A gas is trapped inside a rigid steel cylinder. The cylinder is placed in a freezer and left until the gas reaches a much lower temperature. Explain, in terms of the motion and behaviour of gas molecules, why the pressure of the gas decreases as the temperature falls.

WJEC A-Level Physics (Wales) — 4.2 Kinetic theory and gases (A-Level only) · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Gas molecules are in constant random motion.
Pressure is produced by the impact of molecules on the walls of the container.
When the temperature falls the average kinetic energy of the molecules decreases, so they move more slowly.
Consequently the molecules collide with the walls less frequently and with less force per collision.
The net force on the walls is therefore reduced, so the pressure of the gas decreases.

Examiner tips

  • Use the term ‘average kinetic energy’ to link temperature and molecular speed. Show the chain: lower T → lower KE → slower molecules → fewer/ weaker collisions → lower pressure. Keep the answer concise and avoid unnecessary detail.

Common mistakes

  • Failing to mention that pressure comes from collisions with the walls. Using vague phrases like ‘molecules move slower’ without linking to kinetic energy. Including irrelevant information such as changes in volume (the cylinder is rigid).

Mark scheme (5 marks)

  1. Gas molecules are in constant random motion
  2. Pressure is caused by molecules colliding with the walls of the container
  3. At lower temperature, molecules move more slowly / have less kinetic energy
  4. Molecules collide with the walls less frequently OR with less force per collision
  5. Therefore the overall force on the walls is less, so pressure decreases

Key terms in this question

pressure · temperature · molecules

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