A fixed quantity of gas is sealed inside a container at a constant temperature. The volume of the container is gradually reduced. Explain, in terms of particles, why the pressure of the gas increases as the volume decreases.

Eduqas A-Level Physics — 2.3 Capacitance (A-Level only) · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The gas particles collide with the walls of the container, producing pressure.
When the volume is reduced the particles are forced closer together, so the space between them is smaller.
This means that the particles collide with the walls more frequently – there are more collisions per second.
Because the temperature is constant, the average kinetic energy (and therefore the speed) of the particles does not change.
A higher collision frequency gives a greater perpendicular force per unit area on the walls, so the pressure increases.

Examiner tips

  • Use the word "collide" and mention "walls" to show understanding of pressure generation.
  • Explain that volume reduction brings particles closer, increasing collision frequency.
  • State that temperature constancy keeps kinetic energy constant – avoid saying speed changes.
  • Link higher collision frequency to greater force per unit area, giving higher pressure.

Common mistakes

  • Saying the particles move faster when volume decreases – temperature is constant, so speed stays the same.
  • Omitting the step that reduced volume brings particles closer together, so collisions increase.
  • Using vague terms like "more pressure" without explaining the mechanism of collision frequency.

Mark scheme (5 marks)

  1. The gas particles collide with the walls of the container, producing pressure.
  2. When the volume decreases, the particles are closer together / the space between particles is reduced.
  3. The particles collide with the walls more frequently / there are more collisions per second with the walls.
  4. Temperature is constant, so the average kinetic energy / speed of the particles remains the same.
  5. Greater frequency of collisions means a greater (perpendicular) force per unit area on the walls, so pressure increases.

Key terms in this question

pressure · volume · temperature

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