A sealed metal container holds a fixed quantity of gas. The container is placed in a freezer and left until the gas cools to a much lower temperature. Explain what happens to the pressure of the gas inside the container as it cools, using ideas about particles.

WJEC GCSE Physics (Wales) — 2.3 Forces and motion · 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 gas particles decreases.
The particles therefore move more slowly.
When they collide with the walls they do so with less force.
Consequently the pressure inside the container decreases.

Examiner tips

  • Use the word ‘therefore’ to link cause and effect. Mention kinetic energy and particle speed. Show the chain from temperature to pressure. Keep the answer concise and use the exact terminology.
  • common_mistakes
  • :
  • Writing that pressure increases. Forgetting to mention particle speed. Using vague terms like ‘less energy’ without linking to kinetic energy.

Mark scheme (4 marks)

  1. The temperature of the gas decreases, so the average kinetic energy of the particles decreases.
  2. The particles move more slowly / with less speed.
  3. The particles collide with the walls of the container with less force.
  4. Therefore the pressure of the gas decreases.

Key terms in this question

pressure · temperature · particles

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