A sealed metal cylinder contains a fixed mass of gas. The cylinder is placed in a freezer and left until the gas reaches a much lower temperature. Explain what happens to the pressure of the gas as its temperature decreases, in terms of the molecules of the gas.

AQA A-Level Physics (7408) — 3.6.2 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 gas molecules move more slowly, so their kinetic energy is lower. They collide with the cylinder walls less frequently and each collision exerts a smaller force. Because the volume is fixed, the same area of wall is hit. Consequently the pressure decreases.

Examiner tips

  • Use the word ‘explain’ – describe the change in kinetic energy, collision frequency and force, then state the result for pressure.
  • Mention that the volume is constant to justify why the area of impact does not change.
  • Show the logical chain: lower T → slower molecules → fewer, weaker collisions → lower pressure.
  • Use correct terminology: kinetic energy, collision frequency, force, pressure, volume.

Mark scheme (5 marks)

  1. The molecules move more slowly / have less kinetic energy at lower temperature
  2. The molecules hit the walls of the cylinder less frequently / less often
  3. Each collision exerts a smaller force on the wall / molecules hit the walls with less force
  4. The volume of the cylinder remains constant (so the same area of wall is being hit)
  5. Therefore the pressure of the gas decreases

Key terms in this question

pressure · temperature · fixed mass · molecules

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