A sealed syringe contains a fixed mass of gas at room temperature. The plunger is pushed inward, reducing the volume of the gas. The temperature of the gas stays constant throughout. Explain, in terms of the molecules of the gas, why the pressure of the gas increases when its volume is decreased.

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).

Model answer (5 marks)

The gas molecules are in constant random motion and collide with the walls of the syringe. Pressure is produced by the momentum transferred in these collisions. When the plunger is pushed in, the volume available to the same number of molecules is reduced. Consequently, the molecules are confined to a smaller space and they reach the walls more often. The increased frequency of collisions gives a larger total force on the walls per unit area, so the pressure rises.

Examiner tips

  • Use the word "collide" and "momentum" to link motion to pressure.
  • Show the chain: smaller volume → more collisions per unit time → greater force per area → higher pressure.
  • Mention that temperature is constant, so kinetic energy per molecule stays the same.
  • Keep the answer concise – 5 points can be covered in 3–4 sentences.

Common mistakes

  • Failing to mention that pressure comes from collisions, not just force.
  • Confusing volume decrease with temperature increase – students sometimes say kinetic energy rises.
  • Using vague terms like "more molecules" instead of "more collisions".

Mark scheme (5 marks)

  1. The gas molecules are moving (randomly) and collide with the walls of the syringe
  2. Pressure is caused by these collisions (with the walls)
  3. When volume decreases, the same number of molecules are in a smaller space
  4. Molecules collide with the walls more frequently / more often
  5. More frequent collisions means a greater force on the walls, so pressure is higher

Key terms in this question

pressure · volume · molecules

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