1 markMCQSection A

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

Chemical Reactions and Equations

Types of Chemical Reactions

Options

(a)

2\mathrm{FeSO}_4 \rightarrow \mathrm{Fe}_2\mathrm{O}_3 + \mathrm{SO}_2 + \mathrm{SO}_3

(b)

2\mathrm{H}_2\mathrm{O} \rightarrow 2\mathrm{H}_2 + \mathrm{O}_2

(c)

2\mathrm{AgBr} \rightarrow 2\mathrm{Ag} + \mathrm{Br}_2

(d)

\mathrm{CaCO}_3 \rightarrow \mathrm{CaO} + \mathrm{CO}_2

Official Answer

The decomposition reaction that uses light as the source of energy is $2\mathrm{AgBr} \rightarrow 2\mathrm{Ag} + \mathrm{Br}_2$ .

PhotolysisSilver bromideSunlightDecomposition

Marking Scheme

11 mark for identifying the photolytic decomposition reaction (option c).

Hint

Look for the reaction involving silver salts, which are famously sensitive to sunlight.

Quick Oral Answer

Photolytic decomposition is a reaction where a single substance breaks down into two or more substances in the presence of light. An example is the turning of white silver chloride to grey in sunlight.

Analysis & Explanation

Decomposition reactions require energy in the form of heat, light, or electricity to break down reactants.

Option (c) is correct because the decomposition of silver bromide ( $\mathrm{AgBr}$ ) into silver and bromine is triggered by sunlight (photolysis), as described in Activity 1.8 of the NCERT text.
Option (a) is a thermal decomposition reaction of ferrous sulphate which requires heat (Activity 1.5).
Option (b) is an electrolytic decomposition of water which requires electricity (Activity 1.7).
Option (d) is the thermal decomposition of calcium carbonate (limestone) into quick lime, which requires heat (Equation 1.20).

Common Mistakes

1Confusing thermal decomposition (heat) with photolytic decomposition (light).
2Selecting the electrolysis of water as a light-based reaction.

Interesting Facts

The decomposition of silver halides (like $\mathrm{AgCl}$ and $\mathrm{AgBr}$ ) by light was the fundamental principle behind black and white photography.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2024?

This question carries 1 mark in the CBSE Class 10 Science 2024 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Types of Chemical Reactions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2024 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2024 paper.