A solid is heated until it melts and then continues to be heated until it boils. Explain, in terms of the movement and arrangement of particles, the difference between the solid state and the gas state.

Cambridge International IGCSE Physics (0625) — 2.1 Kinetic particle model 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)

In a solid the particles are tightly packed in a regular arrangement and can only vibrate about fixed positions, so they cannot move freely.
In a gas the particles are far apart, spread out with negligible intermolecular forces, and they move rapidly in all directions at high speed.

Examiner tips

  • Use the exact phrases: "tightly packed in a regular arrangement", "vibrate about fixed positions", "far apart, spread out", "move rapidly in all directions".
  • Show the contrast clearly – solid vs gas – to hit all four points.
  • Keep the answer short and to the point to fit the 4‑mark limit.

Common mistakes

  • Mixing up the description of liquids; describing liquids instead of solids or gases.
  • Using vague terms like "loose" or "free" without specifying vibration or rapid motion.
  • Omitting the point about negligible intermolecular forces in the gas state.

Mark scheme (4 marks)

  1. In a solid, particles are tightly packed together / in a regular arrangement
  2. In a solid, particles can only vibrate about fixed positions / cannot move freely
  3. In a gas, particles are far apart / spread out / widely spaced with negligible/very weak intermolecular forces
  4. In a gas, particles move quickly / rapidly in all directions / randomly at high speed

Key terms in this question

solid · gas · particles

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