Explain, using the kinetic model of an ideal gas, why the pressure exerted by a fixed mass of ideal gas decreases when its volume is increased at constant temperature.

IB DP Physics Higher Level (2023 syllabus) — B.3 Gas laws · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Gas molecules collide with the walls of the container, and the rate of these collisions produces the pressure.
At constant temperature the average kinetic energy (and hence average speed) of the molecules remains unchanged.
When the volume increases, molecules must travel a greater distance between successive collisions with the walls.
The frequency of collisions per unit area of the wall therefore decreases, so the pressure decreases.

Examiner tips

  • Use the kinetic theory language (collisions, frequency, area).
  • Show the chain: larger V → longer mean free path → fewer collisions per unit time → lower pressure.
  • Mention constant temperature keeps speed unchanged.

Common mistakes

  • Confusing volume change with temperature change.
  • Failing to state that speed remains constant at constant T.

Mark scheme (4 marks)

  1. Gas molecules collide with the walls of the container, and the rate of these collisions produces the pressure.
  2. At constant temperature, the average kinetic energy (and hence average speed) of the molecules remains unchanged.
  3. When the volume increases, molecules must travel a greater distance between successive collisions with the walls.
  4. The frequency of collisions per unit area of the wall therefore decreases, so the pressure decreases.

Key terms in this question

kinetic model · ideal gas · pressure

Related

More Gas laws questions

▶ Try answering this question with AI marking (free) →