Explain, in terms of molecular motion and collisions, why the pressure of a fixed mass of ideal gas increases when its temperature is raised at constant volume.

IB DP Physics Standard 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 they move faster.

Because the molecules are moving faster, they collide with the walls of the container more frequently.

Each collision now transfers a larger impulse to the wall, since the molecules arrive with greater speed.

The result is a larger force per unit area on the walls, i.e. an increase in pressure, while the volume remains unchanged.

Examiner tips

  • Use the word ‘average kinetic energy’ to link temperature and speed. Show the chain: ↑T → ↑speed → ↑collision frequency & impulse → ↑pressure. Mention that volume is constant, so pressure change is purely molecular.
  • Use concise, exam‑style sentences; avoid unnecessary words.

Mark scheme (4 marks)

  1. Raising temperature increases the average kinetic energy (or speed) of the molecules.
  2. Faster molecules collide with the walls of the container more frequently.
  3. Each collision transfers greater impulse / momentum to the wall because molecules arrive with greater speed.
  4. Greater force per unit area on the walls means pressure increases; volume is unchanged so the increase is due entirely to molecular effects.

Key terms in this question

ideal gas · pressure · constant volume

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