Thermal Decomposition – IGCSE Chemistry Definition and Key Facts

Thermal decomposition is one of the most frequently tested reaction types in the Cambridge 0620 syllabus. It appears in questions on metal carbonates, metal hydroxides, metal nitrates, and practical observations. Understanding the colour changes and gas tests is essential for full marks.

The 0620 definition

Thermal decomposition is the breaking down of a compound into two or more simpler substances by the action of heat. It is a type of decomposition reaction where heat supplies the energy needed to break chemical bonds.

Thermal decomposition of metal carbonates

This is the most heavily tested category. The general equation is:

Metal carbonate → metal oxide + carbon dioxide

Carbonate	Equation	Colour change	Decomposes easily?
Copper(II) carbonate	CuCO₃ → CuO + CO₂	Green → black	Yes (Bunsen burner)
Zinc carbonate	ZnCO₃ → ZnO + CO₂	White → yellow (hot), white (cold)	Yes
Calcium carbonate	CaCO₃ → CaO + CO₂	White → white (no visible change)	Needs strong heating
Sodium carbonate	Does not decompose at Bunsen temperatures	—	No

The trend: carbonates of less reactive metals decompose more easily than carbonates of more reactive metals. This is because more reactive metals form more stable compounds.

Testing for CO₂

The gas produced turns limewater milky (calcium hydroxide solution forms a white precipitate of calcium carbonate). This test confirms CO₂ production and is required in almost every thermal decomposition question.

Thermal decomposition of metal hydroxides

Hydroxide	Equation	Colour change
Copper(II) hydroxide	Cu(OH)₂ → CuO + H₂O	Blue → black
Iron(III) hydroxide	2Fe(OH)₃ → Fe₂O₃ + 3H₂O	Brown → reddish-brown
Zinc hydroxide	Zn(OH)₂ → ZnO + H₂O	White → yellow (hot)

Group I hydroxides (NaOH, KOH) are thermally stable and do not decompose.

Thermal decomposition of metal nitrates

Nitrate	Products	Notes
Group I (e.g. KNO₃)	Metal nitrite + O₂	2KNO₃ → 2KNO₂ + O₂
Group II and transition metals	Metal oxide + NO₂ + O₂	2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂

The brown gas NO₂ is a visible product — examiners often ask for this observation.

Thermal decomposition is endothermic

Energy must be continuously supplied (by heating) for the reaction to continue. If the heat source is removed, the reaction stops. This makes thermal decomposition an endothermic process — heat energy is taken in to break the bonds in the compound.

Practical applications

Application	Reaction
Making quicklime (CaO)	Heating limestone (CaCO₃) in a lime kiln
Cement production	Thermal decomposition of limestone
Extracting metals from ores	Some metal oxides obtained by decomposing carbonates

In Malaysia, lime kilns have historically been used for producing quicklime from limestone deposits in states like Perak and Perlis.

Worked exam question

Copper(II) carbonate is heated in a test tube. (a) Name the type of reaction. [1] (b) Write the equation. [1] (c) Describe two observations you would make. [2] (d) How would you test for the gas produced? [2]

(a) Thermal decomposition [1]

(b) CuCO₃ → CuO + CO₂ [1]

(c) The green powder turns black [1]; gas/bubbles are produced [1]

(d) Pass the gas through/bubble it into limewater [1]; the limewater turns milky/cloudy [1]

Common exam mistakes

1. Writing “decomposition” alone when the question asks for the specific type — write “thermal decomposition” for full marks when heat is involved.
2. Forgetting the colour change for CuCO₃ → CuO (green to black). This observation is almost always worth a mark.
3. Stating that ZnO is yellow — it is yellow when hot but turns back to white on cooling. Examiners accept either if qualified.
4. Not mentioning the limewater test when asked to confirm CO₂. Simply saying “CO₂ is produced” without describing the test loses the practical marks.