Explain why a gas exerts a pressure on the walls of its container, and why this pressure increases when the gas is heated at constant volume.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.1 States of matter · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Gas particles move randomly in all directions and collide with the walls of the container. Each collision exerts a force on the wall, and the cumulative effect of many such forces produces pressure. When the gas is heated at constant volume, the particles gain kinetic energy and move faster. The faster particles collide with the walls more frequently and with greater force, so the pressure increases.

Examiner tips

Use the word "exert" to link collisions to pressure. Mention both frequency and force of collisions when explaining the rise in pressure. Keep the answer concise – 4 marks can be earned with 2–3 short sentences. Use UK spelling (e.g. "exert" not "exerts").

Common mistakes

Forgetting to mention that the volume is constant. Confusing temperature with pressure. Using vague phrases like "the gas expands" instead of describing kinetic energy and collision frequency.

Mark scheme (4 marks)

Gas particles move randomly (in all directions) and collide with the walls of the container
These collisions (with the walls) exert / create a force on the walls, which produces pressure
When heated, particles gain (kinetic) energy and move faster
Faster-moving particles collide with the walls more frequently and/or with greater force, so pressure increases

Key terms in this question

gas · pressure