The energy involved in change of state
3 marks · NormalExplain, in terms of particles, what happens to the thermal energy and movement of particles in a pure substance as it melts at its melting point.
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As a pure substance melts, the thermal energy supplied is used to weaken or break the forces holding the particles in place in the solid. The particles move apart slightly and gain enough energy to move around each other, so the substance changes from solid to liquid. The temperature remains the same, but the particles' arrangement and movement change.
Also accept: Reference to 'overcoming forces' rather than bonds
Common mistakes: Saying temperature increases at melting point; Not mentioning particle movement
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Marking points
M1 · 1 mark · Explains thermal energy breaks/weakens forces between solid particles
B1 · 1 mark · States particles move more freely/change arrangement
B2 · 1 mark · Mentions temperature remains constant during the process
Worked solution
When a pure substance reaches its melting point, the particles have enough thermal energy to overcome the strong forces holding them in a regular pattern. As a result, the structure breaks down, particles start to move past each other, and the solid turns into a liquid. During this process, the temperature does not increase further until all the solid has melted.