Class 10 Science

Chemical Reactions and Equations NCERT Exercise Solution

Part 1

Question 1: Which of the statements about the reaction below are incorrect?

2PbO (s) + C (s) ⇨ 2PbS (s) + CO2 (g)

  1. Lead is getting reduced.
  2. Carbon dioxide is getting oxidised.
  3. Carbon is getting oxidised.
  4. Lead oxide is getting reduced.
  1. a and b
  2. a and c
  3. a, b and c
  4. All

Answer: (1) a and b


Here, Pb is not present among reactants, so option a is incorrect. Similarly, option b is incorrect because carbon dioxie is not present among reactants. In this reaction, PbO (Lead oxide) is getting reduced and carbon is getting oxidised, because oxygen is removed from PbO and added to carbon.

Question 2: The following reaction is an example of:

Fe2O3 + 2Al ⇨ Al2O3 + 2Fe

  1. Combination reaction
  2. Double displacement reaction
  3. Decomposition reaction
  4. Displacement reaction

Answer: (d) Displacement reaction


Here, Aluminium is dispacing iron from iron oxide. So, it is an example of displacement reaction.

Question 3: What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

  1. Hydrogen gas and iron chloride are produced
  2. Chlorine gas and iron hydroxide are produced
  3. No reaction takes place
  4. Iron salt and water are produced

Answer: (a) Hydrogen gas and iron chloride are produced


You have read that when a dilute acid reacts with metal then metal salt and hydrogen gas are formed.

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

Answer: When the number of atoms of reactants is equal to the number of atoms of products, the reaction is called a balanced chemical equation.

According to the Law of conservation of mass, we cannot create or destroy mass.

So, the total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That's why a chemical equations should always be balanced.

Question 5: Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

Answer: 3H2 + N2 ⇨ 2NH3

(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.

Answer: 2H2S + 3O2 ⇨ 2H2O + 2SO2

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

Answer: 3BaCl2 + Al2(SO4 )3 ⇨ 2AlCl3 + 3BaSO4

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer: 3K + 2H2O ⇨ 2KOH + H2

Question 6: Balance the following chemical equations.

(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O

Answer: 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

Note: Just write 2 before HNO3 and H2O

(b) NaOH + H2SO4 → Na2SO4 + H2O

Answer: 2NaOH + H2SO4 → Na2SO4 + 2H2O

Note: Just write 2 before NaOH and H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

Answer: NaCl + AgNO3 → AgCl + NaNO3

Note: This is balanced equation, so there is no need to change anything.

(d) BaCl2 + H2SO4 → BaSO4 + HCl

Answer: BaCl2 + H2SO4 → BaSO4 + 2HCl

Note: Just write 2 before HCl

Question 7: Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Answer: Ca(OH)2 + CO2 → CaCO3 + H2O

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

Answer: Zn + 2AgNO3 → Zn(NO3)2 + 2Ag

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

Answer: 2Al + 3CuCl2 → 2AlCl2 + 3Cu

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer: BaCl2 + K2SO4 → BaSO4 + 2KCl

Question 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)

Answer: 2KBr (aq) + BaI2 (aq) ⇨ 2KI (aq) + BaBr2 (s)

It is a double displacement reaction.


Here, K and Ba are displacing each other from their respective compounds. So, it is a double displacement reaction.

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

Answer: ZnCO3 (s) ⇨ ZnO (s) + CO2 (g)

It is a decomposition reaction.


Here, a given molecule is giving 2 different molecules after reaction. In other words, the reactant is breaking up or decomposing into two products.

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

Answer: H2 (g) + Cl2 (g) ⇨ 2HCl

It is a combination reaction.


Here, 2 reactants are combining to give one product.

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Answer: Mg (s) + 2HCl (aq) ⇨ MgCl2 (aq) + H2 (g)

It is a displacement reaction.


Here, Mg is displacing hydrogen from HCl.

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

Answer: Exothermic reaction: A reaction in which heat is released is called the exothermic reaction.

Example: Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water.

CH4 (g) + 2O2 (g)⇨ CO2 (g) + 2H2O

Endothermic reaction: A reaction in which heat is absorbed is called endothermic reaction.

Example: When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal.

2AgCl (s) ⇨ 2Ag (s) + Cl2 (g)

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

Answer: In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction.

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