Explain, using the kinetic molecular model, why the pressure exerted by a fixed mass of ideal gas decreases when some of the gas is allowed to escape from a rigid container at constant temperature.

IB DP Physics Standard 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)

Pressure is produced by collisions of gas molecules with the walls of the container. When some gas escapes from a rigid container the number of molecules remaining inside decreases. With fewer molecules there are fewer collisions per unit time per unit area with the walls. Because the temperature is kept constant the average speed (and kinetic energy) of the remaining molecules is unchanged, so the only change that lowers the pressure is the reduced collision frequency.

Examiner tips

  • Use the kinetic‑molecular explanation of pressure
  • Show that temperature constancy means speed is unchanged
  • Explain that fewer molecules = fewer collisions per area per time
  • Keep the answer concise and use the exact wording of the mark scheme

Common mistakes

  • Failing to mention that temperature is constant so speed is unchanged
  • Confusing the escape of gas with a change in temperature
  • Giving a general explanation of pressure without linking it to collisions

Mark scheme (4 marks)

  1. Pressure arises from collisions of gas molecules with the container walls
  2. When gas escapes, the number of molecules inside the container decreases
  3. With fewer molecules, there are fewer collisions per unit time per unit area with the walls
  4. Since temperature is constant, average molecular speed / kinetic energy is unchanged, so the reduction in collision frequency alone accounts for the lower pressure

Key terms in this question

kinetic molecular model · pressure · constant temperature · fixed mass

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