A sealed metal container holds a fixed amount of gas. The container is placed in hot water and the temperature of the gas increases. Explain why the pressure of the gas inside the container increases.

Eduqas GCSE Physics — 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)

Increasing the temperature raises the kinetic energy of the gas particles, so they move faster.
The faster particles collide with the walls more often.
Each collision exerts a larger perpendicular force because the particles have higher momentum.
The greater force per unit area on the walls raises the pressure (volume is fixed, so pressure must increase).

Examiner tips

  • Use the word ‘explain’ to show cause–effect chain; include kinetic energy, collision frequency, force, and pressure relation.
  • Mention that volume is constant, so pressure change is due to force change, not volume change.
  • Use correct terms: kinetic energy, momentum, force, pressure, unit area.

Common mistakes

  • Saying only that temperature increases pressure without linking to kinetic energy and collisions.
  • Forgetting that the volume is fixed, so pressure change is not due to volume change.
  • Using vague terms like ‘more energy’ instead of ‘higher kinetic energy’ or ‘faster particles’.

Mark scheme (4 marks)

  1. Increasing temperature increases the kinetic energy (and/or speed/velocity) of the gas particles
  2. The faster-moving particles collide with the walls of the container more frequently
  3. Each collision exerts a greater (perpendicular) force on the walls because the particles 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

pressure · temperature

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