A sealed metal canister contains a fixed mass of gas. The canister is placed in a freezer and left until the gas reaches a much lower temperature. Explain what happens to the average kinetic energy of the gas molecules and to the pressure of the gas inside the canister as it cools, in terms of the behaviour of the molecules.

Eduqas A-Level Physics — 4.1 Thermal physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The average kinetic energy of the gas molecules decreases as the temperature falls.
The molecules move more slowly, so their average speed is lower.
They collide with the walls of the canister less frequently.
Each collision exerts a smaller force because the molecules are moving more slowly and have less momentum.
Consequently the pressure of the gas inside the canister decreases.

Examiner tips

  • Use the word ‘decreases’ for kinetic energy and pressure. Show the chain: lower temperature → lower KE → slower molecules → fewer, weaker collisions → lower pressure. Mention ‘force of collision’ to link to pressure.

Common mistakes

  • Confusing pressure with temperature. Failing to explain why collisions become weaker. Using vague terms like ‘less energy’ without linking to speed or force.

Mark scheme (5 marks)

  1. The average kinetic energy of the gas molecules decreases (as temperature falls).
  2. The molecules move more slowly (lower average speed).
  3. The molecules collide with the walls of the canister less frequently.
  4. Each collision with the wall exerts a smaller force (because molecules are moving more slowly / have less momentum).
  5. Therefore the pressure of the gas decreases (because pressure is caused by the force of molecular collisions on the walls).

Key terms in this question

pressure · kinetic energy · molecules · temperature

Related

More Thermal physics questions

▶ Try answering this question with AI marking (free) →