Calcium hydroxide solution is used to test for carbon dioxide gas. Explain what is observed when carbon dioxide is bubbled through calcium hydroxide solution, and describe what happens if excess carbon dioxide continues to be passed through the same solution.

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).

Model answer (4 marks)

When CO₂ is bubbled through Ca(OH)₂(aq) a white, milky precipitate of CaCO₃ forms:
1. CO₂ + Ca(OH)₂ → CaCO₃(s) + H₂O
The solution turns cloudy because insoluble calcium carbonate is produced.
If excess CO₂ is passed through, the precipitate dissolves and the solution becomes clear again:
2. CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂(aq)
The calcium carbonate reacts with the additional CO₂ and water to give soluble calcium bicarbonate, so the cloudiness disappears.

Examiner tips

  • Show the two equations to justify the colour change and the clearing.
  • Use the terms ‘precipitate’, ‘soluble’ and ‘cloudy’ as expected by the mark scheme.
  • Explain the role of excess CO₂ in forming calcium bicarbonate.

Mark scheme (4 marks)

  1. The calcium hydroxide solution turns milky/cloudy/white precipitate forms
  2. Because calcium carbonate is produced/formed
  3. When excess carbon dioxide is added, the solution clears/goes clear again
  4. Because calcium carbonate reacts with water and excess carbon dioxide to form calcium hydrogencarbonate, which is soluble

Key terms in this question

calcium hydroxide

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