A sealed metal canister contains a fixed mass of gas. The canister is placed in a freezer and left until the temperature of the gas has fallen significantly. The volume of the canister does not change. Explain why the pressure of the gas inside the canister decreases.

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)

The temperature of the gas falls, so the average kinetic energy of the molecules decreases. The molecules move more slowly and have a lower average speed. They collide with the walls of the container less frequently, and each collision exerts a smaller force on the wall. Consequently the total force per unit area (pressure) is lower.

Examiner tips

  • Use the word "decrease" to show cause and effect
  • Mention both lower speed and fewer collisions
  • Show the link to pressure = force/area
  • Keep answer concise and to the point

Common mistakes

  • Confusing pressure with volume or temperature
  • Failing to mention both fewer collisions and smaller force per collision
  • Using vague terms like "less" without explaining the kinetic energy change

Mark scheme (4 marks)

  1. The temperature of the gas falls, so the average kinetic energy of the molecules decreases
  2. The molecules move more slowly / have lower average speed
  3. The molecules collide with the walls of the container less frequently
  4. Each collision exerts a smaller force on the wall, so the total force per unit area (pressure) is lower

Key terms in this question

pressure · temperature

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