A fixed mass of gas is compressed to a smaller volume at constant temperature. Explain why the pressure of the gas increases.

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)

A fixed mass of gas is compressed to a smaller volume at constant temperature.

1. The gas molecules collide with the walls of the container, producing pressure.
2. When the volume is reduced, the molecules are confined to a smaller space and are closer together.
3. Because they are closer together, the molecules collide with the walls more frequently.
4. More frequent collisions give a greater force per unit area on the walls, so the pressure increases.

Examiner tips

  • Use the kinetic particle model terms: collisions, frequency, force per unit area.
  • Show the logical chain: smaller volume → closer molecules → more collisions → higher pressure.
  • Keep the answer concise and directly linked to the mark scheme points.

Common mistakes

  • Failing to mention that collisions with walls produce pressure.
  • Confusing temperature change with volume change; ignoring the constant temperature condition.
  • Giving a vague explanation such as “the gas gets denser” without linking to collisions and pressure.

Mark scheme (4 marks)

  1. Gas molecules collide with the walls of the container (to produce pressure)
  2. At smaller volume, molecules are closer together / the space is smaller
  3. Molecules collide with the walls more frequently
  4. More frequent collisions means a greater force per unit area / greater pressure on the walls

Key terms in this question

pressure · volume · temperature

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