Class 10 Science | Chapter-end Exercise & Answers | Chapter 1 | Chemical Reactions and Equations

1.   Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) 2Pb(s) + CO2(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)

2.   Fe2O3 + 2Al Al2O3 + 2Fe
The above reaction is an example of a

(a)   combination reaction.

(b)   double displacement reaction.   

(c)   decomposition reaction.

(d)   displacement reaction.

Answer: (d) displacement reaction.

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 & iron chloride are produced.

4.   What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: It is an equation which has an equal number of atoms of all the elements on both sides of the chemical equation. Chemical reaction should be balanced to follow law of conservation of mass.

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.       


(a)   3H2 (g) + N2 (g) 2NH3 (g)

(b)   2H2S (g) + 3O2 (g) 2H2O (l) + 2SO2 (g)

(c)   3BaCl2 (aq) + Al2(SO4)3 (aq) 2AlCl3 (aq) + 3BaSO4 (s)

(d)   2K (s) + 2H2O (l) 2KOH (aq) + H2 (g)

6.   Balance the following chemical equations.       

(a)   HNO3+Ca(OH)2 Ca(NO3)2 + H2O

(b)   NaOH + H2SO4 Na2SO4 + H2O

(c)   NaCl + AgNO3 AgCl + NaNO3

(d)   BaCl2 + H2SO4 BaSO4 + HCl


(a)    2HNO3 + Ca(OH)2  Ca(NO3)2 + 2H2O

(b)   2NaOH + H2SO4  Na2SO4 + 2H2O

(c)    NaCl + AgNO3  AgCl + NaNO3

(d)   BaCl2 + H2SO4  BaSO4 + 2HCl

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


(a)   Ca(OH)2 + CO2  CaCO3 + H2O

(b)   Zn + 2AgNO3  Zn(NO3)2 + 2Ag

(c)   Al + 3CuCl2  2AlCl3 + 3Cu

(d)   BaCl2+ K2SO4  BaSO4 + 2KCl

8.   Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide(aq) + Barium iodide(aq) Potassium iodide(aq) +Barium bromide(s)

(b) Magnesium(s) + Hydrochloric acid(aq) Magnesium chloride(aq) + Hydrogen(g)

(c) Zinc carbonate(s) Zinc oxide(s) + Carbon dioxide(g)

(d) Hydrogen(g) + Chlorine(g) Hydrogen chloride(g)


(a) 2KBr (aq) + BaI2 (aq) 2KI (aq) + BaBr2 (s).

It is Double displacement reaction.

(b) ZnCO3 (s) ZnO (s) + CO2 (g)

It is Decomposition reaction.

(c) H2 (g) + Cl2 (g) 2HCl (g)

It is Combination reaction.

(d) Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

It is Displacement Reaction.

9.   What does one mean by exothermic and endothermic reactions? Give examples.


Exothermic reactions: Chemical reactions that release energy in the form of heat, light, or sound. E.g.

C (g) + O2 (g) CO2 + Heat

Endothermic reactions: Reactions that absorb energy or require energy in order to proceed. E.g.

N2 (g) + O2 (g) + Heat  2NO

10. Why is respiration considered an exothermic reaction? Explain.

Answer: In respiration, oxidation of glucose takes place which produces large amount of heat energy.

C6H12O6 (aq) + 6O2 (g) 6CO2 (g) + 6H2O (l) + Energy

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.


In decomposition reactions, a compound breaks down to form two or more substances. These reactions need energy to proceed. In combination reactions, two or more substances combine to give a new substance with the release of energy.

Decomposition Reaction: E.g.

CaCO3 (s) + Heat  CaO (s) + CO2 (g)

Combination Reaction: E.g.

CaO (s) + H2O (l) Ca(OH)2 (aq)

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.


CaCO3 (s) + Heat  CaO (s) + CO2 (g)

2AgCl (s) + Sunlight  2AgS (s) + Cl2 (g)

2H2O (l) + Electricity  2H2 (g) + O2 (g)

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer: In a displacement reaction, a more reactive element replaces a less reactive element from a compound. E.g.

CuSo4 (aq) + Zn (s) ZnSO4 (aq) + Cu (s)

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds. E.g.

Na2SO4 (aq) + BaCl2 (aq) BaSO4(s) + 2NaCl (aq)

14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.


2AgNO3 (aq) + Cu (s) Cu(NO3)2 (aq) + 2Ag (s)

Silver Nitrate + Copper Copper Nitrate + Silver

15.  What do you mean by a precipitation reaction? Explain by giving examples.


It is a reaction in which an insoluble solid (precipitate) is formed. E.g.

Na2CO3 (aq) + CaCl2 (aq) CaCO3 (s) + 2NaCl (aq)

Here, calcium carbonate is a precipitate.

16.  Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation                     

(b) Reduction


(a) Oxidation: It is a chemical reaction in which gain of oxygen or loss of hydrogen takes place. E.g.

2Mg (s) + O2 (g) + Heat  2MgO (s)

2Cu (s) + O2 (g) + Heat  2CuO (s)

(b) Reduction: It is a chemical reaction in which loss of oxygen or gain of hydrogen takes place. E.g.

CuO + H2 + Heat  Cu + H2O

ZnO + C + Heat  Zn + CO

17.  A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer: X is copper and black-coloured compound is copper oxide (CuO).

18.  Why do we apply paint on iron articles?

Answer: Iron articles are painted to prevent rusting. When painted, the contact of iron articles from moisture and air is cut off.

19.  Oil and fat containing food items are flushed with nitrogen. Why?

Answer: Nitrogen acts as an antioxidant. It prevents oxidation of food items.

20.  Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity


(a) Corrosion: A process where materials, usually metals, deteriorate due to a chemical reaction with air, moisture, chemicals, etc. E.g. Rusting of iron.

4Fe + 3O2 + nH2O 2Fe2O3.2H2O

(b) Rancidity: A process of oxidation of fats and oils resulting in the change in taste and smell.

E.g. taste and smell of butter changes when kept for long.

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