Explain why a gas exerts a pressure on the walls of its container, and why this pressure increases when the same mass of gas is compressed into a smaller volume at constant temperature.

AQA A-Level Physics (7408) — 3.6.2 Thermal Physics · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Gas molecules are in continuous random motion. They collide with the walls of the container, and each collision exerts a force on the wall. Pressure is the force per unit area, so the many collisions give rise to a pressure on the walls. If the same mass of gas is compressed into a smaller volume while the temperature remains constant, the molecules are closer together and have less space to move. Consequently they collide with the walls more frequently, increasing the force per unit area and therefore the pressure.

Examiner tips

  • Use the phrase "continuous random motion" and "collide with the walls" to show understanding of kinetic theory.
  • Explain that pressure is force per unit area and link collisions to force.
  • Show that reducing volume at constant temperature increases collision frequency, not just density.
  • Keep the answer concise and use correct UK spelling.

Common mistakes

  • Failing to mention that pressure is force per unit area.
  • Confusing temperature change with volume change – stating pressure rises because temperature rises.
  • Omitting the role of collision frequency in the pressure increase.

Mark scheme (5 marks)

  1. Gas molecules are in continuous (random) motion
  2. Molecules collide with the walls of the container
  3. Each collision exerts a force on the wall, and pressure arises from these forces (over the area of the wall)
  4. When volume decreases (at constant temperature), molecules are closer together / have less space to move
  5. Molecules collide with the walls more frequently, so pressure increases

Key terms in this question

pressure · volume · temperature

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