A student heats calcium carbonate in an open crucible. The student notices that the mass of the crucible and its contents decreases during the reaction. Explain why the mass decreases, even though the law of conservation of mass states that mass is conserved.

AQA GCSE Chemistry (8462) — 4.3.1 Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations · Explain · 4 marks · View as Markdown

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

Calcium carbonate undergoes thermal decomposition when heated: calcium carbonate → calcium oxide + carbon dioxide

Model answer (4 marks)

The reaction produces carbon dioxide gas.
The CO₂ leaves the crucible and goes into the atmosphere.
Because the gas is not weighed, the recorded mass of the crucible and its contents falls.
Mass is still conserved – the total mass of CaO plus the CO₂ that escaped equals the original mass of CaCO₃.

Examiner tips

  • State that CO₂ is a gas and escapes; mention the crucible is open; explain why the measured mass drops; note that conservation of mass still holds for the whole system.

Common mistakes

  • Failing to mention that CO₂ is a gas; confusing the mass loss with a violation of conservation of mass; not recognising that the crucible is open and the gas can leave.

Mark scheme (4 marks)

  1. Carbon dioxide (gas) is produced as a product
  2. The carbon dioxide gas escapes / is released into the atmosphere / surroundings
  3. The mass of the gas is not measured / not included in the recorded mass
  4. Mass is still conserved because the total mass of calcium oxide AND carbon dioxide equals the original mass of calcium carbonate

Key terms in this question

conservation of mass

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