Thermal Decomposition

Summary: The breaking down of a compound into simpler substances by heating. Tags: igcse chemistry definition reactions Created: 2026-07-14 Last Updated: 2026-07-14

Thermal decomposition is the chemical breakdown of a compound into two or more simpler substances when heated, and it is an endothermic process that requires a continuous input of thermal energy to overcome the lattice enthalpy and break chemical bonds within the compound. A classic IGCSE example is the decomposition of calcium carbonate (limestone) into calcium oxide (quicklime) and carbon dioxide gas when heated strongly: CaCO₃(s) → CaO(s) + CO₂(g), and the carbon dioxide produced can be confirmed by bubbling it through limewater which turns milky. Most metal carbonates follow this general pattern, decomposing to the metal oxide and carbon dioxide, although the ease of decomposition varies significantly — copper(II) carbonate decomposes readily with gentle heating to produce black copper(II) oxide, while Group 1 metal carbonates such as sodium carbonate and potassium carbonate are thermally stable and do not decompose under the heat of a Bunsen burner because their cations have a lower charge density and polarise the carbonate ion less. Thermal decomposition is not limited to carbonates; other examples at IGCSE include the dehydration of hydrated salts such as CuSO₄·5H₂O losing their water of crystallisation, and the breakdown of hydrogen peroxide into water and oxygen. In IGCSE exams, students must be able to predict the products of thermal decomposition, recall the trend in thermal stability of metal carbonates (it increases down a group), and use the limewater test to confirm the presence of carbon dioxide as a gaseous product.

Key term — IGCSE Chemistry (0620/0971) glossary.