A sealed rigid container holds a fixed mass of gas. The temperature of the gas is increased at constant volume. Explain why the pressure of the gas increases.

Pearson Edexcel International GCSE Physics (4PH1) — 5.3 Ideal gas molecules · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The temperature rise makes the gas molecules move faster. Faster molecules collide with the walls more often and with greater momentum change per collision. Because pressure is the perpendicular force per unit area on the walls, the increased force from the more frequent, harder collisions raises the pressure while the volume remains constant.

Examiner tips

  • Use the definition of pressure (force/area) to link collision force to pressure.
  • Show the chain: higher T → higher speed → more frequent & harder collisions → greater force on walls → higher pressure.
  • Mention that volume is fixed, so area of walls does not change the pressure calculation.

Common mistakes

  • Failing to explain that pressure is force per unit area, not just force.
  • Confusing temperature with pressure directly without mentioning molecular motion.
  • Omitting that the volume is constant, so the area of the walls remains unchanged.

Mark scheme (4 marks)

  1. Gas molecules move faster (at higher temperature)
  2. Molecules collide with the walls more frequently
  3. Molecules collide with the walls harder / with greater force (greater change in momentum per collision)
  4. Pressure is the perpendicular force per unit area on the container walls, so greater force from collisions means greater pressure (volume/area unchanged)

Key terms in this question

pressure

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