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Science — Third Preparatory

Science — Third Preparatory

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Science — Third Preparatory 2026/2027

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Unit 1 - Force, Motion, Acceleration and Physical Quantities

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Unit 1 — Chemical Reactions and Their Environmental Impacts

Science — Third Preparatory

Unit 1 — Chemical Reactions and Their Environmental Impacts

1.3 Decomposition reactions

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:

  1. 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.

  2. Calcium carbonate (limestone, marble, chalk) decomposes on strong heating into calcium oxide (quicklime) and carbon dioxide.
    CaCO₃ → CaO + CO₂

  3. 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).
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