03 1.3 Decomposition reactions
- A decomposition reaction is the opposite of combination.
- One compound breaks down into two or more simpler substances, usually when energy such as heat (thermal decomposition) or light is supplied.
General pattern: AB → A + B
Examples:
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Red mercury(II) oxide decomposes when heated into silvery liquid mercury and oxygen gas. 2HgO → 2Hg + O₂ (Check: 2 Hg and 2 O on each side.) This is the experiment in which Joseph Priestley obtained oxygen.
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Calcium carbonate (limestone, marble, chalk) decomposes on strong heating into calcium oxide (quicklime) and carbon dioxide. CaCO₃ → CaO + CO₂
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In a matchstick, potassium chlorate decomposes by heat to give oxygen: 2KClO₃ → 2KCl + 3O₂. (More on this in Lesson 2.)
Do not confuse the two: a combination reaction has one product from several reactants; a decomposition reaction has one reactant that yields several products.
Check your understanding. Magnesium ribbon burning in air is a combination reaction (Mg + O₂ → MgO, two reactants, one product). The heating of mercury oxide is a decomposition reaction (one reactant, two products).