A student fills a rigid metal container with air and seals it. The container is then placed in a pan of boiling water. Explain why the pressure of the gas inside the container increases as the temperature rises.

WJEC A-Level Physics (Wales) — 4.1 Thermal physics · Explain · 5 marks · View as Markdown

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

A rigid metal container is sealed at room temperature and then heated in boiling water.

Model answer (5 marks)

The gas molecules gain kinetic energy as the temperature rises, so they move faster.
They collide with the walls of the rigid container more frequently.
Each collision is more forceful because the molecules have higher momentum.
The volume of the container does not change – it is rigid.
The increased frequency and force of collisions per unit area raise the pressure inside the container.

Examiner tips

  • Use the term ‘kinetic energy’ and ‘momentum’ to show understanding of molecular motion. Explain that the container is rigid so volume is constant – this is key to the pressure change. Show the link between faster, more frequent, and more forceful collisions and pressure.
  • common_mistakes
  • :
  • Confusing temperature with pressure or volume. Forgetting to mention that the container is rigid. Using vague phrases like ‘the gas gets hotter’ without linking to kinetic energy.

Mark scheme (5 marks)

  1. The gas molecules move faster (have greater kinetic energy) when temperature increases
  2. The molecules collide with the walls of the container more frequently
  3. The molecules hit the walls of the container with greater force
  4. The volume of the container stays constant (because it is rigid)
  5. More frequent and more forceful collisions per unit area of the walls result in greater pressure

Key terms in this question

pressure · temperature

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