NCERT Class 10 Science – Wide View Edition

NCERT CLASS 10 SCIENCE • CHAPTER 1 • WIDE VIEW EDITION • SWIPE EQUATIONS TO READ • PREPARED FOR 2025-26 EXAMS

Chemical Reactions

Detailed Solutions Q1 – Q20

Which of the statements about the reaction below are incorrect?

$$2\text{PbO}_{(s)} + \text{C}_{(s)} \rightarrow 2\text{Pb}_{(s)} + \text{CO}_{2(g)}$$

(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidized.
(c) Carbon is getting oxidized.
(d) Lead oxide is getting reduced.

Detailed Explanation:

1. Oxidation: Addition of oxygen. Here, Carbon (C) gains oxygen to form CO₂. So, Carbon is oxidized. (Statement ‘c’ is correct).

2. Reduction: Removal of oxygen. Here, Lead Oxide (PbO) loses oxygen to form Lead (Pb). So, Lead Oxide is reduced. (Statement ‘d’ is correct).

Therefore, incorrect statements are (a) and (b).

$$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$$

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

Reason: Aluminum (Al) is more reactive than Iron (Fe). Therefore, Aluminum displaces Iron from its oxide ($\text{Fe}_2\text{O}_3$). This type of reaction where a more reactive element removes a less reactive element is called a displacement reaction.

What happens when dilute hydrochloric acid is added to iron filings? 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 and iron chloride are produced.

↔ Swipe Equation

$$\text{Fe}(s) + 2\text{HCl}(aq) \rightarrow \text{FeCl}_2(aq) + \text{H}_2(g)$$

Metals generally react with acids to displace Hydrogen gas and form a corresponding salt (Iron Chloride in this case).

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

Definition: An equation in which the number of atoms of each element remains the same on both the reactant (left) and product (right) sides is called a balanced chemical equation.

Reason: We must balance equations to satisfy the Law of Conservation of Mass. This law states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of reactants must equal the total mass of products.

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

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

$$3\text{H}_2(g) + \text{N}_2(g) \rightarrow 2\text{NH}_3(g)$$

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

$$2\text{H}_2\text{S}(g) + 3\text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l) + 2\text{SO}_2(g)$$

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

$$3\text{BaCl}_2(aq) + \text{Al}_2(\text{SO}_4)_3(aq) \rightarrow 2\text{AlCl}_3(aq) + 3\text{BaSO}_4(s)$$

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

$$2\text{K}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{KOH}(aq) + \text{H}_2(g)$$

Balance the following chemical equations:

$\text{(a) HNO}_3 + \text{Ca(OH)}_2 \rightarrow \text{Ca(NO}_3)_2 + \text{H}_2\text{O}$

Balanced:

$$2\text{HNO}_3 + \text{Ca(OH)}_2 \rightarrow \text{Ca(NO}_3)_2 + 2\text{H}_2\text{O}$$

$\text{(b) NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}$

Balanced:

$$2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$$

$\text{(c) NaCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{NaNO}_3$

Balanced: (Already balanced)

$$\text{NaCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{NaNO}_3$$

$\text{(d) BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + \text{HCl}$

Balanced:

$$\text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{HCl}$$

Write the balanced chemical equations for the following reactions:

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

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$

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

$$\text{Zn} + 2\text{AgNO}_3 \rightarrow \text{Zn(NO}_3)_2 + 2\text{Ag}$$

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

$$2\text{Al} + 3\text{CuCl}_2 \rightarrow 2\text{AlCl}_3 + 3\text{Cu}$$

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

$$\text{BaCl}_2 + \text{K}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{KCl}$$

Write the balanced chemical equation for the following and identify the type of reaction.

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

$$2\text{KBr} + \text{BaI}_2 \rightarrow 2\text{KI} + \text{BaBr}_2$$

Type: Double Displacement Reaction

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

$$\text{ZnCO}_3(s) \rightarrow \text{ZnO}(s) + \text{CO}_2(g)$$

Type: Decomposition Reaction

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

$$\text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2\text{HCl}(g)$$

Type: Combination Reaction

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

$$\text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g)$$

Type: Displacement Reaction

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

Exothermic Reactions:
Reactions in which heat is released along with the formation of products are called exothermic reactions.

Example (Burning of Natural Gas):

$$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) + \text{Heat}$$

Endothermic Reactions:
Reactions which require energy in the form of heat, light or electricity to break reactants are called endothermic reactions.

Example (Decomposition of Silver Chloride):

$$2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)$$

Q10

Why is respiration considered an exothermic reaction? Explain.

We need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, rice and potatoes contain carbohydrates which are broken down to form glucose.

This glucose combines with oxygen in the cells of our body and provides energy. Since energy is released in this process, respiration is considered an exothermic reaction.

↔ Swipe

$$\text{C}_6\text{H}_{12}\text{O}_6(aq) + 6\text{O}_2(g) \rightarrow 6\text{CO}_2(aq) + 6\text{H}_2\text{O}(l) + \text{Energy}$$

Q11

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

In a Combination Reaction, two or more substances combine to form a single new substance.

$$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$$

In a Decomposition Reaction, a single substance breaks down into two or more simpler substances.

$$2\text{H}_2\text{O} \xrightarrow{\text{Elec.}} 2\text{H}_2 + \text{O}_2$$

Since the processes are exactly reverse (joining vs breaking), they are called opposite reactions.

Q12

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

(i) By Heat (Thermal Decomposition):

$$\text{CaCO}_3(s) \xrightarrow{\text{Heat}} \text{CaO}(s) + \text{CO}_2(g)$$

(ii) By Light (Photolytic Decomposition):

$$2\text{AgBr}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Br}_2(g)$$

(iii) By Electricity (Electrolytic Decomposition):

$$2\text{H}_2\text{O}(l) \xrightarrow{\text{Electric Current}} 2\text{H}_2(g) + \text{O}_2(g)$$

Q13

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

Displacement Reaction: A more reactive element displaces a less reactive element from its compound.

$$\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Cu}(s)$$

Double Displacement Reaction: There is an exchange of ions between the reactants to form new compounds.

$$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq)$$

Q14

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

Copper is more reactive than Silver. When Copper is added to Silver Nitrate solution, it displaces silver.

$$2\text{AgNO}_3(aq) + \text{Cu}(s) \rightarrow \text{Cu(NO}_3)_2(aq) + 2\text{Ag}(s)$$

Q15

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

Any reaction that produces an insoluble solid (known as a precipitate) which separates from the solution is called a precipitation reaction.

Example: Mixing Sodium Sulphate and Barium Chloride.

$$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(\downarrow) + 2\text{NaCl}(aq)$$

The white precipitate formed is Barium Sulphate ($\text{BaSO}_4$).

[Image of precipitation reaction example]

Q16

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

(a) Oxidation: It is defined as the gain of oxygen by a substance.

1. $\text{2Cu} + \text{O}_2 \rightarrow \text{2CuO}$ (Copper gains Oxygen)

2. $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$ (Carbon gains Oxygen)

(b) Reduction: It is defined as the loss of oxygen from a substance.

1. $\text{CuO} + \text{H}_2 \xrightarrow{\text{Heat}} \text{Cu} + \text{H}_2\text{O}$ (Copper oxide loses Oxygen)

2. $\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$ (Zinc oxide loses Oxygen)

Q17

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

1. The shiny brown element ‘X’ is Copper (Cu).

2. The black coloured compound formed is Copper Oxide (CuO).

Reaction:

$$2\text{Cu}(s) + \text{O}_2(g) \xrightarrow{\text{Heat}} 2\text{CuO}(s)$$

Q18

Why do we apply paint on iron articles?

Iron articles rust when exposed to moist air (Oxygen + Water vapor). Painting the surface covers the iron and cuts off its direct contact with the air and moisture. This prevents the oxidation of iron, thus preventing rusting (corrosion).

Q19

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

When fats and oils are exposed to air for a long time, they get oxidized and become rancid (their smell and taste change). Nitrogen is an inert gas (unreactive). Flushing food packets (like chips) with nitrogen removes oxygen and prevents oxidation, keeping the food fresh for longer.

Q20

Explain the following terms with one example each:
(a) Corrosion
(b) Rancidity

(a) Corrosion:
The process where metals are slowly eaten away by the reaction with air, water, or acids present in the atmosphere.
Example: Rusting of Iron (formation of reddish-brown coating).

(b) Rancidity:
The condition produced by the aerial oxidation of fats and oils in foods marked by unpleasant smell and taste.
Example: The bad taste of potato chips left open for days.