(a) (i) Define the term decomposition reaction. Write one chemical equation each for decomposition reaction where energy is supplied in the form of heat, light or electricity.

(ii) Decomposition of vegetable matter into compost is considered an exothermic reaction. Why?

OR

(b) Why are decomposition reactions called the opposite of combination reactions? Write one chemical equation each for these two types of reactions mentioning the name of the reactant(s) and the product(s) involved in the reactions.

This is an internal-choice 3-mark question testing the definition and examples of decomposition reactions, and the complementary relationship between combination and decomposition reactions, both grounded directly in Section 1.2.1 and 1.2.2 of the NCERT chapter.

In part (a)(i), the chapter explicitly states: 'In this reaction you can observe that a single reactant breaks down to give simpler products. This is a decomposition reaction,' and later: 'We have seen that the decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants.' The three examples chosen (thermal decomposition of $\mathrm{CaCO_3}$ per Eq. 1.20, photochemical decomposition of $\mathrm{AgCl}$ per Eq. 1.22, and electrolytic decomposition of water as demonstrated in Activity 1.7) are grounded in the chapter's own worked examples for the three energy forms, ensuring the answer is fully grounded in the text.

Part (a)(ii) draws on the chapter's specific remark under exothermic reactions: 'The decomposition of vegetable matter into compost is also an example of an exothermic reaction.' Students often wrongly assume all decomposition reactions are endothermic (since thermal/photochemical/electrolytic decomposition need supplied energy); the composting case is a deliberate exception the chapter highlights, and the answer explains the reasoning (slow microbial oxidation releasing heat) consistent with the chapter's classification of it alongside other exothermic processes such as respiration.

In part (b) (the alternative), the chapter's summary explicitly states: 'Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances,' directly following the definition of combination reactions ('two or more substances combine to form a single new product'). The chosen equations (Eq. 1.13 for combination, Eq. 1.20 for decomposition) are both textbook equations, and naming reactants/products for each fulfils the question's explicit requirement.