When a solid is heated to its melting point, it turns into a liquid. Explain, in terms of particles, what happens to the arrangement and movement of particles during melting, and why the substance can now flow.

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)

During melting the particles gain kinetic energy from the heat supplied. This energy overcomes the forces that keep the particles in a fixed, regular arrangement, so the ordered structure breaks down. The particles remain close together but are no longer held in fixed positions; they can move, slide and flow past one another. Because of this ability to move relative to each other, the substance can flow and takes the shape of its container.

Examiner tips

  • Use the word "gain" for kinetic energy and "overcome" the forces; mention the loss of a fixed arrangement; state that particles remain close but can move; link this to the ability to flow and take the shape of the container.
  • Include all four points in a logical order: energy gain → loss of order → close but mobile particles → flow.
  • Use the exact terminology "kinetic energy", "fixed arrangement", "flow".

Common mistakes

  • Writing that particles become far apart or that the liquid is a gas; confusing melting with boiling; omitting the link between particle movement and the ability to flow.

Mark scheme (4 marks)

  1. During melting, the particles gain (kinetic) energy (from heating)
  2. The regular/fixed arrangement of particles breaks down / particles are no longer held in fixed positions
  3. Particles in a liquid are still close together but can move/slide/flow past one another
  4. Because particles can move/slide past one another, the liquid can flow / takes the shape of its container

Key terms in this question

melting point · liquid · particles

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