Explain, using the kinetic model of an ideal gas, why the pressure of a fixed mass of gas increases when its temperature is raised at constant volume.

IB DP Physics Higher Level (2023 syllabus) — B.3 Gas laws · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Raising the temperature increases the average kinetic energy of the gas molecules, so their average speed rises.
Because the speed is higher, a molecule reaches a wall in a shorter time, so collisions with the wall occur more frequently.
Each collision now imparts a larger impulse to the wall, as the molecule arrives with greater momentum.
The net effect is a larger force per unit area on the walls – i.e. higher pressure – and with volume fixed no opposing effect occurs.

Examiner tips

  • Use the kinetic theory terms (average kinetic energy, momentum, impulse).
  • Show the chain: ↑T → ↑speed → ↑collision frequency & impulse → ↑pressure.
  • Keep the answer concise – 4 points only.

Common mistakes

  • Confusing temperature with pressure; stating that pressure rises because volume decreases.
  • Using vague phrases like ‘more energy’ without linking to speed or momentum.
  • Omitting the point that volume is constant, so no counter‑effect.

Mark scheme (4 marks)

  1. Raising the temperature increases the average kinetic energy (or average speed) of the gas molecules.
  2. Molecules collide with the container walls more frequently (because they travel between walls in less time).
  3. Each collision transfers greater impulse (or force) to the wall because the molecules arrive with greater momentum.
  4. The total force per unit area (pressure) on the walls therefore increases; since volume is constant, no competing effect reduces the pressure.

Key terms in this question

kinetic model · pressure

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