---
title: "Chemical Reactions and Equations: CBSE Class 10 Science previous year questions"
url: https://www.swavid.com/cbse/class-10/science/pyq/chemical-reactions-and-equations
---

# Chemical Reactions and Equations: CBSE Class 10 Science previous year questions

14 questions from CBSE Class 10 board papers, newest first, each with full working.

## CBSE Class 10 Science Question Paper 2026 (Set 31/1/1) with Solutions

### Question 17

*1 mark · MCQ*

(i) $\text{AgNO}_3 + \text{NaCl} \longrightarrow \text{NaNO}_3 + \text{AgCl}$
(ii) $\text{K}_2\text{SO}_4 + \text{BaCl}_2 \longrightarrow \text{BaSO}_4 + 2\text{KCl}$
Which of the following options clearly describes both the reactions ?

- (i) is double displacement, (ii) is displacement reaction.
- Both, (i) and (ii) are displacement reactions and precipitation reactions.
- Both, (i) and (ii) are double displacement reactions and precipitation reactions.
- (i) is displacement, (ii) is double displacement reaction.

**Solution**

1. In reaction (i), $\text{AgNO}_3$ and $\text{NaCl}$ exchange ions to form $\text{AgCl}$ precipitate and $\text{NaNO}_3$, making it a double displacement and precipitation reaction.
2. In reaction (ii), $\text{K}_2\text{SO}_4$ and $\text{BaCl}_2$ exchange ions to form a white precipitate of $\text{BaSO}_4$, making it also a double displacement and precipitation reaction.

**Answer:** (c) Both, (i) and (ii) are double displacement reactions and precipitation reactions.

> Common mistake: Confusing double displacement reactions with single displacement reactions.

### Question 19

*1 mark · MCQ*

The gases evolved on heating lead (II) nitrate crystals are :

- NO and $\text{O}_2$
- $\text{N}_2$ and $\text{NO}_2$
- $\text{NO}_2$ and $\text{H}_2$
- $\text{NO}_2$ and $\text{O}_2$

**Solution**

1. When lead (II) nitrate crystals are heated, they undergo thermal decomposition to give lead oxide, nitrogen dioxide, and oxygen gas.
2. The brown fumes evolved are of nitrogen dioxide ($\text{NO}_2$) along with oxygen ($\text{O}_2$) gas.

**Answer:** (d) $\text{NO}_2$ and $\text{O}_2$

> Common mistake: Confusing the gases with nitric oxide or nitrogen gas.

### Question 26

*3 marks · Short answer*

(a) Name the substance oxidised and reduced in the following reaction :
$\text{ZnO} + \text{C} \longrightarrow \text{Zn} + \text{CO}$
(b) Balance the following chemical reaction :
$\text{Pb}(\text{NO}_3)_2 + \text{KI} \longrightarrow \text{PbI}_2 + \text{KNO}_3$
(c) Give one example each of electrolytic decomposition and decomposition by sunlight.

**Part (a)**

1. Carbon (C) is oxidised to carbon monoxide (CO) as it gains oxygen.
2. Zinc oxide (ZnO) is reduced to zinc (Zn) as it loses oxygen.

Answer (a): Substance oxidised: Carbon (C); Substance reduced: Zinc oxide (ZnO).

**Part (b)**

1. Write the unbalanced equation: $\text{Pb}(\text{NO}_3)_2 + \text{KI} \longrightarrow \text{PbI}_2 + \text{KNO}_3$.
2. Balance potassium and nitrate ions by placing coefficient 2 before $\text{KI}$ and $\text{KNO}_3$: $\text{Pb}(\text{NO}_3)_2 + 2\text{KI} \longrightarrow \text{PbI}_2 + 2\text{KNO}_3$.

Answer (b): $\text{Pb}(\text{NO}_3)_2 + 2\text{KI} \longrightarrow \text{PbI}_2 + 2\text{KNO}_3$

**Part (c)**

1. Electrolytic decomposition: $2\text{H}_2\text{O} \xrightarrow{\text{Electricity}} 2\text{H}_2 + \text{O}_2$.
2. Decomposition by sunlight: $2\text{AgCl} \xrightarrow{\text{Sunlight}} 2\text{Ag} + \text{Cl}_2$.

Answer (c): Electrolysis of acidified water and photolytic decomposition of silver chloride.

**Answer:** Substance oxidised is C, reduced is ZnO. Balanced equation: $\text{Pb}(\text{NO}_3)_2 + 2\text{KI} \longrightarrow \text{PbI}_2 + 2\text{KNO}_3$.

> Common mistake: Confusing the substance oxidised with the oxidising agent.

## CBSE Class 10 Science Question Paper 2025 (Set 31/1/1) with Solutions

### Question 1

*1 mark · MCQ*

Electrolysis of water is a decomposition reaction. The mass ratio ($M_H : M_O$) of hydrogen and oxygen gases liberated at the electrodes during electrolysis of water is :

- 8 : 1
- 2 : 1
- 1 : 2
- 1 : 8

**Solution**

1. Water ($H_2O$) decomposes into hydrogen and oxygen gases in the ratio of $2:1$ by volume.
2. Since the molecular formula is $H_2O$, the mass ratio of hydrogen to oxygen is $M_H : M_O = 2 \times 2 : 16 = 4 : 16 = 1 : 8$.
3. Students often confuse volume ratio ($2:1$) with mass ratio ($1:8$), making option (b) a common wrong guess.

**Answer:** (d) 1 : 8

> Common mistake: Confusing the volumetric ratio of gases with their mass ratio.

### Question 17

*1 mark · Assertion and reason*

Assertion (A) : Decomposition reactions are generally endothermic reactions.
Reason (R) : Decomposition of organic matter into compost is an exothermic process.

- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Assertion (A) is false, but Reason (R) is true.

**Solution**

1. Decomposition reactions require energy in the form of heat, light or electricity for breaking down the reactants, making them generally endothermic.
2. The decomposition of organic matter into compost is an exothermic process because it releases heat.
3. Both statements are correct facts from NCERT, but the fact that organic decomposition is exothermic does not explain why general decomposition reactions are endothermic.

**Answer:** Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

> Common mistake: Confusing general inorganic decomposition reactions with biochemical exothermic processes like composting.

### Question 21

*2 marks · Very short answer*

A student performs the following experiment in his school laboratory (dilute sulphuric acid with Zn granules). List two observations to justify that in this experiment a chemical change has taken place.

**Solution**

1. Evolution of hydrogen gas is observed as bubbles formed around zinc granules.
2. Change in temperature is observed as the conical flask becomes warm due to an exothermic reaction.

**Answer:** Evolution of hydrogen gas and change in temperature (or evolution of heat).

> Common mistake: Writing physical changes instead of chemical change characteristics.

## CBSE Class 10 Science Question Paper 2024 (Set 31/1/1) with Solutions

### Question 1

*1 mark · MCQ*

When $2\text{ mL}$ of sodium hydroxide solution is added to few pieces of granulated zinc in a test tube and then warmed, the reaction that occurs can be written in the form of a balanced chemical equation as :

- $\text{NaOH} + \text{Zn} \rightarrow \text{NaZnO}_2 + \text{H}_2\text{O}$
- $2\text{NaOH} + \text{Zn} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2$
- $2\text{NaOH} + \text{Zn} \rightarrow \text{NaZnO}_2 + \text{H}_2$
- $2\text{NaOH} + \text{Zn} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2\text{O}$

**Solution**

1. When sodium hydroxide reacts with zinc metal, it produces sodium zincate and hydrogen gas.
2. The balanced chemical equation is $2\text{NaOH} + \text{Zn} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2$.

**Answer:** (b) $2\text{NaOH} + \text{Zn} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2$

> Common mistake: Confusing sodium zincate formula as $\text{NaZnO}_2$ instead of $\text{Na}_2\text{ZnO}_2$.

### Question 2

*1 mark · MCQ*

Select from the following a decomposition reaction in which source of energy for decomposition is light :

- $2\text{FeSO}_4 \rightarrow \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$
- $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$
- $2\text{AgBr} \rightarrow 2\text{Ag} + \text{Br}_2$
- $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$

**Solution**

1. Decomposition reactions that require light energy are called photolytic decomposition reactions.
2. Silver bromide decomposes into silver and bromine in the presence of sunlight: $2\text{AgBr} \rightarrow 2\text{Ag} + \text{Br}_2$.

**Answer:** (c) $2\text{AgBr} \rightarrow 2\text{Ag} + \text{Br}_2$

> Common mistake: Choosing water electrolysis which uses electrical energy instead of light.

### Question 7

*1 mark · MCQ*

$\text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$
The reaction given above is a redox reaction because in this case :

- $\text{MnO}_2$ is oxidised and $\text{HCl}$ is reduced.
- $\text{HCl}$ is oxidised.
- $\text{MnO}_2$ is reduced.
- $\text{MnO}_2$ is reduced and $\text{HCl}$ is oxidised.

**Solution**

1. In the given reaction, $\text{MnO}_2$ loses oxygen to form $\text{MnCl}_2$, so it is reduced.
2. $\text{HCl}$ loses hydrogen to form $\text{Cl}_2$ (or is oxidized), so $\text{HCl}$ is oxidized.

**Answer:** (d) $\text{MnO}_2$ is reduced and $\text{HCl}$ is oxidised.

> Common mistake: Confusing oxidation and reduction in terms of addition/removal of oxygen and hydrogen.

### Question 21

*2 marks · Very short answer*

Name the type of chemical reaction in which calcium oxide reacts with water. Justify your answer by giving balanced chemical equation for the chemical reaction.

**Solution**

1. The reaction between calcium oxide and water is a combination reaction because two or more reactants combine to form a single product.
2. The balanced chemical equation is: $\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat}$

**Answer:** Combination reaction with $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$

> Common mistake: Forgetting to mention the release of heat or failing to balance the chemical equation.

### Question 27

*3 marks · Short answer*

Write one chemical equation each for the chemical reaction in which the following have taken place :
(i) Change in colour
(ii) Change in temperature
(iii) Formation of precipitate
Mention colour change/temperature change (rise/fall)/compound precipitated along with equation.

**Part (i)**

1. Reaction: $\text{Fe} (s) + \text{CuSO}_4 (aq) \to \text{FeSO}_4 (aq) + \text{Cu} (s)$
2. Colour change: The blue colour of copper sulphate solution fades and a reddish-brown coating of copper is deposited on iron.

Answer (i): $\text{Fe} + \text{CuSO}_4 \to \text{FeSO}_4 + \text{Cu}$, blue colour changes to light green with reddish-brown deposit.

**Part (ii)**

1. Reaction: $\text{Zn} (s) + \text{H}_2\text{SO}_4 (aq) \to \text{ZnSO}_4 (aq) + \text{H}_2 (g)$
2. Temperature change: There is a rise in temperature as it is an exothermic reaction.

Answer (ii): $\text{Zn} + \text{H}_2\text{SO}_4 \to \text{ZnSO}_4 + \text{H}_2$, rise in temperature.

**Part (iii)**

1. Reaction: $\text{BaCl}_2 (aq) + \text{Na}_2\text{SO}_4 (aq) \to \text{BaSO}_4 (s) + 2\text{NaCl} (aq)$
2. Precipitate formed: A white precipitate of barium sulphate is formed.

Answer (iii): $\text{BaCl}_2 + \text{Na}_2\text{SO}_4 \to \text{BaSO}_4 \downarrow + 2\text{NaCl}$, white precipitate of barium sulphate.

**Answer:** Chemical equations representing change in colour, change in temperature, and formation of precipitate are given in the respective parts.

> Common mistake: Forgetting to write physical states or failing to mention the specific observation such as colour change or temperature rise.

## CBSE Class 10 Science Question Paper 2023 (Set 31/1/1) with Solutions

### Question 2

*1 mark · MCQ*

The emission of brown fumes in the given experimental set-up is due to

[Diagram showing thermal decomposition of lead nitrate producing brown fumes]

- thermal decomposition of lead nitrate which produces brown fumes of nitrogen dioxide.
- thermal decomposition of lead nitrate which produces brown fumes of lead oxide.
- oxidation of lead nitrate forming lead oxide and nitrogen dioxide.
- oxidation of lead nitrate forming lead oxide and oxygen.

**Solution**

1. On heating lead nitrate, it undergoes thermal decomposition to form lead oxide, oxygen gas, and brown fumes of nitrogen dioxide.

**Answer:** (a) thermal decomposition of lead nitrate which produces brown fumes of nitrogen dioxide.

> Common mistake: Mistaking thermal decomposition for simple oxidation or confusing nitrogen dioxide fumes with lead oxide.

### Question 3

*1 mark · MCQ*

$\text{MnO}_2 + x\text{HCl} \rightarrow \text{MnCl}_2 + y\text{H}_2\text{O} + z\text{Cl}_2$
In order to balance the above chemical equation, the values of $x, y$ and $z$ respectively are :

- 6, 2, 2
- 4, 1, 2
- 4, 2, 1
- 2, 2, 1

**Solution**

1. The given equation is $\text{MnO}_2 + x\text{HCl} \rightarrow \text{MnCl}_2 + y\text{H}_2\text{O} + z\text{Cl}_2$.
2. Balancing oxygen and hydrogen gives 4 $\text{HCl}$ and 2 $\text{H}_2\text{O}$, and balancing chlorine gives 1 $\text{Cl}_2$, so $x=4, y=2, z=1$.

**Answer:** (c) 4, 2, 1

> Common mistake: Incorrectly balancing chlorine atoms on both sides of the reaction.

### Question 27

*3 marks · Short answer*

(a) Identify the reducing agent in the following reactions :
(i) $4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}$
(ii) $\text{H}_2\text{O} + \text{F}_2 \rightarrow \text{HF} + \text{HOF}$
(iii) $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$
(iv) $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$
(b) Define a redox reaction in terms of gain or loss of oxygen.

**Part (a)(i)**

1. Ammonia ($\text{NH}_3$) loses hydrogen and gets oxidized to NO, so it acts as the reducing agent.

Answer (a)(i): $\text{NH}_3$

**Part (a)(ii)**

1. Water ($\text{H}_2\text{O}$) loses hydrogen to form HOF, so it acts as the reducing agent.

Answer (a)(ii): $\text{H}_2\text{O}$

**Part (a)(iii)**

1. Carbon monoxide ($\text{CO}$) gains oxygen to form $\text{CO}_2$, so it acts as the reducing agent.

Answer (a)(iii): $\text{CO}$

**Part (a)(iv)**

1. Hydrogen ($\text{H}_2$) gains oxygen to form $\text{H}_2\text{O}$, so it acts as the reducing agent.

Answer (a)(iv): $\text{H}_2$

**Part (b)**

1. A reaction in which one reactant gains oxygen (gets oxidized) while another reactant loses oxygen (gets reduced) is called a redox reaction.

Answer (b): A reaction involving gain of oxygen by one substance and loss of oxygen by another is called a redox reaction.

**Answer:** (a) Identifying reducing agents in the given reactions, and (b) defining redox reaction in terms of gain or loss of oxygen.

> Common mistake: Confusing oxidizing agent with reducing agent.

## Related pages

- [Chemical Reactions and Equations: NCERT solutions](https://www.swavid.com/science/class/10/chapter/chemical-reactions-and-equations/ncert-solutions)
- [All CBSE Class 10 Science papers](https://www.swavid.com/cbse/class-10/science/previous-year-papers)

Solutions written by SwaVid, a personal AI tutor for Class 6 to 10 Maths and Science. Practise this chapter free: https://www.swavid.com/start/student