A student seals a fixed quantity of gas inside a rigid metal container and places it in a freezer. Explain what happens to the pressure of the gas inside the container as its temperature decreases.

WJEC A-Level Physics (Wales) — 2.2 DC circuits · 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 temperature of the gas decreases, so the average kinetic energy of the gas molecules falls. Consequently, the molecules collide with the container walls less frequently and with less force. Therefore the pressure of the gas decreases. In fact, for a fixed volume the pressure is directly proportional to the absolute temperature (P∝T).

Examiner tips

  • Use the P∝T relationship to justify the final statement
  • Show the chain: ↓T → ↓KE → ↓collision frequency/force → ↓P
  • Keep the explanation concise and in student voice

Common mistakes

  • Failing to mention the kinetic energy change
  • Confusing temperature units (C vs K)
  • Not linking the pressure change to the P∝T proportionality

Mark scheme (5 marks)

  1. The temperature of the gas decreases / the gas cools down
  2. The average kinetic energy of the gas molecules decreases
  3. The gas molecules collide with the container walls less frequently / with less force
  4. Therefore the pressure of the gas decreases
  5. The pressure is directly proportional to the absolute (Kelvin) temperature / pressure–temperature relationship stated

Key terms in this question

pressure · temperature

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