Explain why the pressure inside a sealed gas container increases when the volume of the container is decreased at constant temperature.

AQA GCSE Physics (8463) — 4.3.3 Particle model and pressure (Phys only) · 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. The gas molecules move randomly and collide with the walls of the container.
2. When the volume is reduced, the molecules have less space and a smaller area to travel.
3. This causes more frequent collisions with the walls – there are more collisions per unit area per unit time.
4. More frequent collisions give a greater force per unit area on the walls, so the pressure increases.

Examiner tips

  • Use the word "collide" and mention "force per unit area" to show understanding of pressure definition.
  • Show the logical sequence: less volume → fewer paths → more collisions → higher pressure.

Common mistakes

  • Confusing temperature change with volume change; forgetting to state constant temperature.
  • Using vague terms like "more gas" instead of "more collisions" or mis‑defining pressure as force alone.

Mark scheme (4 marks)

  1. The gas molecules are in constant random motion and collide with the walls of the container
  2. Decreasing the volume means the molecules have less space / a smaller area to travel through
  3. The molecules collide with the walls more frequently / there are more collisions per unit area per unit time
  4. More frequent collisions means a greater force per unit area on the walls, so pressure increases

Key terms in this question

pressure · constant temperature · volume

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