Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas. A student carries out this reaction in an open beaker and observes that the mass of the reaction mixture decreases during the reaction. Explain why the mass decreases, and describe what the student could do to show that mass is conserved in this reaction.

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)

1. Hydrogen gas is produced.
2. The gas leaves the beaker, so the mass of the mixture falls.
3. If the reaction were carried out in a closed, sealed container the gas would be retained.
4. Then the mass before and after would be the same, proving mass conservation.

Examiner tips

  • Use the exact wording from the mark scheme – mention hydrogen gas and a closed container.
  • Show the cause (gas escape) before the solution (sealed vessel).
  • Keep each point brief and to the point.

Mark scheme (4 marks)

  1. Hydrogen gas is produced during the reaction
  2. The hydrogen gas escapes / is released into the surroundings from the open beaker
  3. The student should carry out the reaction in a closed / sealed container
  4. The total mass before the reaction would equal the total mass after the reaction, showing mass is conserved

Key terms in this question

hydrogen gas · reaction mixture

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