A fixed mass of gas is trapped inside a syringe at constant temperature. The piston of the syringe is slowly pushed inward, reducing the volume of the gas. Explain why the pressure of the gas increases as the volume decreases.

Pearson Edexcel International GCSE Physics (4PH1) — 5.3 Ideal gas molecules · Explain · 4 marks · View as Markdown

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

A fixed mass of gas is trapped inside a syringe at constant temperature. The piston is slowly pushed inward so that the volume of the gas is reduced.

Model answer (4 marks)

The gas molecules occupy a smaller volume, so they have less space to travel before colliding with the walls. Consequently, collisions with the walls become more frequent. The piston and syringe walls also present a smaller surface area for the same number of molecules. The increased collision frequency and reduced area both raise the force exerted on the walls per unit area, i.e. the pressure increases.

Examiner tips

  • Use the word ‘collide’ and ‘frequency’ to show understanding of kinetic theory; link volume reduction to collision frequency and area; explain pressure as force per area; keep answer concise and to the point.

Common mistakes

  • Confusing pressure with force or volume; not mentioning collision frequency; using vague terms like ‘more molecules’ instead of ‘same number in less space’.

Mark scheme (4 marks)

  1. The gas molecules collide with the walls of the syringe (and piston) more frequently
  2. Because the same number of molecules occupy a smaller volume / the molecules have less space to travel before hitting a wall
  3. The collisions also occur over a smaller (surface) area
  4. Greater frequency of collisions (and smaller area) means greater force per unit area, so pressure increases

Key terms in this question

pressure · volume · constant temperature

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