# NCERT Solutions for Class 10 Science Chapter 1 Chemical reactions and equations

Contents

## chemical reactions and equations solutions

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

#### 2 PbO(s) + C(s) → 2Pb (s) + CO2(g)

(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. (i) (a) and (b)

### Q 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.

### Q 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.

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

Ans.

Balanced chemical equation :

When the total number of atoms on the reactant side and the product side is equal, the equation is called a balanced equation.

Need for balancing a chemical equation :

A chemical equation needs to be balanced because a chemical reaction is just a rearrangement of atoms.Atoms themselves are neither created nor destroyed during the course of a chemical reaction.The chemical equation needs to be balanced to follow the law of conservation of mass.

### Q 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) 3H2 (g) + N2 (g) → 2NH3 (g)
(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

### Q 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

### Q 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

Ans. (a) Ca (OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag
(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

### Q 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)

Ans.

(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)
Type : Double displacement reaction

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

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

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

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

Ans.

Exothermic reactions :

Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.
Example :
(i) C (s) + O2 (g) → CO2 (g) + Heat
(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat

Endothermic reactions :

Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. These reactions lower the temperature of their surrounding area, thereby creating a cooling effect.
Examples :
(i) C (s) + 2S (s) → CS2 (l) – Heat
(ii) N2 (g) + O2 (g) → 2NO(g) – Heat

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

Ans. Respiration is considered as exothermic reaction because energy is released in this process. Glucose combines with oxygen present in our cells to form carbon dioxide and water along with energy.

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

Ans. In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.

Decomposition reaction CaCO3 (s) ⇨ CaO (s) + CO2 (g)

Combination reaction H2 (g) + Cl2 ⇨ 2HCl (g)

Ans.

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

Ans.

#### Displacement Reaction Examples :

Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)

Cu(s) + 2AgNo3(aq) → 2Ag(s) + Cu(NO3)2(aq)

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

Ans.  In the refining of silver the following chemical reaction is involved. The silver nitrate is in solution and the metallic copper will dissolve to form copper nitrate; as it does, it will precipitate the silver in solution as metallic silver.

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

Ans. When two reactants in solution react and one or more of the products is insoluble or forms a precipitate, the reaction is called a precipitation reaction. For example, when a solution of iron chloride and ammonium hydroxide are mixed, a brown precipitate of iron hydroxide is formed.

Example :

### Q 16. Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction

Ans. It is a chemical reaction in which gain of oxygen or loss of hydrogen takes place. For example in the first reaction copper is oxidised to become copper oxide and in second Magnesium is oxidised to become Magnesium Oxide.

Reduction Reaction: It is a chemical reaction in which loss of oxygen or gain of hydrogen takes place. For example in the first reaction copper oxide is reduced to become copper and in second Zinc Oxide is reduced to become Zinc.

### Q 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.

Ans. Copper is the brown colored element X which on heating in air becomes black colored compound copper oxide.

### Q 18. Why do we apply paint on iron articles?

Ans. When iron is exposed to the environmental condition it terribly reacts with water and air and ends up with iron rusting. Iron articles are therefore painted off so that iron can not react with water and air. This is the way to avoid a reaction which would concurrently stop rusting of iron.

### Q 19. Oil and fat containing food items are flushed with nitrogen. Why? 20. Explain the following terms with one example each. (a) Corrosion (b) Rancidity

Ans. Oil and fat containing food items flushed with nitrogen because nitrogen acts as an antioxidant and it prevent them from being oxidised. 20.

Chemical Reactions and Equations Class 10 Notes

NCERT Solutions for Class 10 Science Chapter 1 Chemical reactions and equations

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