When a sealed container of gas is heated at constant volume, its pressure increases. Explain why, in terms of the behaviour of the gas particles.

Eduqas GCSE Physics — 2.6 Particle physics and using physics to investigate the universe · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Heating the gas raises its temperature, increasing the average kinetic energy and speed of the particles.

The faster particles collide with the walls more often.

They also strike the walls with greater force.

Pressure is the perpendicular force per unit area on the walls, so more frequent and harder collisions give a higher pressure.

Examiner tips

  • Use the term 'average kinetic energy' and link it to temperature. Show the chain: temperature ↑ → speed ↑ → collision frequency ↑ and force ↑ → pressure ↑. Mention that volume is constant, so no change in area of walls.
  • Use concise, exam‑style language and avoid unnecessary words.

Common mistakes

  • Failing to mention that temperature rise increases kinetic energy. Confusing pressure with force or area separately. Omitting the link between collision frequency/force and pressure.

Mark scheme (4 marks)

  1. Heating the gas increases the temperature, which increases the average kinetic energy (and speed) of the gas particles.
  2. The faster-moving particles collide with the walls of the container more frequently.
  3. The particles also collide with the walls with greater force (harder).
  4. Pressure is the perpendicular force per unit area on the container walls, so more frequent and harder collisions result in greater pressure.

Key terms in this question

pressure · gas particles

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