Thermal Decomposition of Metal Carbonates — Gases in the Atmosphere | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Air composition, greenhouse effect and climate implications appear in most series.

What thermal decomposition is

Thermal decomposition is a reaction in which a single compound breaks down into two or more simpler products when heated
Metal carbonates from the lower half of the reactivity series break down readily on heating to give the metal oxide plus carbon dioxide
General equation:

metal carbonate → metal oxide + carbon dioxide

The carbon dioxide given off can be confirmed by bubbling it through limewater (calcium hydroxide solution), which turns milky/cloudy

Other examples

ZnCO₃(s) → ZnO(s) + CO₂(g) (the white solid turns yellow when hot and back to white once cool)
CaCO₃(s) → CaO(s) + CO₂(g) (the same reaction used industrially to make lime from limestone)
Group 1 carbonates (Na₂CO₃, K₂CO₃) do not decompose at Bunsen-burner temperatures because their carbonates are very stable

Exam tip

Thermal decomposition of copper(II) carbonate — colour change and equation

What comes up: "Give the colour change that occurs" when copper(II) carbonate is heated, or "Name this type of reaction."

Write (two marks): (1) The solid changes colour from green to black. The mark scheme requires both colours and they must be stated in the correct order. (2) The reaction type is thermal decomposition.

Watch out: starting from the wrong colour (e.g. "blue to black") is the most common error — copper(II) carbonate is green, not blue (blue is copper(II) sulfate crystals). Write the equation as CuCO₃ → CuO + CO₂; state symbols are not required for the mark.