A fixed quantity of gas is trapped inside a syringe. The plunger is pushed in slowly so that the volume of the gas decreases, while the temperature of the gas stays the same. Explain, using kinetic theory, why the pressure of the gas increases as the volume decreases.

WJEC GCSE Physics (Wales) — 2.2 The kinetic theory and gas pressure · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The gas particles collide with the walls of the syringe, and these collisions cause the pressure of the gas.
As the volume decreases, the particles are confined to a smaller space.
This means the particles collide with the walls more frequently.
More frequent collisions give a greater force per unit area on the walls, so the pressure is higher.

Examiner tips

  • Use the word "collide" and "frequency" to show kinetic theory. Show the link: smaller V → more collisions → higher pressure. Keep the answer concise and use UK spelling.

Common mistakes

  • Failing to mention collisions or frequency. Using vague terms like "more particles" instead of "more collisions". Omitting the link between force per unit area and pressure.

Mark scheme (4 marks)

  1. The gas particles collide with the walls of the syringe, and these collisions cause the pressure of the gas.
  2. As volume decreases, the particles are closer together / confined to a smaller space.
  3. So the particles collide with the walls more frequently / more often.
  4. More frequent collisions means a greater force per unit area on the walls, so the pressure is higher.

Key terms in this question

pressure · kinetic theory · volume · temperature

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