Calcium carbonate, calcium oxide and calcium hydroxide are all useful materials obtained from limestone. Explain why calcium hydroxide solution is used to test for carbon dioxide gas.

WJEC GCSE Chemistry (Wales) — 1.6 Limestone · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Calcium hydroxide dissolves slightly in water to form limewater. When carbon dioxide is bubbled through limewater, a change is observed.

Model answer (4 marks)

Calcium hydroxide reacts with carbon dioxide to give calcium carbonate:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
The product, calcium carbonate, is insoluble in water. Its formation makes the limewater appear milky or cloudy, signalling the presence of CO₂.

Examiner tips

  • Show the reaction equation; include all reactants and products.
  • Explain that CaCO₃ is insoluble, giving the milky appearance.
  • Use the term ‘limewater’ and ‘cloudy’ as examiner expects.
  • Mention the reaction is a precipitation test for CO₂.

Common mistakes

  • Writing the reaction incorrectly (e.g. missing H₂O).
  • Failing to state that CaCO₃ is insoluble.
  • Using vague language like ‘makes it cloudy’ without linking to precipitation.

Mark scheme (4 marks)

  1. Calcium hydroxide reacts with carbon dioxide
  2. Calcium carbonate is produced / formed
  3. Calcium carbonate is insoluble (in water)
  4. This causes the limewater to turn milky / cloudy

Key terms in this question

calcium hydroxide · carbon dioxide · calcium carbonate

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