Thermal Decomposition

Thermal decomposition is a reaction in which a single compound breaks down into two or more simpler substances when heated. It is a common type of chemical change and is used in industry to extract metals and make lime.

What Is Thermal Decomposition?

Thermal decomposition is a chemical reaction in which one substance breaks down into two or more substances when heated. Energy from heat is needed to break the chemical bonds in the original compound.

General form:

$\text{compound} \xrightarrow{\text{heat}} \text{substance A} + \text{substance B}$

Unlike physical changes (like melting), this cannot simply be reversed by cooling — new substances have been formed.

Key Examples

Metal Carbonates

Metal carbonates decompose when heated to form a metal oxide and carbon dioxide:

$\text{CaCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{CO}_2$

Calcium carbonate (limestone) → calcium oxide (quicklime) + carbon dioxide

This reaction is the basis of the cement and steel industries. Calcium oxide is collected and used to make cement and to neutralise acidic soils.

Copper Carbonate

$\text{CuCO}_3 \xrightarrow{\text{heat}} \text{CuO} + \text{CO}_2$

The colour change from green (copper carbonate) to black (copper oxide) is visible evidence that a new substance has formed.

Note

Carbon dioxide produced in these reactions can be identified by bubbling it through limewater — if the limewater turns milky (cloudy), CO₂ is present.

Evidence That Thermal Decomposition Has Occurred

A colour change is observed
A gas is produced (can be tested)
The original substance cannot be recovered simply by cooling
Loss of mass during the reaction (gas escapes)

Industrial Uses

Lime production: burning limestone in a kiln at ~900°C to produce quicklime (CaO)
Metal extraction: some metal oxides are reduced further after thermal decomposition
Food industry: baking powder contains sodium hydrogencarbonate (NaHCO₃), which decomposes on heating to release CO₂ bubbles that make cakes rise

Tip

Thermal decomposition reactions are endothermic — they take in energy from the surroundings (heat). The reaction will only continue as long as sufficient heat is supplied.

Real world: Baking powder making cakes rise

Baking powder contains sodium hydrogencarbonate (NaHCO₃). When heated in an oven, it thermally decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ gas released creates bubbles in the cake batter, making the cake light and spongy. Without this thermal decomposition, cakes would be dense and flat.

Real world: Limestone kilns and cement production

The cement industry burns calcium carbonate (limestone, CaCO₃) in huge rotary kilns at around 900 °C. The thermal decomposition produces calcium oxide (quicklime, CaO) and carbon dioxide: CaCO₃ → CaO + CO₂. The calcium oxide is then used to make cement. This is one of the world's largest industrial thermal decomposition processes — and one of the biggest sources of industrial CO₂ emissions (about 8% of global CO₂ comes from cement production).

Real world: Green verdigris on copper statues

The green coating (verdigris/patina) on copper statues like the Statue of Liberty is copper carbonate. If heated strongly, copper carbonate thermally decomposes: CuCO₃ → CuO + CO₂. The colour changes from green (copper carbonate) to black (copper oxide) — a visible indicator that a new substance has formed through thermal decomposition.

Tip

Where do you see this in real life? Baking powder in cakes (NaHCO₃ decomposing), cement works heating limestone, copper turning black when its carbonate coating is heated — thermal decomposition is behind both industrial chemistry and everyday baking.