A sealed, rigid container holds a fixed quantity of gas. The container is placed in a freezer and the temperature of the gas decreases. Explain, in terms of particles, why the pressure of the gas decreases as its temperature falls.

WJEC A-Level Physics (Wales) — 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)

Temperature is a measure of the average kinetic energy of the particles.
As the temperature falls, the average kinetic energy (and speed) of the particles decreases.
The slower particles collide with the container walls with less force.
They also collide with the walls less frequently.
Pressure is the perpendicular force per unit area on the container walls, so with less force and fewer collisions, the pressure decreases.

Examiner tips

  • Use the word ‘average kinetic energy’ to link temperature and particle speed. Show the chain: lower temperature → lower KE → slower particles → less force & fewer collisions → lower pressure. Mention both force and frequency of collisions to hit all 5 marks.

Common mistakes

  • Forgetting to mention the decrease in collision frequency. Using vague terms like ‘less energy’ without specifying kinetic energy. Mixing up temperature with pressure or volume in the explanation.

Mark scheme (5 marks)

  1. Temperature is a measure of the average kinetic energy of the particles
  2. As temperature decreases, the average kinetic energy (and speed) of the particles decreases
  3. Slower particles collide with the container walls with less force
  4. The particles also collide with the walls less frequently
  5. Pressure is the perpendicular force per unit area on the container walls, so with less force and fewer collisions, the pressure decreases

Key terms in this question

pressure · temperature · particles

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