(i) Decomposition Reaction: A reaction in which a single substance breaks down to give two or more substances is called a decomposition reaction.
I. Electrolysis of water:
$$2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{H}_2(g) + \text{O}_2(g)$$
Water (one substance) decomposes into hydrogen and oxygen. Hence it is a decomposition reaction. Energy involved: electrical energy.
II. Blackening of silver bromide:
$$2\text{AgBr}(s) \xrightarrow{\text{sunlight}} 2\text{Ag}(s) + \text{Br}_2(g)$$
Silver bromide decomposes into silver and bromine. Hence it is a decomposition reaction. Energy involved: light energy.
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(ii) Formation of calcium oxide involves decomposition (CaCO₃ heated → CaO + CO₂), while formation of calcium hydroxide involves combination (CaO + H₂O → Ca(OH)₂). These are opposite reactions:
$$\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g) \quad \text{(Decomposition)}$$
$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat} \quad \text{(Combination)}$$
In decomposition, one reactant splits into two products; in combination, two reactants form one product — hence they are opposite reactions.
Source: Chapter 1, Section 1.2 — Types of Chemical Reactions
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