Use the kinetic theory of matter to explain why a gas fills any container it is placed in, but a liquid only fills the bottom of a container.

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)

In a gas the particles move in random directions and have almost no forces of attraction between them, so they are not held together and can spread out to fill the whole container.
In a liquid the particles are still attracted to each other by significant intermolecular forces, which keep them close together. The particles can move or flow past one another but cannot overcome the attractive forces to spread out and fill the whole container, so the liquid only occupies the bottom of the vessel.

Examiner tips

  • Use the phrase "negligible intermolecular forces" for gases and "significant intermolecular forces" for liquids. Show the contrast between free movement and restricted movement. Mention that liquids can flow but not disperse.
  • common_mistakes
  • :
  • Confusing the behaviour of solids with liquids. Saying that liquids have no forces of attraction. Failing to explain why gases fill the whole container.

Mark scheme (4 marks)

  1. In a gas, particles move in random directions / randomly in all directions
  2. Gas particles have (almost) no forces of attraction between them / negligible intermolecular forces, so they are not held together
  3. In a liquid, particles are still attracted to each other / have forces of attraction between them that keep them close together
  4. Liquid particles can move/flow/slide past each other but are not free to move far apart / cannot overcome the attractive forces to fill the whole container

Key terms in this question

kinetic theory

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