Use the kinetic theory of matter to explain what happens to the particles in a gas when the gas is compressed into a smaller volume at constant temperature.

WJEC GCSE Physics (Wales) — 1.8 Kinetic theory · 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 are forced closer together, so the space between them decreases.
The particles collide with the walls more frequently, giving more collisions per second.
Because the temperature is constant, the average speed (and kinetic energy) of the particles remains unchanged.
Consequently, the pressure of the gas increases.

Examiner tips

  • Use the phrase "forced closer together" to show compression. Mention "more collisions per second" to link to pressure. State that temperature constancy keeps kinetic energy constant. Finish with "pressure increases" as the conclusion.

Common mistakes

  • Failing to mention that the average speed stays the same. Confusing volume decrease with temperature rise. Using vague terms like "more particles" instead of "more collisions".

Mark scheme (4 marks)

  1. The gas particles are closer together / the space between particles decreases
  2. The particles collide with the walls more frequently / more collisions per second with the walls
  3. Because temperature is constant, the average speed / kinetic energy of the particles remains the same
  4. Therefore the pressure of the gas increases

Key terms in this question

kinetic theory · temperature · particles

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