A sealed syringe contains a fixed quantity of air at room temperature. The plunger is slowly pushed inward, reducing the volume of the air. The temperature of the air stays constant throughout. Explain what happens to the pressure of the air inside the syringe as the volume decreases.

WJEC GCSE Physics (Wales) — 2.3 Forces and motion · 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 air particles collide with the walls of the syringe more frequently because the same number of particles are now confined to a smaller volume. This increases the force exerted on the walls per unit area. Consequently, the pressure inside the syringe rises – pressure and volume are inversely proportional.

Examiner tips

  • Use the term ‘collide more frequently’ to show understanding of kinetic theory.
  • Explain that the number of particles is constant and the volume decreases.
  • Show that force per area (pressure) increases.
  • Link the result to Boyle’s law (P∝1/V).

Common mistakes

  • Confusing pressure with force – forgetting to divide by area.
  • Saying the temperature changes – it remains constant.
  • Using vague terms like ‘more pressure’ without explaining why.

Mark scheme (4 marks)

  1. The particles collide with the walls of the syringe (more frequently / more often)
  2. This is because the same number of particles are now in a smaller volume / space
  3. The force on the walls per unit area increases
  4. Therefore the pressure increases / pressure and volume are inversely proportional

Key terms in this question

pressure · volume

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