The process of breaking and forming new bonds in chemical substances is referred to as a chemical reaction.
In simple words: Breaking and making chemical bonds is called a chemical reaction.

Exam Tip: Define a chemical reaction as the fundamental process involving the breaking and formation of chemical bonds.

Question 20. Name the chemical used in black and white photography.
Answer: Silver bromide is the chemical used.
In simple words: Silver bromide is the substance that helps black and white photos appear.

Exam Tip: Remember that silver bromide is light-sensitive, which is why it is used in photography.

Question 21. Name the ions present in barium sulphate.
Answer: The ions present in barium sulphate are \( Ba^{2+} \) and \( SO_4^{2-} \).
In simple words: Barium sulphate has a barium ion with a +2 charge and a sulphate ion with a -2 charge.

Exam Tip: Always remember the charges of common polyatomic ions like sulphate (\( SO_4^{2-} \)).

Question 22. Name the reaction which forms insoluble salts.
Answer: The reaction that forms insoluble salts is called a precipitation reaction.
In simple words: A reaction that creates a solid substance that doesn't dissolve in liquid is known as a precipitation reaction.

Exam Tip: Precipitation reactions are often double displacement reactions where one product is a solid precipitate.

Question 23. State the reaction in which hydrogen acts as a reducing agent.
Answer: In the reaction \( CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l) \), hydrogen acts as a reducing agent.
In simple words: Hydrogen helps copper oxide lose its oxygen, making hydrogen the reducing agent in this reaction.

Exam Tip: A reducing agent is a substance that causes another substance to be reduced (gain electrons or lose oxygen/gain hydrogen) while it itself gets oxidized.

Question 24. Give one use of quick lime.
Answer: Quick lime is used in the manufacturing of cement.
In simple words: Quick lime is a material used to make cement.

Exam Tip: Quick lime (calcium oxide) has many industrial uses, with cement manufacturing being a key one.

Question 25. Give one example of decomposition reaction in which solid and gas are two products obtained.
Answer: An example is the thermal decomposition of calcium carbonate:
\( CaCO_3(s) \rightarrow CaO(s) + CO_2(g) \). Here, calcium oxide is a solid, and carbon dioxide is a gas.
In simple words: When calcium carbonate breaks down, it forms solid calcium oxide and carbon dioxide gas.

Exam Tip: Remember to include the state symbols (s) for solid and (g) for gas when writing chemical equations.

Question 26. Consider the above mentioned two chemical equations with two different kinds of arrows (\( \uparrow \) and \( \downarrow \)) along with product. What do these two different arrows indicate?
Answer: The \( \uparrow \) arrow indicates that a gas is evolved, while the \( \downarrow \) arrow shows that an insoluble substance (a precipitate) is formed.
In simple words: An upward arrow means gas comes out, and a downward arrow means a solid forms that doesn't dissolve.

Exam Tip: These arrows are important symbols in chemical equations to convey the physical state changes of products, especially for gases and precipitates.

Short Answer Type Questions

Question 1. What happens when, carbon dioxide and water react in the same ratio?
Answer: When 6 molecules of carbon dioxide and 6 molecules of water react together, glucose is formed with the evolution of oxygen gas. The balanced equation is:
\( 6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 \).
In simple words: If carbon dioxide and water combine in a specific amount, they make sugar (glucose) and release oxygen.

Exam Tip: This reaction is a simplified representation of photosynthesis, a key biological process.

Question 2. How can you chemically remove the black coating of copper oxide?
Answer: The black coating of copper oxide can be chemically removed by passing hydrogen gas over heated copper oxide. The black coating then turns brown as oxygen is removed by hydrogen.
\( CuO(s) + H_2(g) \xrightarrow{Heat} Cu(s) + H_2O(l) \).
In simple words: You can take off the black copper oxide layer by heating it and letting hydrogen gas pass over it, which will turn it back into brown copper.

Exam Tip: This is a reduction reaction where copper oxide loses oxygen, and hydrogen gains oxygen, acting as a reducing agent.

Question 3. Products formed when A and B react together are zinc chloride and hydrogen gas. Find reactants A and B.
Answer: The reactant A is zinc metal, and reactant B is hydrochloric acid. The reaction is \( Zn + 2HCl \rightarrow ZnCl_2 + H_2 \).
In simple words: Zinc metal and hydrochloric acid react to make zinc chloride and hydrogen gas.

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

Question 4. Name the products obtained and type of reaction given below:
\( Na_2SO_4 + BaCl_2 \rightarrow \)
Answer: The products obtained in the above reaction are barium sulphate and sodium chloride.
\( Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl \). The reaction is a double displacement reaction.
In simple words: When sodium sulphate and barium chloride react, they produce barium sulphate and sodium chloride. This is a double displacement type of reaction.

Exam Tip: In a double displacement reaction, ions exchange partners to form two new compounds, often one of which is a precipitate.

Question 5. When quick lime is added to water a hissing sound is produced. Write the chemical reaction and name the type of reaction taking place.
Answer: When quick lime (calcium oxide) is added to water, it forms calcium hydroxide, producing a hissing sound and releasing heat. The reaction is:
\( CaO + H_2O \rightarrow Ca(OH)_2 + \Delta \). This is an exothermic and combination reaction.
In simple words: Adding quick lime to water creates calcium hydroxide, releases heat, and makes a hissing sound. This is a combination reaction that gives off heat.

Exam Tip: A combination reaction forms a single product from two or more reactants. An exothermic reaction releases heat.

Question 6. Three test tubes are taken and labelled as A, B and C. In test tube A iron nail is dipped in water. In test tube B iron nail is dipped in mixture of water and oil and in test tube C iron nail is added with dry calcium chloride. Name the test tube in which I the iron nail will rust and why?
Answer: The iron nail will rust in test tube A because it provides the conditions required for rusting, as both moisture and air are present in it.
In simple words: The iron nail in test tube A will rust because it has both water and air, which are needed for rusting.

Exam Tip: Rusting (corrosion of iron) requires the presence of both oxygen (from air) and moisture (water).

Question 7. Metal X becomes green when left in air, turns black when heated in air. Name the metal and the compounds formed in both the cases.
Answer: Metal X is copper. When left in air, the green compound formed is copper carbonate. When heated in air, the black colored compound formed is copper oxide.
In simple words: The metal is copper. When left out, it turns green (copper carbonate), and when heated, it turns black (copper oxide).

Exam Tip: Copper's reactions with atmospheric components lead to distinct color changes, which are good indicators for identification.

Question 8. Give two examples of a reaction which is both endothermic and decomposition in nature.
Answer: Two examples are:
\( CaCO_3(s) \xrightarrow{heat} CaO(s) + CO_2(g) \)
\( ZnCO_3(s) \xrightarrow{heat} ZnO(s) + CO_2(g) \)
In simple words: Heating calcium carbonate breaks it into calcium oxide and carbon dioxide. Similarly, heating zinc carbonate breaks it into zinc oxide and carbon dioxide. Both need heat to break down.

Exam Tip: Thermal decomposition reactions are typically endothermic as they require energy (heat) to break bonds.

Question 9. Explain why most of the metal articles become dull when kept exposed to air?
Answer: Metal articles react with the gases present in the air, and their surface gets coated with a layer of compound it forms, making them look dull and lose their luster. For example, aluminium reacts with oxygen to form aluminium oxide, and its surface becomes dull.
In simple words: Metal items get dull when left in the air because they react with air to form a thin layer of a new substance on their surface, which makes them lose their shine.

Exam Tip: This process is a form of corrosion, where the metal surface gradually deteriorates due to chemical reactions with its environment.

Question 10. What is rancidity? What is the general name of chemicals which are added to fat and oil containing food so as to prevent the rancidity?
Answer: Rancidity is the process where oil and fat-containing food, when exposed to air, reacts with oxygen, gets oxidized, and becomes rancid, changing its smell and taste. The general name for chemicals added to prevent this oxidation is antioxidants. For instance, nitrogen gas is an antioxidant.
In simple words: Rancidity is when fatty foods spoil and taste bad due to air exposure. Antioxidants are added to stop this from happening.

Exam Tip: Antioxidants work by preventing the oxidation of fats and oils, thus preserving the food's freshness and flavor.

Question 11. Define displacement reaction. Give one example of it, how is it different from double displacement reaction?
Answer: In a displacement reaction, a more reactive metal displaces (takes the place of) a less reactive metal from its compound. An example is:
\( Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s) \).
It differs from a double displacement reaction because, in a double displacement reaction, two different atoms or groups of atoms (ions) exchange for each other. An example is:
\( Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl \).
In simple words: A displacement reaction is when a stronger metal pushes a weaker metal out of its compound. A double displacement reaction is when two compounds swap their parts.

Exam Tip: Remember the reactivity series of metals to determine if a displacement reaction will occur. Double displacement reactions typically involve ionic compounds.

Question 12. Give differences between the exothermic and endothermic reactions.
Answer: In an exothermic reaction, heat is given out during the reaction. In contrast, an endothermic reaction requires heat to break bonds and form new compounds.
In simple words: Exothermic reactions release heat, making things warmer. Endothermic reactions absorb heat, making things colder.

Exam Tip: A good way to remember is 'exo' means 'out' (heat out), and 'endo' means 'in' (heat in).

Question 13. Explain and name the type of reaction seen when iron reacts with hydrochloric acid.
Answer: When iron reacts with hydrochloric acid, it forms hydrogen gas and iron chloride. This reaction is called a displacement reaction.
\( Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g) \).
In simple words: Iron reacting with hydrochloric acid makes hydrogen gas and iron chloride. This is a displacement reaction.

Exam Tip: Metals above hydrogen in the reactivity series will displace hydrogen from dilute acids.

Question 14. Why is photosynthesis considered as endothermic reaction?
Answer: Photosynthesis is a reaction where energy is required to form glucose from carbon dioxide and water. Energy, in the form of sunlight, is needed to break the bonds of hydrogen and oxygen. Hence, it is termed an endothermic reaction.
In simple words: Photosynthesis needs sunlight energy to make sugar, so it takes in heat and is called an endothermic reaction.

Exam Tip: Endothermic reactions absorb energy from their surroundings, often causing a decrease in temperature or requiring an external energy source like light.

Question 15. What is electrolytic decomposition? Give two uses of electrolytic decomposition reaction.
Answer: Electrolytic decomposition occurs when electricity passes through a molten compound, which is ionic in nature. The compound's ions then separate into their components, thereby decomposing the compound. It is used in separating hydrogen and oxygen gas from water, and it also helps to decompose sodium chloride into sodium metal and chlorine gas.
In simple words: Electrolytic decomposition uses electricity to break down a substance into simpler parts. It helps separate hydrogen and oxygen from water and also separates sodium and chlorine from salt.

Exam Tip: Electrolytic decomposition requires an external power source to drive non-spontaneous reactions, making it crucial for industrial production of certain elements.

Question 16. Give one example for each of the following reactions:
1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
Answer:
1. Combination reaction: \( 2Mg + O_2 \rightarrow 2MgO \)
2. Decomposition reaction: \( CaCO_3 \xrightarrow{heat} CaO + CO_2 \)
3. Displacement reaction: \( Zn + CuSO_4 \rightarrow ZnSO_4 + Cu \)
In simple words: For combination, magnesium and oxygen combine to form magnesium oxide. For decomposition, calcium carbonate breaks down with heat into calcium oxide and carbon dioxide. For displacement, zinc pushes copper out of copper sulphate to form zinc sulphate and copper.

Exam Tip: Practice identifying the distinct characteristics of each reaction type to easily recall examples.

Question 17. Give three ways used to prevent rusting.
Answer: Three ways to prevent rusting are:
1. Oiling of metals
2. Applying paints
3. Making an alloy
In simple words: You can stop rust by oiling metals, painting them, or turning them into alloys.

Exam Tip: These methods work by creating a barrier between the iron surface and the oxygen and moisture in the air.

Question 18. Define corrosion, rusting and rancidity.
Answer:
**Corrosion:** It is a process where a metal reacts with substances like moisture and gases present in the atmosphere, forming surface compounds and getting corroded.
**Rusting:** It is the process in which iron metal, when exposed to air and moisture, reacts to form a reddish-brown substance called rust.
**Rancidity:** It is when food containing fats and oil gets oxidized upon exposure to air, resulting in a bad taste and smell.
In simple words: Corrosion is when metals get damaged by air or water. Rusting is specific corrosion of iron, turning it reddish-brown. Rancidity is when fatty foods spoil and smell bad.

Exam Tip: Remember that rusting is a specific type of corrosion that only applies to iron and its alloys, while corrosion is a general term for the deterioration of metals.

Question 19. Name the type of reaction for the following:
1. Vegetable matter changing into compost.
2. Burning of natural gas.
3. Adding water to quick lime to form slaked lime.
Answer:
1. Vegetable matter changing into compost is an exothermic and decomposition reaction.
2. Burning of natural gas is an exothermic reaction.
3. Adding water to quick lime to form slaked lime is a combination reaction and exothermic in nature.
In simple words: Making compost from plants is decomposition and releases heat. Burning natural gas releases heat. Mixing water and quick lime makes slaked lime and also releases heat, which is a combination reaction.

Exam Tip: Focus on whether a reaction breaks down (decomposition) or builds up (combination), and whether it releases (exothermic) or absorbs (endothermic) heat.

Question 20. How can one make an equation more informative?
Answer: To make a chemical equation more informative:
1. The physical state of the reactants and products is mentioned, such as gaseous (g), liquids (l), aqueous (aq), or solid (s).
2. Conditions such as temperature, pressure, or a catalyst are mentioned above or below the arrow in the equation.
In simple words: You can make equations better by showing if things are solid, liquid, gas, or dissolved in water, and by writing down any special conditions like heat or pressure needed.

Exam Tip: Including state symbols and reaction conditions makes chemical equations complete and accurately reflect the actual process.

Question 21. Give the chemical equations (balanced) for the following:
1. Reaction used in black and white photography.
2. Reaction when glucose is oxidised.
3. Formation of water from H2 and O2.
Answer:
1. Reaction used in black and white photography:
\( 2AgBr(s) \xrightarrow{Sunlight} 2Ag(s) + Br_2(g) \)
2. Reaction when glucose is oxidized:
\( C_6H_{12}O_6(s) + 6O_2(aq) \rightarrow 6CO_2(g) + 6H_2O(g) + Energy \)
3. Formation of water from \( H_2 \) and \( O_2 \):
\( 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \)
In simple words: Silver bromide breaks down in light for photos. Glucose reacts with oxygen to give carbon dioxide, water, and energy. Hydrogen and oxygen combine to form water.

Exam Tip: Always balance chemical equations to obey the law of conservation of mass, ensuring the same number of atoms of each element on both sides.

Question 22. Give equations to show the chemical reactions of zinc and lead where it displaces copper from its compound.
Answer:
Zinc displacing copper:
\( Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s) \)
Lead displacing copper:
\( Pb(s) + CuCl_2(aq) \rightarrow PbCl_2(aq) + Cu(s) \)
In simple words: Zinc and lead can both take copper's place in its compounds, forming zinc sulphate or lead chloride and leaving copper behind.

Exam Tip: Metals higher in the reactivity series can displace metals lower in the series from their salt solutions.

Question 23. Give an example each for thermal decomposition and photochemical decomposition reactions. Write relevant balanced chemical equations also.
Answer:
**Thermal decomposition reaction:**
\( CuCO_3(s) \xrightarrow{Heat} CuO(s) + CO_2(g) \)
**Photochemical decomposition reaction:**
\( 2AgCl(s) \xrightarrow{Sunlight} 2Ag(s) + Cl_2(g) \)
In simple words: Thermal decomposition uses heat to break down copper carbonate. Photochemical decomposition uses sunlight to break down silver chloride.

Exam Tip: Thermal decomposition requires heat energy, while photochemical decomposition requires light energy.

Long Answer Type Questions

Question 1. Different types of chemical reactions are:
(a) Combination reactions: The reaction in which two or more than two substances combine together to form a single compound.
Example:
\( 2Mg + O_2 \rightarrow 2 MgO \)
(b) Decomposition reaction: The reaction in which a compound decomposes to form two or more substances is called decomposition reaction.
Example:
\( CaCO_3 \xrightarrow{heat} CaO + CO_2 \)
(c) Displacement reaction: The reaction in which more reactive metal displaces the less reactive metal is called displacement reaction.(d) Double displacement reaction: The reaction in which two different atoms or groups of atoms exchange for each other is called double displacement reaction.
Example:
\( Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq) \)
(e) Oxidation – reduction reaction: The reaction in which oxygen is added/hydrogen is removed is called oxidation reaction. The reaction in which hydrogen is added/oxygen is removed is called reduction.
Example:
\( CuO + H_2 \xrightarrow{heat} Cu + H_2O \)
Answer: Different types of chemical reactions include:
(a) **Combination reactions:** These reactions occur when two or more substances combine to form a single, more complex compound. For example, magnesium and oxygen combine to form magnesium oxide:
\( 2Mg + O_2 \rightarrow 2 MgO \).
(b) **Decomposition reaction:** In this type, a compound breaks down into two or more simpler substances. For instance, calcium carbonate decomposes with heat to form calcium oxide and carbon dioxide:
\( CaCO_3 \xrightarrow{heat} CaO + CO_2 \).
(c) **Displacement reaction:** This happens when a more reactive metal takes the place of a less reactive metal in a compound. (d) **Double displacement reaction:** In this case, two different atoms or groups of atoms (ions) swap places with each other. For example, sodium sulfate and barium chloride react to form barium sulfate and sodium chloride:
\( Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq) \).
(e) **Oxidation – reduction reaction:** Oxidation is a process where a substance gains oxygen or loses hydrogen (or loses electrons). Reduction is a process where a substance gains hydrogen or loses oxygen (or gains electrons). An example is when copper oxide reacts with hydrogen to form copper and water; here, copper oxide is reduced, and hydrogen is oxidized:
\( CuO + H_2 \xrightarrow{heat} Cu + H_2O \).
In simple words: Combination reactions join things, like magnesium and oxygen. Decomposition reactions break things apart, like calcium carbonate with heat. Displacement is when one element replaces another. Double displacement is when two elements swap partners. Oxidation means adding oxygen or losing hydrogen, and reduction means losing oxygen or gaining hydrogen.

Exam Tip: Understand the core definition and provide a balanced chemical equation for each type to demonstrate full comprehension.

Question 2. Give an activity to prove that water contains H : O in the ratio of 2 : 1.
Answer: Here is an activity to prove that water contains hydrogen and oxygen in a 2:1 ratio:
1. Take a plastic mug. Drill two holes at its base and fit rubber stoppers into these holes. Insert carbon electrodes into these rubber stoppers.
2. Connect these electrodes to a 6-volt battery.
3. Fill the mug with water such that the electrodes are immersed. Add a few drops of dilute sulphuric acid to the water to make it conductive.
4. Take two test tubes filled with water and invert them over the two carbon electrodes.
5. Switch on the current and leave the apparatus undisturbed for some time.
6. You will notice bubbles forming at both electrodes. These bubbles displace the water in the test tubes.
7. The gas collected in the test tube attached to the cathode is twice the volume of the gas collected at the anode.
8. Once the test tubes are filled with the respective gases, remove them carefully. Bring a burning matchstick or candle near the test tube containing the gas obtained at the cathode; it burns with a pop sound, indicating hydrogen gas. The gas at the anode relights a glowing splinter, indicating oxygen. This shows that the ratio of hydrogen to oxygen in water is 2:1 by volume.

In simple words: You can break water into hydrogen and oxygen using electricity. The hydrogen gas collects twice as much as oxygen gas, proving the 2:1 ratio.

Exam Tip: When describing an experiment, clearly list the steps, observations, and the conclusion, ensuring that the volume ratio of hydrogen to oxygen is accurately stated.

Question 3. What is rancidity? Suggest two methods to reduce the problem of rancidity. How is corrosion different from rusting?
Answer:
1. **Rancidity:** Fat and oil-containing food, when kept open in the air, gets oxidized and becomes rancid, which changes its taste and smell.
2. **Two methods to reduce rancidity:** Keep the food in closed containers and use antioxidants.
3. **Corrosion vs. Rusting:** Corrosion is seen in all metals when kept exposed, forming a layer of compound by reaction of metal with moisture, acid, or gases present in it. Rusting is a specific process in which iron metal reacts with air and moisture to form a brownish powder called rust.
In simple words: Rancidity makes fatty foods taste and smell bad; you can stop it by sealing food or using special chemicals. Corrosion is when any metal gets damaged by the environment, but rusting is only for iron, turning it brown.

Exam Tip: Clearly distinguish between corrosion (general) and rusting (specific to iron). For rancidity, focus on oxidation and prevention methods.

Question 4. What is meant by exothermic and endothermic reaction? Give examples to explain the same.
Answer:
**Exothermic reactions:** These are reactions in which heat is released during chemical reactions.
Example: The burning of natural gas (methane) releases a large amount of heat, forming carbon dioxide and water.
\( CH_4(g) + O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + heat \).
**Endothermic reactions:** These are reactions in which heat is absorbed or required.
Example: When calcium carbonate is heated, it forms calcium oxide and carbon dioxide, requiring heat.
\( CaCO_3(s) \xrightarrow{heat} CaO(s) + CO_2(g) \).
In simple words: Exothermic reactions give off heat, like burning gas. Endothermic reactions take in heat, like heating calcium carbonate to break it down.

Exam Tip: Remember that heat is a product in exothermic reactions and a reactant in endothermic reactions. Use state symbols in your examples.

Question 5. What is a redox reaction? When a magnesium ribbon burns in air with a dazzling flame, is magnesium oxidized or reduced? Why?
Answer: A redox reaction is a reaction that shows both oxidation and reduction reactions occurring simultaneously. When a magnesium ribbon burns in the air, magnesium is oxidized because oxygen combines with it. The reaction is:
\( 2Mg + O_2 \rightarrow 2MgO \).
In simple words: A redox reaction involves both gaining and losing oxygen at the same time. When magnesium burns, it gains oxygen, so it gets oxidized.

Exam Tip: Recall that oxidation is the gain of oxygen (or loss of hydrogen/electrons), and reduction is the loss of oxygen (or gain of hydrogen/electrons). These two processes always occur together.

Questions On Higher Order Thinking Skills (Hots)

Question 1. Name the term used for the solution of the reactant or product when dissolved in water.
Answer: The term used is 'Aqueous'.
In simple words: When something dissolves in water, we call its solution 'aqueous'.

Exam Tip: The state symbol (aq) in chemical equations denotes an aqueous solution.

Question 2. State one advantage and one disadvantage of corrosion.
Answer:
**Advantage:** In some metals, a protective layer is formed on their surface, which helps to prevent further corrosion. For example, aluminium forms a layer of aluminium oxide (\( Al_2O_3 \)) by corrosion, and this layer prevents further damage.
**Disadvantage:** A major disadvantage is the loss of the metal itself, leading to structural weakening and economic costs.
In simple words: Corrosion can be good because it forms a protective shield on some metals, like aluminum, stopping more damage. But it's bad because it wears away the metal, costing money and making things weaker.

Exam Tip: Focus on real-world examples (like aluminum passivation) for advantages and common problems (like rusting of iron) for disadvantages.

Question 3. List four changes which help us to determine whether a chemical reaction has taken place.
Answer: Four changes that help determine if a chemical reaction has taken place are:
1. Change in state
2. Change in colour
3. Change in temperature
4. Evolution of a gas
In simple words: You can tell a chemical reaction happened if you see a change in what it looks like (solid, liquid, gas), its color, its temperature, or if a gas is made.

Exam Tip: These observable changes are key indicators often used in qualitative analysis in chemistry experiments.

Question 4. Ahmad took a magnesium ribbon (cleaned) and burned it on a flame. The white powder formed was taken in a test tube and water was added to it. He then tested the solution formed with red and blue litmus paper. What change was seen and why?
Answer: The red litmus paper turned blue, while the blue litmus paper remained blue. This occurred because when magnesium ribbon burns in air, it forms white magnesium oxide. When magnesium oxide is dissolved in water, it creates magnesium hydroxide, which is basic in nature. Bases turn red litmus blue and have no effect on blue litmus.
In simple words: The red litmus turned blue because the burned magnesium formed magnesium oxide, which then made magnesium hydroxide in water, and that's a basic substance. Blue litmus stayed blue because it was already basic.

Exam Tip: Recall that metal oxides are generally basic and form hydroxides with water, which are detectable using litmus paper (red to blue).

Question 5. Give one example of a combination reaction in which an element combines with a compound to give you a new compound.
Answer: An example of an element combining with a compound is when sulfur dioxide reacts with oxygen to form sulfur trioxide.
\( 2SO_2(g) + O_2(g) \rightarrow 2SO_3(g) \). Another example is \( 2CO(g) + O_2(g) \rightarrow 2CO_2(g) \).
In simple words: When sulfur dioxide, which is a compound, joins with oxygen, an element, they make sulfur trioxide. This is a combination reaction.

Exam Tip: Ensure your example clearly shows one reactant as a single element and the other as a compound, forming a new single compound.

Question 6. Deepa added magnesium into a test tube containing dilute hydrochloric acid. She saw some gas coming out of it. She took a burning match stick near the mouth of the test tube and she heard a popping sound while the match stick extinguished. Deepa concluded that the gas evolved is hydrogen and it is not combustible. Find the error in her conclusion and support your answer with one valid reason.
Answer: Deepa's conclusion that hydrogen gas is not combustible is wrong. Hydrogen gas is highly combustible and burns with an explosion, producing a large amount of heat. The popping sound and the extinguishing of the matchstick are direct evidence of its combustibility, as the flame ignites the hydrogen and then goes out due to the rapid burning.
In simple words: Deepa was wrong; hydrogen gas actually burns very easily. The popping sound and the matchstick going out showed that the hydrogen was burning quickly.

Exam Tip: The 'pop' sound test is a characteristic test for hydrogen gas, indicating its highly combustible nature.

Question 7. Arnav took magnesium and reacted it with dil. HCl to record the observation. Then Deepak took the same piece of magnesium and reacted it with cone. HNO3 and dil. H2SO4 but did not see a reaction. Explain this behavior.
Answer: Arnav initially reacted magnesium with dilute HCl, which would form magnesium chloride (\( MgCl_2 \)) and hydrogen gas. The piece of metal that Deepak later used was not magnesium but magnesium chloride (\( MgCl_2 \)), which is a stable salt and thus did not react with concentrated \( HNO_3 \) or dilute \( H_2SO_4 \).
In simple words: Arnav used up the magnesium. Deepak tried to react magnesium chloride (the product) with acids, but it wouldn't react because it was no longer magnesium metal.

Exam Tip: Always consider what products are formed in a reaction and how they might behave in subsequent tests. The original reactant might have been completely consumed.

Question 8. Name the type of reaction seen in the following cases:
1. Garbage producing foul smell
2. Black and white photograph film when exposed to sunlight
3. Carbon dioxide gas passed through lime water
Answer:
1. Garbage producing foul smell: Decomposition reaction
2. Black and white photograph film when exposed to sunlight: Decomposition reaction
3. Carbon dioxide gas passed through lime water: Combination reaction
In simple words: Garbage rotting and film reacting to sunlight are both breakdown reactions. Carbon dioxide reacting with lime water is a combination reaction.

Exam Tip: For each scenario, identify whether substances are breaking down or combining to correctly classify the reaction type.

Question 9. Four beakers with chemicals are shown below. Name the beakers which will show which will be endothermic in nature.
Answer:
Exothermic reactions: Beaker A (\( H_2O + K_2SO_4 \)) and Beaker B (\( H_2O + NH_4NO_3 \)).
Endothermic reactions: Beaker C (\( H_2O + CuSO_4(\text{anhydrous}) \)) and Beaker D (\( H_2O + CuSO_4(\text{crystals}) \)).

In simple words: The beakers that show endothermic reactions (meaning they get colder by absorbing heat) are C and D, containing copper sulphate. The others are exothermic (meaning they get warmer by releasing heat).

Exam Tip: Dissolution of many salts can be either exothermic (like \( K_2SO_4 \)) or endothermic (like \( NH_4NO_3 \)). Anhydrous salts often release heat when hydrating, but dissolving crystals might absorb it.

Question 10. Give two examples in which reactants react to show combination reaction and a new product is formed. This new product decomposes to give the initial reactants. Do all combination reactions get decomposed to produce the same reactants?
Answer: The reactions in which reactants combine to form products, and then these new products decompose to give the initial reactants, are called reversible reactions.
Example:
\( X + Y \xrightarrow{\text{combination}} Z \)
\( Z \xrightarrow{\text{decomposition}} X + Y \)
No, not all combination reactions decompose to produce the same initial reactants. Many combination reactions are irreversible under normal conditions, or they may decompose into different products.
In simple words: Some reactions can go both ways: they combine to form a new substance, and that substance can break down to form the original parts again. But not all combination reactions can be undone to get the exact same starting materials back.

Exam Tip: Understand that reversible reactions involve a dynamic equilibrium where both forward (combination) and reverse (decomposition) processes occur, but this is not universal for all combination reactions.

Question 12. Why are certain reagents like silver bromide stored in dark bottles in the labs?
Answer: Reagents and chemicals such as silver bromide decompose when exposed to light. Hence, they are kept in dark bottles in labs to prevent exposure to light and their decomposition.
In simple words: Silver bromide is kept in dark bottles because light makes it break down, so dark bottles protect it.

Exam Tip: Light-sensitive compounds, especially those used in photography (like silver halides), must be protected from light to maintain their chemical integrity.

Question 14. Mohan took pure water for the electrolytic decomposition of water but did not see any bubble near the electrodes. Explain why?
Answer: Pure water has covalent bonds and therefore does not allow electricity to flow through it easily, so it does not decompose. However, by adding a few drops of dilute acid, free ions are obtained, allowing electricity to flow through the water to dissociate hydrogen and oxygen.
In simple words: Pure water doesn't conduct electricity well, so it won't break apart. But if you add a little acid, it helps the electricity flow and the water splits into bubbles.

Exam Tip: Pure water is a poor conductor of electricity. Adding an electrolyte (like an acid or a base) increases its conductivity, enabling electrolysis.

Question 15. A teacher took few crystals of sugar in a dry test tube and heated the test tube over flame. The colour of sugar turned black. Explain why?
Answer: Sugar is a complex compound that undergoes decomposition when heated. Water from the sugar evaporates, leaving behind only black carbon in the test tube, which causes the color change.
In simple words: When sugar is heated, it breaks down. The water in the sugar boils away, and what's left behind is black carbon.

Exam Tip: This is a dehydration reaction where the components of water are removed from the sugar molecule, leaving behind carbon.

Question 16. Blue crystals of copper sulphate on heating in a dry test tube become colourless. Give reasons.
Answer: The blue color of copper sulphate is due to its crystalline nature, which holds five water molecules (water of crystallization). Upon heating, these water molecules disappear, and anhydrous copper sulphate, which is white in color, is left behind.
In simple words: Blue copper sulphate crystals turn white when heated because the water trapped inside them evaporates, leaving only the white copper sulphate powder.

Exam Tip: Many hydrated salts have distinct colors, and losing their water of crystallization upon heating often results in a color change and the formation of an anhydrous (water-free) salt.

Question 17. \( FeSO_4.7H_2O \), green colour crystals on heating, changes colour. Why?
Answer: The green color of ferrous sulfate heptahydrate (\( FeSO_4.7H_2O \)) crystals is due to the presence of seven water molecules of crystallization. When heated, it loses these water molecules, leading to a change in color.
In simple words: Green ferrous sulfate crystals change color when heated because they lose the water molecules that give them their green color.

Exam Tip: Similar to copper sulfate, ferrous sulfate is a hydrated salt whose color depends on the presence of water of crystallization.

Practical Based Questions (Solved)

Question 1. Predict what would happen if you add zinc coated iron nail into the solution of copper sulphate. Give reason for your prediction.
Answer: The zinc layer on the iron nail will combine with the copper sulphate liquid, and the liquid's blue shade will gradually lighten. Later, the liquid will become green because the iron nail will then engage with copper sulphate. This happens as both iron and zinc are more active than copper in copper sulphate, so a displacement process takes place.
In simple words: A zinc-coated iron nail in copper sulphate solution will make the blue liquid turn green because zinc and iron are more reactive and displace copper.

Exam Tip: Remember to explain both the color changes and the underlying reactivity principle in displacement reactions to score full marks.

Question 2. How can you differentiate sodium metal and zinc metal given in the test tubes in the lab. You are advised not to touch any of the metals. Name the type of reaction seen.
Answer: In both test tubes, I will add cold water and watch the reaction. The tube where the reaction finishes more quickly will contain sodium, and the process is a combination and releases heat.
In simple words: To tell sodium and zinc apart, add cold water to each. The one that reacts faster is sodium. Both show combination reactions that give off heat.

Exam Tip: Always note the reaction rate and heat evolution when identifying highly reactive metals like sodium.

Question 3. A student wants to find how fast the reaction is completed and to do so he places a 'X' marked on the tile, above it he places the conical flask with sodium sulphate in it. After some time he adds the solution of barium chloride and records the time taken for the "X' to disappear. Explain what makes the cross disappear. Name the type of reaction.
Answer: In this reaction, barium chloride and sodium sulphate liquids combine to form a white solid of barium sulphate. This solid makes it hard to see through the newly formed substance, which causes the 'X' mark to disappear. This type of process is termed a double displacement reaction.
In simple words: When barium chloride and sodium sulphate mix, they create a white solid (precipitate) of barium sulphate. This solid blocks the view, making the 'X' disappear. It's a double displacement reaction.

Exam Tip: The formation of an insoluble precipitate is key in double displacement reactions and often causes visual changes like cloudiness or disappearance of marks.

Question 4. Name the chemicals and the materials required to do this experiment.
Answer: The necessary chemicals are: calcium carbonate for heat-induced breakdown, silver bromide for light-triggered breakdown, and tap water for electrical breakdown. Materials include beakers, test tubes, a burner, tripod stand, tongs, a battery, crocodile clips, wires, carbon electrodes, hand gloves, safety goggles, and a lab coat.
In simple words: You need calcium carbonate, silver bromide, and tap water. You also need lab equipment like beakers, test tubes, a burner, battery, wires, and safety gear like gloves and goggles.

Exam Tip: When listing lab requirements, separate chemicals from equipment and include safety gear for practical-based questions.

Question 5. A student wants to study the decomposition reaction of iron sulphate in lab, what precautions should the student take and why?
Answer: Students must wear a lab coat, gloves, and safety goggles because heating a test tube with iron sulphate can cause burns. They should turn on the exhaust fan or conduct heating in a fume cupboard since sulphur dioxide gas, freed during breakdown, has a strong, suffocating odor. Students can also wear a mask or prevent breathing in the lab gas.
In simple words: Wear safety gear like a lab coat, gloves, and goggles because heating iron sulphate can cause burns. Use an exhaust fan or fume cupboard, and a mask, to avoid inhaling the strong-smelling sulphur dioxide gas.

Exam Tip: Always prioritize safety in lab experiments; common precautions include protective clothing, ventilation, and avoiding direct inhalation of fumes.

Question 6. The teacher wants to show that all the crystals in the lab will decompose to give water on heating which is collected on the upper surface of inside of the test tube. Name any four compounds the teacher should use in the lab to perform this activity.
Answer: The teacher could use blue vitriol, which is copper sulphate (a blue crystal salt), green vitriol, or iron sulphate (a green crystal salt), white sodium carbonate crystals, and white calcium sulphate crystals.
In simple words: The teacher can use blue copper sulphate, green iron sulphate, white sodium carbonate, and white calcium sulphate crystals. All these release water when heated.

Exam Tip: Remember common hydrated salts like copper sulphate, iron sulphate, and calcium sulphate as examples of crystals that release water upon heating.

Question 7. Give one example of the combination reaction that is also exothermic in nature and how can you show its exothermic nature in the lab.
Answer: When lime (calcium oxide) reacts with water, it creates a very exothermic reaction. This process gives off warmth that can be measured with a thermometer in the laboratory.
In simple words: When calcium oxide (lime) mixes with water, it's a combination reaction that produces a lot of heat. You can show this in the lab by measuring the temperature rise with a thermometer.

Exam Tip: The reaction of quicklime with water is a classic example of both a combination and an exothermic reaction, easily demonstrable by temperature change.