Thermal decomposition of carbonates

Thermal decomposition is a reaction driven by heat alone, and metal carbonates are the classic example in 4541. On strong heating, a carbonate splits into the metal oxide and carbon dioxide. How easily this happens depends on how reactive the metal is, which links the reaction directly to the reactivity series.

The balanced equation

The general word equation is: metal carbonate → metal oxide + carbon dioxide, on heating. With correct formulae and state symbols, common examples are:

CaCO₃(s) → CaO(s) + CO₂(g)

CuCO₃(s) → CuO(s) + CO₂(g)

ZnCO₃(s) → ZnO(s) + CO₂(g)

Each carbonate loses carbon dioxide and leaves the oxide behind, and the equations are already balanced as written. An important exception must be learned: the carbonates of very reactive Group 1 metals, such as sodium carbonate (Na₂CO₃) and potassium carbonate (K₂CO₃), are stable to heat and do not decompose in an ordinary Bunsen flame.

Conditions required

Strong, direct heating is required, a hot Bunsen flame on the solid carbonate in a boiling tube or crucible. No catalyst and no other reactant is involved; heat is the only driving force. The less reactive the metal, the more easily its carbonate decomposes, so copper(II) carbonate decomposes readily while calcium carbonate needs stronger heating and Group 1 carbonates do not decompose at all.

What you observe

The gas released is carbon dioxide, confirmed because it turns limewater milky. Colour changes make the reaction easy to follow: green copper(II) carbonate turns to black copper(II) oxide; white zinc carbonate gives zinc oxide, which is yellow while hot and white when cold; and white calcium carbonate gives white calcium oxide with no dramatic colour change. No flame or glow is needed from the solid itself, the change is purely thermal.

Where it appears in the SPM exam

In 4541/1 this is tested as the products of heating a carbonate, the limewater test for carbon dioxide, and the stability trend linked to the reactivity series. In 4541/2 it appears as equation writing and as questions comparing the ease of decomposition of different carbonates. In the practical paper 4541/3, heating a carbonate and testing the gas, or recording the colour change of copper(II) carbonate, is a straightforward observation task.

How we teach it

We tie the equation to the reactivity idea: a stable carbonate belongs to a reactive metal, an easily decomposed carbonate belongs to a less reactive metal. Students most often lose marks by trying to decompose sodium or potassium carbonate, by forgetting the limewater test, or by giving the wrong residue colour. Learning the copper (green to black) and zinc (yellow hot, white cold) colour changes gives you memorable, exam-ready observations.

Quick summary

The pattern to remember is simple: heat a carbonate, get the oxide plus carbon dioxide, and confirm the gas with limewater. Copper(II) carbonate is the model, green solid to black solid, gas turns limewater milky, while sodium and potassium carbonates are the exceptions that stay unchanged. Pair each equation with its colour change and its place in the reactivity series, and every version of this question becomes predictable.