Decomposition – IGCSE Chemistry Definition and Key Facts

Decomposition is one of the fundamental reaction types in the Cambridge 0620 syllabus. Recognising a decomposition is straightforward: one reactant appears on the left of the arrow and two or more products appear on the right.

The 0620 definition

A decomposition reaction is a reaction in which a single compound breaks down to form two or more simpler substances. The general pattern is:

AB → A + B

The reverse of decomposition is a combination (synthesis) reaction.

Types of decomposition

Thermal decomposition

The compound is broken down by heating. This is the most commonly tested form in IGCSE papers.

Substance	Equation	Observation
Calcium carbonate	CaCO₃ → CaO + CO₂	White solid remains; CO₂ turns limewater milky
Copper(II) carbonate	CuCO₃ → CuO + CO₂	Green solid turns black
Zinc carbonate	ZnCO₃ → ZnO + CO₂	White solid turns yellow when hot, white when cold
Potassium manganate(VII)	2KMnO₄ → K₂MnO₄ + MnO₂ + O₂	Purple crystals produce oxygen

See thermal decomposition for a deeper treatment.

Catalytic decomposition

A catalyst speeds up the decomposition without being consumed. The classic IGCSE example is the decomposition of hydrogen peroxide using manganese(IV) oxide:

2H₂O₂ → 2H₂O + O₂ (catalyst: MnO₂)

This reaction is used in the laboratory preparation of oxygen.

Electrolytic decomposition

Passing a direct electric current through a compound in the liquid or molten state breaks it down. For example, electrolysis of molten lead(II) bromide:

PbBr₂ → Pb + Br₂

This method is essential for extracting reactive metals such as aluminium from their ores.

Decomposition of metal carbonates — exam pattern

The ease of thermal decomposition of a metal carbonate relates to the position of the metal in the reactivity series. Carbonates of less reactive metals (copper, zinc) decompose easily with a Bunsen burner. Carbonates of more reactive metals (calcium, sodium) need stronger heating — sodium carbonate does not decompose at Bunsen temperatures.

This trend is a popular Paper 2 and Paper 4 question.

Decomposition of metal hydroxides

Some metal hydroxides decompose on heating. For example:

Cu(OH)₂ → CuO + H₂O

Copper(II) hydroxide (a blue solid) breaks down to copper(II) oxide (black) and water. Hydroxides of Group I metals (NaOH, KOH) are stable and do not decompose at Bunsen temperatures.

Decomposition vs other reactions

Feature	Decomposition	Displacement	Neutralisation
Number of reactants	1	2	2
Energy input usually needed?	Yes (heat, electricity, light)	Not always	No (exothermic)
Example	CaCO₃ → CaO + CO₂	Zn + CuSO₄ → ZnSO₄ + Cu	HCl + NaOH → NaCl + H₂O

Worked exam question

Copper(II) carbonate is heated strongly. (a) Name the type of reaction. [1] (b) Write the equation. [1] (c) State two observations. [2]

(a) Thermal decomposition [1]

(b) CuCO₃ → CuO + CO₂ [1]

(c) Green solid turns black [1]; gas produced turns limewater milky / effervescence [1]

Common exam mistakes

1. Writing two reactants on the left and calling the reaction decomposition — decomposition has only one reactant.
2. Confusing thermal decomposition with combustion. Combustion requires oxygen as a second reactant; thermal decomposition uses heat alone to break a single compound apart.
3. Forgetting the colour change observations, especially CuCO₃ (green → black) and ZnCO₃ (white → yellow when hot).