Calcium oxide (CaO) is made by heating calcium carbonate (CaCO₃) in a kiln. The equation for this reaction is: CaCO₃ → CaO + CO₂. The relative atomic masses (Ar) are: Ca = 40, C = 12, O = 16. Explain why the mass of solid remaining in the kiln is less than the mass of calcium carbonate that was added. Use the relative formula masses of the substances in the equation to support your answer.

AQA GCSE Chemistry (8462) — 4.3.2 Use of amount of substance in relation to masses of pure substances · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The reaction releases CO₂ as a gas:
1. CaCO₃ (Mr = 100) → CaO (Mr = 56) + CO₂ (Mr = 44).
2. CO₂ leaves the kiln, so it is not part of the remaining solid.
3. The law of conservation of mass means the total mass of products equals the mass of reactants.
4. The mass lost equals the mass of CO₂ (44 g per 100 g CaCO₃), so the solid mass is less.

Examiner tips

  • Show the balanced equation and the formula masses; link the 44 g CO₂ loss to the 56 g CaO remaining.
  • Use the word ‘released’ or ‘escapes’ to show CO₂ is a gas.
  • Mention the conservation of mass to justify that no atoms are lost.
  • Keep the answer concise and use UK spelling.

Common mistakes

  • Writing the wrong formula mass for CaCO₃ or CO₂.
  • Failing to state that CO₂ is a gas that leaves the kiln.
  • Not linking the mass difference to the 44 g CO₂ loss.

Mark scheme (4 marks)

  1. Carbon dioxide (CO₂) is a gas / is released into the atmosphere / escapes from the kiln
  2. The law of conservation of mass still holds / no atoms are lost or made / total mass of products equals mass of reactants
  3. Correct relative formula mass of CaCO₃ = 100 (or CaO = 56, or CO₂ = 44)
  4. Links the difference in Mr values to mass loss — the mass of CO₂ (Mr = 44) accounts for the difference between the mass of CaCO₃ (Mr = 100) and CaO (Mr = 56)

Key terms in this question

relative formula mass

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