A sealed rigid container holds a fixed quantity of gas. The container is placed in a freezer, causing the temperature of the gas to decrease. Explain, in terms of particles, what happens to the pressure of the gas as its temperature 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)

As the temperature falls, the average kinetic energy of the gas particles decreases.
The particles move more slowly.
They collide with the walls less frequently.
Each collision exerts a smaller force on the walls (particles hit with less force).
Consequently the pressure decreases, because pressure is force per unit area and both the force and the collision frequency are reduced.

Examiner tips

  • Use the word ‘decreases’ for both kinetic energy and pressure. Show the chain: T↓ → KE↓ → speed↓ → collision frequency↓ → force↓ → P↓. Mention that pressure is force per unit area.
  • Use the exact wording from the mark scheme – e.g. ‘average kinetic energy’, ‘collide with the container walls’, ‘force per unit area’.
  • Include all five points in the correct order.

Mark scheme (5 marks)

  1. As temperature decreases, the average kinetic energy of the gas particles decreases
  2. The particles move more slowly
  3. The particles collide with the container walls less frequently
  4. Each collision exerts a smaller force on the walls (particles hit with less force)
  5. Therefore the pressure decreases (because pressure is force per unit area and both force and collision frequency are reduced)

Key terms in this question

pressure · temperature · particles

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