A sealed syringe contains a fixed amount of gas. The plunger is pushed in slowly so that the volume of the gas decreases, but the temperature of the gas stays the same. Explain why the pressure of the gas increases.

Eduqas GCSE Physics — 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. Pressure is the perpendicular force per unit area produced by these collisions. When the volume is reduced, the same number of particles occupies a smaller space, so they collide with the walls more frequently. The increased collision frequency raises the pressure.

Examiner tips

  • Use the kinetic theory explanation of pressure as collision force per area
  • Show that volume reduction keeps particle number constant and raises collision frequency
  • Mention temperature constant so kinetic energy unchanged

Common mistakes

  • Failing to state that temperature is constant, implying kinetic energy changes
  • Confusing volume change with temperature change
  • Using vague terms like "more force" without linking to collision frequency

Mark scheme (4 marks)

  1. The gas particles collide with the walls of the syringe (container)
  2. Pressure is caused by these collisions / pressure is the perpendicular force per unit area from particle collisions
  3. Decreasing the volume means the same number of particles is contained in a smaller space
  4. So particles collide with the walls more frequently, increasing the pressure

Key terms in this question

pressure · volume · temperature

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