A student carries out a chemical reaction and notices that the total mass of the products is less than the total mass of the reactants she started with. She claims this proves the law of conservation of mass is wrong. Explain why the student's claim is incorrect, and explain how the law of conservation of mass applies to all chemical reactions.

WJEC A-Level Chemistry (Wales) — 3.6 Amines and polymers (A-Level only) · Explain · 5 marks · View as Markdown

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

A student heats a metal carbonate in an open crucible. She records the mass before and after heating and finds that the mass has decreased.

Model answer (5 marks)

No atoms are lost or made during a chemical reaction. The mass of the products equals the mass of the reactants. In the heating of a metal carbonate a gas (carbon dioxide) is produced and escapes into the atmosphere, so the mass measured after heating is lower. If the mass of the escaped gas were included, or if the reaction were carried out in a closed container, the total mass would remain the same. Therefore the student’s claim is incorrect because the decrease in mass is explained by the loss of a gaseous product, not by atoms being destroyed.

Examiner tips

  • Use the phrase "no atoms are lost or made" to show understanding of conservation of mass.
  • Explain the role of the gaseous product and why it is not measured.
  • Mention that a closed system would give equal masses.
  • Show the logical link between loss of gas and apparent mass loss.

Common mistakes

  • Confusing the loss of mass with a violation of conservation of mass.
  • Failing to mention that the gas escapes and is not measured.
  • Assuming the reaction is in a closed system without stating it.

Mark scheme (5 marks)

  1. No atoms are lost or made during a chemical reaction
  2. The mass of the products equals the mass of the reactants
  3. A gas (carbon dioxide) is produced and escapes into the atmosphere / is not collected
  4. If the mass of the escaped gas were included / the reaction were carried out in a closed container, the total mass would remain the same
  5. The student's claim is incorrect because the decrease in mass is explained by the loss of a gaseous product, not by atoms being destroyed

Key terms in this question

conservation of mass

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