Use the kinetic theory of matter to explain why solids have a fixed shape and cannot be compressed, whereas gases have no fixed shape and can be compressed easily.

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 solid the particles are held in fixed positions by strong forces in a regular lattice. They can only vibrate about their fixed points, so the solid has a fixed shape and cannot be compressed.
In a gas the particles are far apart with large gaps between them. They move freely in all directions and can be pushed closer together into the gaps, so a gas has no fixed shape and can be compressed easily.

Examiner tips

  • Use the terms "fixed positions", "lattice", "vibrate", "far apart", "freely move", "compressible"
  • Show the contrast: solids have fixed shape because particles cannot move, gases have no fixed shape because particles can move and be pushed together

Common mistakes

  • Confusing the behaviour of liquids with solids; liquids can flow but still have fixed volume
  • Forgetting to mention the strong forces or the lattice structure in solids
  • Using vague terms like "tight" or "loose" instead of "fixed positions" and "far apart"

Mark scheme (4 marks)

  1. In a solid, particles are held in fixed positions (by strong forces / in a regular arrangement / lattice)
  2. This means solids have a fixed shape because particles cannot move from their positions / can only vibrate about fixed points
  3. In a gas, particles are far apart / widely spaced with large gaps between them
  4. Gas particles can be pushed closer together (into the gaps) so the gas can be compressed / has no fixed shape because particles move freely in all directions

Key terms in this question

kinetic theory · compress · fixed shape

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