A sealed metal can contains a fixed amount of gas. The can is placed in a freezer and the temperature of the gas decreases. Explain, in terms of the movement of gas molecules, why the pressure of the gas inside the can decreases as the temperature falls.

WJEC A-Level Physics (Wales) — 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)

1. Gas molecules are in constant random motion.
2. Pressure is caused by molecules colliding with the walls of the container.
3. As the temperature falls, the molecules move more slowly and have less kinetic energy.
4. The slower molecules collide with the walls less frequently.
5. Each collision exerts a smaller force on the walls, so the overall pressure is lower.

Examiner tips

  • Use the word "collide" and "kinetic energy" to show understanding of the mechanism.
  • Show the chain: lower T → slower molecules → fewer, weaker collisions → lower pressure.
  • Keep the answer concise – 5 points, one sentence each.
  • Remember to mention both frequency and force of collisions.

Common mistakes

  • Confusing temperature with pressure; stating that pressure causes temperature change. Using vague terms like "less energy" without linking to kinetic energy. Omitting the role of collision frequency or force in the explanation.

Mark scheme (5 marks)

  1. Gas molecules are in constant random motion / are moving
  2. Pressure is caused by molecules colliding with the walls of the container
  3. As temperature decreases, the molecules move more slowly / have less kinetic energy
  4. The molecules collide with the walls less frequently / less often
  5. Each collision exerts a smaller force on the walls, so overall pressure is lower

Key terms in this question

pressure · temperature · molecules

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