NCERT Solution Class 10 Science Chapter-1 Chemical reaction and equation



1. Why should a magnesium ribbon be cleaned before burning in air?

  1. A magnesium ribbon should be cleaned before burning in air to remove any oxide layer that may have formed on its surface, which could interfere with the reaction and affect the accuracy of the results.

2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate +
Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Ans : (i) H2 + Cl2 → 2HCl

(ii) BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3

(iii) 2Na + 2H2O → 2NaOH + H2
3. Write a balanced chemical equation with state symbols for the
following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react
to give insoluble barium sulphate and the solution of sodium
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric
acid solution (in water) to produce sodium chloride solution and

Ans : (i) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)

(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)



1. 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

Ans :

(i) The substance ‘X’ used for white washing is calcium oxide (CaO). (ii) The reaction of calcium oxide with water is as follows: CaO + H2O → Ca(OH)2

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas

Ans :

In Activity 1.7, hydrogen gas and oxygen gas are collected separately by the process of electrolysis of water. The amount of hydrogen gas collected is double the amount of oxygen gas collected because the ratio of hydrogen to oxygen in water is 2:1. Therefore, when water is electrolysed, twice the amount of hydrogen gas is produced as compared to oxygen gas. The gas collected in one of the test tubes is hydrogen gas.

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1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

  1. When an iron nail is dipped into a copper sulphate solution, the iron nail displaces the copper ions in the solution, forming iron sulphate and metallic copper. The displaced copper ions form a reddish-brown solid on the surface of the nail, while the blue colour of the copper sulphate solution fades. This happens because copper is less reactive than iron, so iron can displace copper from its compound.

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

One example of a double displacement reaction is the reaction between barium chloride (BaCl2) and sodium sulphate (Na2SO4) to form barium sulphate (BaSO4) and sodium chloride (NaCl):

BaCl2 + Na2SO4 → BaSO4 + 2NaCl

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2 (g) → Cu(s) + H2O(l)

Ans :

(i) In the reaction 4Na(s) + O2(g) → 2Na2O(s), sodium (Na) is oxidised because it gains oxygen (O) and oxygen is more electronegative than sodium. Oxygen is reduced because it gains electrons from sodium to form oxide ions.

(ii) In the reaction CuO(s) + H2(g) → Cu(s) + H2O(l), copper oxide (CuO) is reduced because it gains hydrogen (H) to form metallic copper (Cu) and water (H2O). Hydrogen is oxidised because it loses electrons to form water molecules.


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

Ans : The incorrect statements about the given reaction are (c) Carbon is getting oxidised, and (b) Carbon dioxide is getting oxidised. The correct statements are (a) Lead is getting reduced and (d) Lead oxide is getting reduced.

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.

Ans : (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.

Ans : a) Hydrogen gas and iron chloride are produced.
4. What is a balanced chemical equation? Why should chemical equations be balanced?

Ans : A balanced chemical equation represents a chemical reaction with the same number of atoms of each element on both the reactant and product side. Chemical equations should be balanced because of the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction, but can only be rearranged.

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 sulpur 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.

Ans : (a) N2(g) + 3H2(g) → 2NH3(g)

(b) H2S(g) + 3O2(g) → 2H2O(g) + 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

Ans : (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


7. (a) Ca(OH)2 + CO2 → CaCO3 + H2O

(b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag

(c) 2Al + 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) 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)

8. (a) 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s) (Double displacement reaction)

(b) ZnCO3(s) → ZnO(s) + CO2(g) (Decomposition reaction)

(c) H2(g) + Cl2(g) → 2HCl(g) (Combination reaction)

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

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

Ans :Exothermic reactions are those in which energy is released in the form of heat, light, or sound. Examples include combustion reactions, such as burning wood or paper, and neutralisation reactions, such as the reaction between an acid and a base to form water and salt. Endothermic reactions are those in which energy is absorbed from the surroundings, and the temperature of the reaction mixture decreases. An example is the reaction between baking soda and vinegar.

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

Ans :  Respiration is considered an exothermic reaction because it releases energy in the form of heat and ATP. In respiration, glucose is broken down into carbon dioxide and water, releasing energy that is used by cells.


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

Ans :Combination reactions involve the joining of two or more substances to form a single substance, whereas decomposition reactions involve the breakdown of a single substance into two or more substances. The reactants and products are reversed in these two types of reactions. For example, the combination reaction of hydrogen and oxygen to form water is the opposite of the decomposition reaction of water into hydrogen and oxygen.

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