A fixed quantity of gas is stored in a sealed container at constant temperature. Explain what happens to the pressure of the gas when the volume of the container is decreased.

Eduqas GCSE Physics — 2.6 Particle physics and using physics to investigate the universe · 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 gas particles collide with the walls of the container.

When the volume is decreased the particles are forced into a smaller space, so they are closer together and have less room to move.

Because they are closer together, they collide with the walls more frequently.

Hence the pressure rises – at constant temperature pressure is inversely proportional to volume (P∝1/V).

Examiner tips

  • Use the word ‘collide’ to show understanding of kinetic theory.
  • Explain the link between volume, collision frequency and pressure.
  • Mention the inverse relationship P∝1/V to show depth.
  • Keep each point concise – 4 marks total.

Common mistakes

  • Saying the pressure decreases – opposite of the correct trend.
  • Omitting the collision‑frequency explanation.
  • Using ‘temperature changes’ when the question states temperature is constant.

Mark scheme (4 marks)

  1. The gas particles collide with the walls of the container
  2. Decreasing the volume means the particles are closer together / have less space to move
  3. The particles collide with the walls more frequently / more often
  4. So the pressure increases / pressure-volume relationship: pressure is inversely proportional to volume at constant temperature

Key terms in this question

volume

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