Explain why the pressure of a fixed mass of gas increases when its temperature is raised at constant volume.

Cambridge International IGCSE Physics (0625) — 2.1 Kinetic particle model of matter · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Higher temperature increases the average kinetic energy of the gas molecules.

The molecules therefore move faster.

At constant volume they collide with the walls more frequently.

Each collision gives a larger change in momentum, so the force on the walls – and the pressure – is greater.

Examiner tips

Use the word ‘average kinetic energy’ to link temperature and speed. Show the chain: temperature → kinetic energy → speed → collision frequency → pressure. Mention ‘constant volume’ to justify that the number of collisions per unit area increases, not the area itself.
Avoid vague phrases such as ‘more energy’ without specifying kinetic energy or speed. Do not state ‘pressure rises because the gas expands’ – volume is fixed.
Do not omit any of the four key points – each is worth a mark.

Common mistakes

Confusing temperature with pressure. Misstating that volume changes. Failing to mention the change in momentum or force per collision.

Mark scheme (4 marks)

Higher temperature means greater average kinetic energy of the molecules
Molecules move faster / with greater speed
Molecules collide with the walls of the container more frequently
Each collision exerts a greater force on the walls / molecules rebound with greater change in momentum

Key terms in this question

pressure · temperature