A sealed metal container holds a fixed quantity of gas. When the container is placed in hot water, the pressure of the gas inside increases. Use kinetic theory to explain why the pressure of the gas increases when its temperature is raised.

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)

Increasing the temperature raises the average kinetic energy of the gas molecules, so they move faster.
The faster molecules collide with the walls more often.
Each collision delivers a larger impulse because the molecules are moving faster.
The increased impulse per unit area gives a higher pressure (volume is fixed).

Examiner tips

  • Use the phrase "average kinetic energy" and link it to temperature.
  • Show the chain: higher KE → faster speed → more frequent collisions → greater force → higher pressure.
  • Mention that volume is constant, so pressure must rise.
  • Use the word "exert" or "deliver" to describe the force on the walls.

Common mistakes

  • Failing to mention that volume is constant, so pressure must increase.
  • Using vague terms like "more energy" without linking to speed or collisions.
  • Confusing pressure with force instead of force per unit area.

Mark scheme (4 marks)

  1. Increasing temperature increases the average kinetic energy (and speed) of the gas molecules
  2. The gas molecules collide with the walls of the container more frequently
  3. Each collision exerts a greater force on the walls because the molecules are moving faster
  4. Greater force per unit area on the walls means pressure increases (and volume is constant so pressure must rise)

Key terms in this question

kinetic theory · pressure · temperature

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