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3.3.1Chemical Changes

Thermal Decomposition of Metal Carbonates

5 min read 5 parts Lesson 11 of 24 Unit quiz

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Section 1 of 5The Decomposition Reaction

3.3.1 Thermal Decomposition of Metal Carbonates

Thermal decomposition is a reaction in which heat breaks a single compound into simpler substances.

Thermal decomposition
  • A compound breaks down into simpler substances when it is heated.
General equation
  • Metal carbonate breaks down to form a metal oxide and carbon dioxide gas.
Reactivity link
  • Carbonates of metals low in the reactivity series decompose readily when heated.
Thermal stability
  • A carbonate that needs strong heating before it decomposes is described as thermally stable.

Section 2 of 5Heating Copper(II) Carbonate

Copper(II) carbonate is a useful example because its colour change and gas test are both clear.

1
Heat a sample of green copper(II) carbonate in a test tube, using a Bunsen burner.
2
The powder gradually darkens to black as copper(II) oxide forms, showing that the reaction has occurred.
3
Pass the gas produced through limewater, which turns milky, confirming that the gas is carbon dioxide.
4
The equation for the reaction is CuCO₃(s) → CuO(s) + CO₂(g), showing copper(II) carbonate forming copper(II) oxide and carbon dioxide.

Section 3 of 5Thermal Stability

Thermal Stability
Group 1 carbonates
  • These are very stable and need strong heating; lithium carbonate is the least stable of them.
Less reactive metals
  • Carbonates of less reactive metals are less thermally stable, so they decompose at lower temperatures.
Bunsen burner
  • Some carbonates decompose at the temperature a Bunsen burner can reach, while others need far more heat.
Gas test
  • Limewater turns milky when carbon dioxide is passed through it, confirming the gas.

Section 4 of 5Worked Example

Question Describe the observations when copper(II) carbonate is heated and the gas produced is tested.

  1. Note the colour of the solid before heating: it is green.
  2. Observe the solid darkening to black as copper(II) oxide forms.
  3. Pass the gas through limewater and observe the limewater turn milky.

Answer The green powder turns black and the limewater turns milky, showing that copper(II) oxide and carbon dioxide were formed.

Section 5 of 5Exam Checklist & Tips

3.3.1 – Key Points for the Exam

  • Thermal decomposition A reaction in which a compound breaks down into simpler substances when heated.
  • Thermal stability The resistance of a compound to decomposition when it is heated.
  • Limewater A solution of calcium hydroxide that turns milky in the presence of carbon dioxide.
  • Metal oxide A compound of a metal with oxygen, which is the product of heating most metal carbonates.

Common Mistakes

  • Writing that a metal carbonate decomposes to form the metal; the product is a metal oxide.
  • Forgetting that copper(II) carbonate changes colour from green to black when heated.
  • Saying limewater turns milky because of oxygen rather than carbon dioxide.
  • Confusing thermal stability with reactivity;
    • stability refers to how resistant a compound is to decomposition on heating.
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