A student burns a piece of wood in a closed container. The student notices that the total mass of the container and its contents does not change during the reaction. The student then opens the container and finds that the mass of solid remaining is less than the original mass of the wood. Explain why the total mass does not change during burning in the closed container, and why the mass of solid remaining appears less after the container is opened.

Edexcel GCSE Chemistry (1CH0) — 1.8 Calculations involving masses · 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 law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. In a closed container no substances can enter or leave, so all products of the reaction are retained and the total mass of the container and its contents remains the same. When wood burns it reacts with oxygen to produce gaseous products such as carbon dioxide and water vapour. When the container is opened these gases escape into the surroundings, leaving only the solid ash or residue. Therefore the measured solid mass is less than the original mass of the wood.

Examiner tips

  • Use the phrase "law of conservation of mass" to show understanding of the principle.
  • Explain that a closed system retains all products, keeping total mass constant.
  • Mention that burning wood produces gases that leave the system when opened.
  • Show the logical sequence: closed system → no mass loss → open system → gases escape → solid mass appears reduced.

Common mistakes

  • Failing to mention the law of conservation of mass or the closed system condition.
  • Confusing the loss of mass with chemical change rather than physical escape of gases.
  • Not recognising that the solid mass is less because gases have left, not because the wood itself has lost mass.

Mark scheme (4 marks)

  1. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction
  2. In a closed container, no substances can enter or leave, so all products are retained and the total mass remains the same
  3. When wood burns it reacts with oxygen to produce gaseous products (such as carbon dioxide and water vapour)
  4. When the container is opened, the gaseous products escape into the surroundings, so only the solid ash/residue remains, making the measured solid mass less than the original

Key terms in this question

closed container

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