CBSE Class 10 Chemistry Chemical Reactions And Equations Worksheet Set E

Question. In which of the following, the identity of initial substance remains unchanged ?
(a) Curdling of milk
(b) Formation of crystals by process of crystallisation
(c) Fermentation of grapes
(d) Digestion of food
Answer: (b)

Question. To balance \( Al(OH)_3 + HNO_3 \rightarrow Al(NO_3)_3 + H_2O \) number of \( HNO_3 \) molecules required will be :
(a) 2
(b) 4
(c) 3
(d) 8
Answer: (c)

Question. The reaction between calcium oxide and water is :
(a) Combination reaction
(b) Decomposition reaction
(c) Displacement reaction
(d) Double-decomposition reaction
Answer: (a)

Question. The colour of ferrous sulphate crystals is :
(a) Deep green
(b) Light green
(c) Bluish green
(d) None of the above
Answer: (b)

Question. What is the chemical name for slaked lime ?
(a) Sodium hydroxide
(b) Calcium oxide
(c) Calcium hydroxide
(d) Sodium chloride
Answer: (c)

Question. Heating of ferrous sulphate is a type of :
(a) Decomposition reaction
(b) Combination reaction
(c) Displacement reaction
(d) All of the above
Answer: (a)

Question. Which metal is displaced when lead is put in the solution of copper chloride ?
(a) Lead
(b) Copper
(c) Chlorine
(d) All of the above
Answer: (b)

Question. On the basis of evolution or absorption of heat, chemical reactions can be divided in how many types ?
(a) Two
(b) Three
(c) Four
(d) One
Answer: (a)

Question. Which of the following metal is protected by a layer of its oxide ?
(a) Copper
(b) Iron
(c) Aluminium
(d) Sodium
Answer: (c)

Question. The oxidation reaction which produces heat and light is called :
(a) Endothermic
(b) Photochemical
(c) Exothermic
(d) Combustion
Answer: (d)

Question. Match the following
Column I
(a) \( BaCl_{2(aq)} + ZnSO_{4(aq)} \rightarrow BaSO_4 \downarrow + ZnCl_{2(aq)} \)
(b) \( 2AgCl_{(s)} \rightarrow 2Ag_{(s)} + Cl_{2(g)} \)
(c) \( CuSO_{4(aq)} + Fe_{(s)} \rightarrow FeSO_{4(aq)} + Cu_{(s)} \)
(d) \( H_2O_{(l)} + CO_{2(g)} \rightarrow H_2CO_{3(aq)} \)
Column II
(i) Displacement
(ii) Combination
(iii) Decomposition
(iv) Double displacement
Answer: (a) (iv), (b) (iii), (c) (i), (d) (ii).

Fill in the blanks

Question. (a) The conversion of ferrous sulphate to ferric sulphate is .................. reaction.
(b) Boiling of water to give water vapour is a .................. change.
(c) The slow process of decay or destruction of a metal due to effect of air, moisture and acids on its own is known as .................. .
Answer: (a) Oxidation, (b) Physical, (c) Corrosion.

Very Short Answers Type Questions

Question. What is observed when a solution of sodium sulphate is added to a solution of barium chloride in a test tube? Write equation for the chemical reaction involved and name the type of reaction in this case.
Answer: \( Na_2SO_{4(aq)} + BaCl_{2(aq)} \rightarrow BaSO_{4(s)} + 2NaCl_{(aq)} \). This is a Precipitation reaction or double displacement reaction. Observation: White coloured precipitate is formed.

Question. What is a chemical reaction ?
Answer: A process in which a new substance is formed and original substance lose its identity is called a chemical change.

Question. Identify the type of reaction : \( Zn + CuSO_4 \rightarrow ZnSO_4 + Cu \)
Answer: Displacement reaction.

Question. Which substance is used in white washing ?
Answer: Calcium hydroxide (slaked lime) is used in white washing.

Question. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.
Answer: Calcium oxide. \( CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) \)

Question. Why should an equation be balanced ?
Answer: An equation should be balanced because according to the law of conservation of mass, mass can neither be created nor be destroyed. Therefore, the number of atoms on both the sides of equation must be equal.

Question. Why is hydrogen included in activity series of metals ?
Answer: Hydrogen is included in activity series of metals because like metals, hydrogen can also lose electrons to form positive ions.

Question. Why is a copper vessel covered with a green coating in rainy season ?
Answer: Copper vessel is covered with a green coating in rainy season due to the attack of oxygen, carbon dioxide and water vapours of the air on copper forming green coloured basic copper carbonate.

Question. Name the most reactive metal and least reactive metal of the activity series of metals.
Answer: Most reactive metal is potassium and least reactive metal is gold.

Question. What is the reason behind various metals such as iron get wasted every year in our country ?
Answer: Due to corrosion various metals such as iron get wasted every year in our country.

Question. What happens when a dilute solution of ferrous sulphate was gradually added to a beaker containing acidified permanganate solution ? Explain the reason.
Answer: The light purple colour of the solution fades and finally disappears. The colour of the solution disappears because of dilution. No reaction is involved in this.

Question. When the powder of common metal is heated in open china dish, its colour turns black. However, when hydrogen is passed over the hot black substance so formed, it regains its original colour. Based on the above information answer the following questions :
(a) What type of chemical reaction takes place in each of the two given steps ?
(b) Name the metal initially taken in the powder form.
Answer: (a) In the first step, oxidation takes place and in the second step, redox reaction takes place. (b) The metal initially taken in powder form was copper.

Short Answers Type Questions

Question. Mention with reason the colour changes observed when :
(a) silver chloride is exposed to sunlight.
(b) copper powder is strongly heated in the presence of oxygen.
(c) a piece of zinc is dropped in copper sulphate solution.
Answer: (a) When silver chloride (white) is exposed to sunlight, it undergoes photochemical decomposition forming black metallic coloured silver and greenish yellow gas chlorine. Thus, the change in colour is observed due to the oxidation of silver chloride in the presence of sunlight.
(b) Heated copper metal reacts with oxygen to form the black copper oxide. Thus, the change in colour is observed when copper powder is strongly heated. \( 2Cu(s) + O_2(g) \rightarrow 2CuO(s) \)
(c) On adding zinc to \( CuSO_4 \) solution, zinc displaces copper from copper sulphate and forms zinc sulphate solution. This is indicated by colour change from blue to colourless. \( CuSO_4 \) solution has a blue colour while \( ZnSO_4 \) solution is colourless. \( Zn(s) + CuSO_4(aq) \rightarrow Cu(s) + ZnSO_4(aq) \)

Question. 2 g of silver chloride is taken in a china dish and the china dish is placed in sunlight for sometime. What will be your observation in this case ? Write the chemical reaction involved in the form of a balanced chemical equation. Identify the type of chemical reaction.
Answer: \( 2AgCl \xrightarrow{Sunlight} 2Ag + Cl_2 \). Initially the colour of silver chloride is white but when it is kept in sunlight it breaks down to give silver and chlorine. Hence, the colour changes to grey. It is an example of photo-chemical decomposition.

Question. Identify the type of reactions taking place in each of the following cases and write the balanced chemical equation for the reactions.
(a) Zinc reacts with silver nitrate to produce zinc nitrate and silver.
(b) Potassium iodide reacts with lead nitrate to produce potassium nitrate and lead iodide.
Answer: (a) The given reaction is a displacement reaction. \( Zn(s) + 2 AgNO_3(aq) \rightarrow Zn(NO_3)_2 (aq) + 2Ag(s) \)
(b) It is a double displacement reaction. \( 2KI(aq) + Pb(NO_3)_2(aq) \rightarrow 2 KNO_3(aq) + PbI_2(s) \)

Question. You might have noted that when copper powder is heated in a China dish, the reddish brown surface of copper powder becomes coated with a black substance.
(a) Why has this black substance formed ?
(b) What is this black substance?
(c) Write the chemical equation of the reaction that takes place.
(d) How can the black coating on the surface be turned reddish brown?
Answer: (a) This black substance has been formed due to the oxidation of copper powder.
(b) This black substance is copper (II) oxide (CuO).
(c) \( 2Cu + O_2(g) \xrightarrow{Heat} 2CuO(s) \)
(d) This black coating can be removed by treating the black compound with hydrogen. \( CuO + H_2(g) \rightarrow Cu + H_2O(l) \). Copper oxide (black) + Hydrogen -> Copper metal (shiny brown) + water.

Question. Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electrictiy.
Answer: (i) Thermal decomposition reaction: \( 2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g) \). (ii) Photo decomposition reaction: \( AgCl(s) \xrightarrow{Sunlight} Ag(s) + Cl_2(g) \). (iii) Electrolytic decomposition reaction: \( 2H_2O(l) \xrightarrow{Electricity} 2H_2(g) + O_2(g) \).

Question. What does one mean by exothermic and endothermic reactions ? Give examples.
Answer: Chemical reactions that release energy in the form of heat, light or sound are called exothermic reactions. Combination reactions are exothermic. Example : Sodium oxide dissolves in water to form sodium hydroxide and releases large amount of energy. \( Na_2O + H_2O \rightarrow 2NaOH + Energy \). Reactions that absorb energy or require energy in order to proceed are called endothermic reactions. Example : Combination of nitrogen and oxygen to form nitric oxide. \( N_2 + O_2 + Heat \rightarrow 2NO \).

Question. What do you mean by a precipitation reaction ? Explain by giving examples.
Answer: A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction. For example : \( Na_2CO_3(s) + CaCl_2(s) \rightarrow CaCO_3(s) + 2NaCl (aq) \). In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Question. Identify the oxidising agent (oxidant) in the following reactions :
(a) \( Pb_3O_4 + 8HCl \rightarrow 3PbCl_2 + Cl_2 + 4H_2O \)
(b) \( 2Mg + O_2 \rightarrow 2MgO \)
(c) \( CuSO_4 + Zn \rightarrow Cu + ZnSO_4 \)
(d) \( V_2O_5 + 5Ca \rightarrow 2V + 5CaO \)
(e) \( 3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2 \)
(f) \( CuO + H_2 \rightarrow Cu + H_2O \)
Answer: (a) \( Pb_3O_4 \) (b) \( O_2 \) (c) \( CuSO_4 \) (d) \( V_2O_5 \) (e) \( H_2O \) (f) \( CuO \)

Question. Why should a magnesium ribbon be cleaned before burning in air ?
Answer: Magnesium is a very reactive metal, it reacts with oxygen to form a layer of magnesium oxide on its surface when stored. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. Hence, this layer is removed by cleaning magnesium ribbon by sand paper so that the underlying metal can be exposed into air.

Question. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.
(a) Why do silver articles turn black when kept in the open for a few days ? Name the phenomenon involved.
(b) Name the black substance formed and give its chemical formula.
Answer: (a) Metals such as silver when attacked by substances around it such as moisture, acids, gases etc., are said to corrode and this phenomenon is called corrosion. (b) The black substance is formed because silver (Ag) reacts with \( H_2S \) present in air. It forms thin black coating of silver sulphide (\( Ag_2S \)).

Question. Oil and fat containing food items are flushed with nitrogen. Why ? OR Explain why, food products containing fats and oils (like potato chips) are packaged in nitrogen.
Answer: Oil and fat containing food items are flushed with nitrogen because it is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Question. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form ammonia gas.
(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.
(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated \( H_2SO_4 \).
(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.
Answer: (a) \( N_2(g) + 3H_2 \rightarrow 2NH_3(g) \) Combination Reaction (b) \( NaOH(aq) + CH_3COOH(aq) \rightarrow CH_3COONa(aq) + H_2O(l) \) Double displacement reaction or neutralisation reaction. (c) \( C_2H_5OH(l) + CH_3COOH(l) \rightarrow CH_3COOC_2H_5(l) + H_2O(l) \) Double displacement reaction or Esterificaton reaction. (d) \( C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) + 2H_2O(g) + Heat + Light \) Redox reaction or combustion reaction.

Question. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case :
(a) Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.
(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.
(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.
(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.
Answer: (a) \( Fe_2O_3(s) + 2Al(s) \rightarrow Al_2O_3(s) + 2Fe(l) + Heat \) Displacement reaction or redox reaction (b) \( 3Mg(s) + N_2(g) \rightarrow Mg_3N_2(s) \) Combination reaction (c) \( 2KI(aq) + Cl_2(g) \rightarrow 2KCl(aq) + I_2(s) \) Displacement reaction (d) \( C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l) + Heat \) Redox reaction or combustion reaction

Question. In the reaction : \( MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2 \)
(a) Name the compound (i) oxidised, (ii) reduced.
(b) Define oxidation and reduction on its basis.
Answer: (a) (i) HCl is oxidised in the reaction. (ii) \( MnO_2 \) is reduced in the reaction. (b) Oxidation is the gain of oxygen or loss of hydrogen whereas reduction is the gain of hydrogen or loss of oxygen.

Long Answers Type Questions

Question. Write balanced chemical equations to explain what happens, when
(i) Mercuric oxide is heated.
(ii) Mixture of cuprous oxide and cuprous sulphide is heated.
(iii) Aluminium is reacted with manganese dioxide.
(iv) Ferric oxide is reduced with aluminium.
(v) Zinc carbonate undergoes calcination.
Answer: (i) Cinnabar (HgS) is an ore of mercury. When it is heated in air, it is first converted into mercuric oxide (HgO). Mercuric oxide is then reduced to mercury on further heating. \( 2HgO(s) \xrightarrow{Heat} 2Hg(l) + O_2(g) \)
(ii) When a mixture of copper oxide and copper sulphide is heated metallic copper is formed along with the evo lution of the sulphur dioxide gas. The reaction equation is as follows: \( 2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2 \)
(iii) When aluminium powder is reacted with manganese dioxide, Manganese and aluminium oxide are formed with the evolution of heat and the following reaction takes place : \( 3MnO_2(s) + 4Al(s) \rightarrow 3Mn(l) + 2Al_2O_3(s) + Heat \)
(iv) When ferric oxide is reduced with aluminium a displacement reaction takes place and this reaction is highly exothermic. The amount of heat evolved is so large that the metals are produced in the molten state. In fact, the reaction of iron (III) oxide (\( Fe_2O_3 \)) with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the thermite reaction. \( Fe_2O_3(s) + 2Al(s) \rightarrow 2Fe(l) + Al_2O_3(s) + Heat \)
(v) The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination. The chemical reaction that takes place during calcination of zinc ores can be shown as follows : \( ZnCO_3(s) \xrightarrow{Heat} ZnO(s) + CO_2(g) \)

Question. (a) State the various characteristics of chemical reactions.
(b) State one characteristic each of the chemical reaction which takes place when :
(i) Dilute hydrochloric acid is added to sodium carbonate.
(ii) Lemon juice is added gradually to potassium permanganate solution.
(iii) Dilute sulphuric acid is added to barium chloride solution.
(iv) Quick lime is treated with water.
(v) Wax is burned in the form of a candle.
Answer: (a) The various characteristics of chemical reactions are : (i) Evolution of a gas (ii) Formation of a precipitate (iii) Change in colour (iv) Change in temperature (v) Change in state. (b) (i) Evolution of carbon dioxide gas. (ii) Change in colour from purple to colourless. (iii) Formation of white precipitate of barium sulphate. (iv) Change in temperature. (v) Change in state from solid to liquid and gas.

Question. Give the characteristic tests for the following gases : (a) \( CO_2 \), (b) \( SO_2 \), (c) \( O_2 \), (d) \( H_2 \).
Answer: (a) Carbon dioxide (\( CO_2 \)) : This gas turns lime water milky when passed through it due to the formation of insoluble calcium carbonate. \( Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \). (b) Sulphur dioxide (\( SO_2 \)) : This gas when passed through acidic potassium permanganate solution (purple in colour) turns it colourless because \( SO_2 \) is a strong reducing agent. \( 2KMnO_4 + 2H_2O + 5SO_2 \rightarrow K_2SO_4 + 2MnSO_4 + 2H_2SO_4 \). (c) Oxygen : The evolution of oxygen (\( O_2 \)) gas during a reaction can be confirmed by bringing a burning candle near the mouth of the test tube containing the reaction mixture. The intensity of the flame increases because oxygen supports burning. (d) Hydrogen (\( H_2 \)) : This gas burns with a pop sound when a burning candle is brought near it. \( 2Cu(NO_3)_2(s) \rightarrow 2CuO(s) + O_2(g) + 4NO_2(g) \)

Assertion and Reasoning Based Questions

Directions : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.

Question. Assertion : Chemical equations should be balanced.
Reason : As per the law of conservation of mass, mass can neither be created nor be destroyed.
(a)
(b)
(c)
(d)
Answer: (a)

Question. Assertion : Hydrogen peroxide is kept in coloured bottles.
Reason : Hydrogen peroxide is a moderately reactive metal that can react with light or heat slowly to produce water.
(a)
(b)
(c)
(d)
Answer: (c)

Question. Assertion : Silver bromide is kept in the coloured bottles.
Reason : Silver bromide decomposes in presence of light.
(a)
(b)
(c)
(d)
Answer: (a)

Question. Assertion : Colour of copper sulphate solution changes when an iron nail is kept immersed in it.
Reason : The decomposition reaction takes place between iron and copper leading to the formation of iron sulphate.
(a)
(b)
(c)
(d)
Answer: (c)

Question. Assertion : Hydrogen is not included in the activity series of metals.
Reason : It is because it can lose electrons to form positive ions.
(a)
(b)
(c)
(d)
Answer: (d)

Creating Based Questions

Question. 5 ml of sulphuric acid was put into a small conical flask during some experiment in a laboratory. A small balloon has been fixed on the glass pipe inserted through the middle of the stopper cork. A student working in the lab adds some zinc granules to the conical flask by mistake and observes the balloon is blown up. Also, the conical flask became hot in the process.
(a) What may be the reason behind blowing up of balloon ?
(b) What may be the chemical representation of the above observation ?
Answer: (a) The zinc granules undergo a chemical reaction with sulphuric acid to give hydrogen gas as a product. This evolved hydrogen gas fills the balloon and balloon gets blown up.
(b) As this is a chemical reaction, this can be represented by the following chemical equation :
\( Zn(s) + H_2SO_4(aq) \longrightarrow ZnSO_4(aq) + H_2(g) \)

Question. Write a balanced chemical equation with state symbols for the following reactions :
(a) Glucose molecule \( (C_6H_{12}O_6) \) reacts with oxygen molecule to give carbon dioxide gas and gaseous water.
(b) Iron dust reacts with gaseous water to give iron oxide \( (Fe_3O_4) \) precipitate and hydrogen gas.
Answer: (a) \( C_6H_{12}O_6(s) + 6O_2(g) \longrightarrow 6CO_2(g) + 6H_2O \)
(b) \( 3Fe(s) + 4H_2O (g) \longrightarrow Fe_3O_4(s) + 4H_2(g) \)

Question. Annie was happy to see her small house getting cleaned and whitewashed. She observed that the workers dissolved some pale white powdery material called quick lime into water (a vigorous reaction takes place while mixing) left it for some time and applied the white suspension obtained to the walls. At first the wall looked pale white and watery but in 2-3 days the walls became shiny bright white in colour. How can you describe what she observed through chemical equations ?
Answer: Quick lime is chemically known as calcium oxide. It reacts vigorously with water to produce slaked lime (calcium hydroxide), releasing a large amount of heat.
\( CaO(s) + H_2O(l) \longrightarrow Ca(OH)_2(aq) + Heat \)
Quick lime Slaked lime
The slaked lime after application to wall reacts slowly with carbon dioxide present in atmosphere, to give a thin layer of calcium carbonate on the walls after 2-3 days giving the walls a shiny bright white colour.
\( Ca(OH)_2(aq) + CO_2(g) \longrightarrow CaCO_3 + H_2O(l) \)
Slaked lime Calcium carbonate

Question. Give reason and name the type of chemical reaction taking place in each case :
(a) Dissolution of ammonium chloride in water leads to cooling of the glass apparatus used for dissolutions.
(b) Silver chloride powder which is white in colour, turns grey when kept in sunlight.
(c) Blue colour of copper sulphate solution fades when an iron nail is dipped inside the solution.
Answer: (a) Dissolution of ammonium chloride in water is an endothermic reaction where heat is absorbed from surroundings hence making the surrounding cooler than before :
\( NH_4Cl(s) + H_2O(l) \longrightarrow NH_4OH(aq) + HCl(aq) \)
Ammonium chloride Ammonium hydroxide Hydrochloric Acid
(b) Silver chloride undergoes decomposition reaction in sunlight to give silver metal and chlorine :
\( 2AgCl(s) \longrightarrow 2Ag(s) + Cl_2(g) \)
Silver chloride (White) Silver metal (Grey)
(c) Iron displaces copper from its solution, hence the colour of copper sulphate solution becomes less blue or fades. This is an example of displacement reaction :
\( Fe(s) + CuSO_4(aq) \longrightarrow FeSO_4(aq) + Cu(s) \)
Copper sulphate (Blue) Ferrous sulphate

Question. Which species out of (1) and (2) is getting oxidised and which one is getting reduced in the below given reactions and why :
(a) \( CuO + H_2 \longrightarrow Cu + H_2O \)
     (1)   (2)
(b) \( MnO_2 + 4HCl \longrightarrow MnCl_2 + 2H_2O + Cl_2 \)
     (1)      (2)
What is the general name for these types of reactions ?
Answer: (a) Here cuprous oxide (1) is getting reduced (to give copper metal) as it is losing oxygen and hydrogen (2) is getting oxidised as it is combining with oxygen to give water.
(b) Here manganese dioxide (1) is getting reduced (to \( MnCl_2 \)) as it is losing oxygen and hydrochloric acid (2) is getting oxidised to water.
\( MnO_2 + 4HCl \longrightarrow MnCl_2 + 2H_2O + Cl_2 \)
Manganese dioxide Hydrochloric acid
Generally, these type of reactions are known as redox reaction.

Paragraph/Table and Case Study Based Questions

Vedanshi and Krati are working in the lab. During the experiment of determining the reactivity of acid and bases, the test tube containing chemical ‘X’ cracked and the liquid inside produced blisters on the skin of the Krati. The pH strip turned blue to red, with the reaction to the liquid. Vedanshi immediately called her teacher for help, and first aid was provided to Krati.

Question. What could be the possible liquid be present in test tube?
(a) Boiling water
(b) Concentrated \( H_2SO_4 \)
(c) Vinegar
(d) Sodium Hydroxide Solution
Answer: (b)

Question. The following list include the possible first aid for the laboratory accidents.
1. Thoroughly rinse the affected area with water.
2. Having medicines without prescribing the doctor.
3. Consult the doctor immediately
4. To go outside the lab, preferably in an open area.
5. Not telling the teacher and other elders or they might scold you.
(a) All of these
(b) 1, 2 and 3 are correct
(c) Only 1 is correct
(d) 1, 3 and 4 are correct
Answer: (d)

Question. In the experiment, Krati and Vedanshi listed the chemicals in list A and B. Which of the following is correct?
(a) List A: \( HCl \), Citric Acid, Oxalic Acid, Nitric Acid, Vinegar; List B: Ammonia, Sodium Hydroxide, Baking soda
(b) List A: Ammonia, Sodium Hydroxide, Baking soda; List B: \( HCl \), Citric Acid, Oxalic Acid, Nitric Acid, Vinegar
(c) List A: \( HCl \), Citric Acid, Oxalic Acid, Nitric Acid,; List B: Ammonia, Sodium Hydroxide, Baking soda, vinegar
(d) List A: Ammonia, Sodium Hydroxide, Vinegar; List B: \( HCl \), Citric Acid, Oxalic Acid, Nitric Acid, Baking Soda
Answer: (b)

Question. Krati detected the pH of four unknown solution A, B, C and D as follows 11, 5, 7 and 2 respectively and note down her results.
1. C is neutral in nature.
2. On reacting A and D, water is liberated.
3. A and C are basic in nature.
4. B and D react to form hydrogen gas
(a) All are correct
(b) 1 and 2 are correct
(c) 1 and 4 are correct
(d) 1, 2 and 3 are correct.
Answer: (b)

Question. What does the following activity represent?
(a) Production of \( HCl \)
(b) Production of \( H_2SO_4 \)
(c) Production of \( CO_2 \)
(d) Reaction of calcium chloride with sulphuric acid.
Answer: (a)

Balanced chemical equation has an equal number of atoms of different elements in the reactants and products. According to law of conservation of mass, matter can neither be created nor be destroyed in a chemical reaction.

Question. Which of the following equation are balanced?
(a) \( 3Fe(s) + 4H_2O(g) \longrightarrow Fe_3O_4(s) + 4H_2(g) \)
(b) \( 2CaO(s) + H_2O(l) \longrightarrow 2Ca(OH)_2(aq) \)
(c) \( CH_4(g) + 2O_2(g) \longrightarrow CO_2(g) + 2H_2O(g) \)
(d) \( 2AgBr(s) \longrightarrow Ag(s) + 2Br_2(g) \)
Answer: (a)

Question. Which of the following involve an exothermic reaction?
(a) Respiration
(b) Oxidation
(c) Combustion
(d) All of these
Answer: (d)

Question. When the powder of a common metal is heated in an open china dish, its colour turns black. However, when hydrogen is passed over the hot black substance so formed, it regains its original colour. What type of chemical reaction takes place in each of the two given steps?
(a) First oxidation reaction then displacement reaction
(b) First combustion reaction, then reduction reaction
(c) First oxidation reaction, then redox reaction
(d) First reduction reaction, then redox reaction
Answer: (c)

Question. Study the given diagram in which white powder of silver chloride was left open in sunlight. After few hours, it was observed that the white powder changed to silvery material. What type of reaction is it?
(a) Displacement reaction
(b) Decomposition reaction
(c) Redox reaction
(d) Corrosion
Answer: (b)

Question. After rain, Shreya observed that the iron lock of the main gate of her house had a reddish-brown powdery covering. The cleaned the lock using sandpaper and found the shiny, non-corroded layer beneath. What could she do to prevent such reaction to occur?
(a) To buy a new similar lock.
(b) To coat the surface with a layer of paint.
(c) To clean the lock using sandpaper regularly.
(d) To buy a gold lock instead of iron lock.
Answer: (b)

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Question. Rahul has been collecting copper coins and silver coins. One day he observed a green coating on copper coins and a black coating on silver coins. State the chemical phenomenon responsible for these coating and also write chemical names of each coating.
Answer: Corrosion is the phenomenon responsible. Green coating on copper: Basic copper carbonate. Black coating on silver: Silver sulphide.

Question. State the relationship between oxidation and oxidising agent in a redox reaction. Give an example of redox reaction showing the relationship between oxidation and oxidising agent.
Answer: In a redox reaction, the substance that undergoes oxidation acts as a reducing agent, while the substance that undergoes reduction is the oxidising agent. Example: \( CuO + H_2 \longrightarrow Cu + H_2O \). Here, \( CuO \) is the oxidising agent as it is reduced to \( Cu \).

Question. State which of the following chemical reactions will take place or not, giving suitable reason for each :
(a) \( Zn(s) + CuSO_{4(aq)} \longrightarrow ZnSO_{4(aq)} + Cu(s) \)
(b) \( Fe(s) + ZnSO_{4(aq)} \longrightarrow FeSO_{4(aq)} + Zn(s) \)
(c) \( Zn(s) + FeSO_{4(aq)} \longrightarrow ZnSO_{4(aq)} + Fe(s) \)
Answer: (a) Reaction takes place because \( Zn \) is more reactive than \( Cu \). (b) Reaction does not take place because \( Fe \) is less reactive than \( Zn \). (c) Reaction takes place because \( Zn \) is more reactive than \( Fe \).

Question. (a) Define corrosion. What name is given to the corrosion of iron ?
(b) Name the colour of coating formed on silver and copper articles, when exposed to air ?
(c) List two damages caused by corrosion and suggest how corrosion can be prevented.
Answer: (a) Corrosion is the slow degradation of metals due to the action of air and moisture. Corrosion of iron is called rusting. (b) Silver: Black; Copper: Green. (c) Damages: weakening of bridges, destruction of car bodies. Prevention: Painting, galvanisation.

Question. Zinc metal is added to dilute hydrochloric acid. The gas evolved is :
(a) Oxygen
(b) Hydrogen
(c) Chlorine
(d) Carbon dioxide
Answer: (b)

Question. Reverse of the following chemical reaction is not possible :
\( Zn(s) + CuSO_{4(aq)} \longrightarrow ZnSO_{4(aq)} + Cu(s) \)
Justify this statement with reason.
Answer: The reverse reaction \( Cu(s) + ZnSO_{4(aq)} \longrightarrow CuSO_{4(aq)} + Zn(s) \) is not possible because copper is less reactive than zinc and cannot displace zinc from its salt solution.

Question. Name the substance oxidised and the substance reduced, and also identify the oxidising agent and reducing agents in the following reaction :
(a) \( 3MnO_2 + 4Al \longrightarrow 3Mn + 2Al_2O_3 \)
(b) \( Fe_2O_3 + 3CO \longrightarrow 2Fe + 3CO_2 \)
(c) \( SO_2 + 2H_2S \longrightarrow 3S + 2H_2O \)
Answer: (a) Oxidised: \( Al \), Reduced: \( MnO_2 \), Oxidising Agent: \( MnO_2 \), Reducing Agent: \( Al \). (b) Oxidised: \( CO \), Reduced: \( Fe_2O_3 \), Oxidising Agent: \( Fe_2O_3 \), Reducing Agent: \( CO \). (c) Oxidised: \( H_2S \), Reduced: \( SO_2 \), Oxidising Agent: \( SO_2 \), Reducing Agent: \( H_2S \).

Question. While studying the double displacement reaction, the solutions of barium chloride and sodium sulphate are mixed together.
(a) What do you observe as soon as the two solutions are mixed together ?
(b) What will happen in the above observation made by you after ten minutes ?
Answer: (a) A white precipitate of barium sulphate is formed immediately. (b) The precipitate settles down at the bottom of the test tube.

Question. (a) While studying the combination reaction on adding water to quick lime, name the product formed and write its colour.
(b) While studying the decomposition reaction by heating ferrous sulphate crystals in a test-tube a product is formed in the test-tube. Name the product and write its colour.
Answer: (a) Calcium hydroxide (slaked lime), Colour: White. (b) Ferric oxide, Colour: Reddish-brown.

Question. When hydrogen gas is passed over heated copper (II) oxide, copper and steam are formed. Write the balanced chemical equation for this reaction and state (a) the substance oxidized and (b) the substance reduced in the reaction.
Answer: \( CuO + H_2 \longrightarrow Cu + H_2O \). (a) Substance oxidised: \( H_2 \). (b) Substance reduced: \( CuO \).

Question. Select combination reaction, decomposition reaction and displacement reaction from the following chemical equations :
(a) \( ZnCO_3(s) \longrightarrow ZnO(s) + CO_2(g) \)
(b) \( Pb(s) + CuCl_2(aq) \longrightarrow PbCl_2(aq) + Cu(s) \)
(c) \( NaBr(aq) + AgNO_3(aq) \longrightarrow AgBr(s) + NaNO_3(aq) \)
(d) \( H_2(g) + Cl_2(g) \longrightarrow 2HCl(g) \)
(e) \( Fe_2O_3 + 2Al \longrightarrow Al_2O_3 + 2Fe \)
(f) \( 3H_2(g) + N_2(g) \longrightarrow 2NH_3(g) \)
(g) \( CaCO_3(s) \xrightarrow{Heat} CaO(s) + CO_2(g) \)
Answer: Combination: (d), (f); Decomposition: (a), (g); Displacement: (b), (e); Double Displacement: (c).

Question. State the kind of chemical reactions in the following examples :
(a) Digestion of food in stomach
(b) Combustion of coal in air
(c) Heating of limestone
Answer: (a) Decomposition reaction. (b) Combination or oxidation reaction. (c) Thermal decomposition reaction.

Question. (a) Define corrosion.
(b) What is corrosion of iron called ?
(c) How will you recognise the corrosion of silver ?
(d) Why corrosion of iron is a serious problem ?
(e) How can we prevent corrosion ?
Answer: (a) The process of slow eating up of metals by air and moisture. (b) Rusting. (c) Formation of black coating of silver sulphide. (d) It damages iron structures like bridges and ships leading to economic loss. (e) By painting, oiling, galvanisation or electroplating.

Question. Write balanced chemical equations for the following statements :
(a) NaOH solution is heated with zinc granules.
(b) Excess of carbon dioxide gas is passed through lime water.
(c) Dilute sulphuric acid reacts with sodium carbonate.
(d) Egg shells are dropped in hydrochloric acid.
(e) Copper (II) oxide reacts with dilute hydrochloric acid.
Answer: (a) \( 2NaOH + Zn \longrightarrow Na_2ZnO_2 + H_2 \). (b) \( Ca(OH)_2 + CO_2 \longrightarrow CaCO_3 + H_2O \); \( CaCO_3 + H_2O + CO_2 \longrightarrow Ca(HCO_3)_2 \). (c) \( Na_2CO_3 + H_2SO_4 \longrightarrow Na_2SO_4 + H_2O + CO_2 \). (d) \( CaCO_3 + 2HCl \longrightarrow CaCl_2 + H_2O + CO_2 \). (e) \( CuO + 2HCl \longrightarrow CuCl_2 + H_2O \).