Melting – IGCSE Chemistry Definition

Melting is the change of state from solid to liquid. It is central to the 0620 syllabus for understanding heating curves, testing purity, and explaining changes of state using the kinetic particle theory. Questions on melting appear regularly across Papers 1, 2, and 4.

The 0620 definition

Melting is the change of state from solid to liquid at a fixed temperature called the melting point. For water, this is 0 degrees C at standard atmospheric pressure.

Particle explanation

In a solid, particles are arranged in a regular pattern and can only vibrate about fixed positions. When heated to the melting point:

1. Particles gain kinetic energy and vibrate more vigorously
2. At the melting point, particles gain enough energy to partly overcome intermolecular forces
3. The regular arrangement breaks down and particles can slide past each other
4. The substance becomes a liquid

During melting, the temperature stays constant because all the energy supplied goes into overcoming intermolecular forces, not into raising kinetic energy.

Melting on heating curves

On a heating curve, the melting section is a flat horizontal plateau at the melting point. During this plateau:

• Energy is continuously supplied
• Temperature does not rise
• The substance exists as both solid and liquid
• Energy overcomes intermolecular forces (endothermic process)

Melting point and purity

Observation	Conclusion
Sharp melting point matching data book	Pure substance
Melting over a range of temperatures	Impure substance / mixture
Melting point lower than data book value	Impure — impurities lower the melting point

This is one of the standard methods for testing purity in the 0620 syllabus. A sharp melting point at the expected value is strong evidence of a pure substance.

Worked exam question

A student heats a solid and records the temperature. The temperature rises, then stays constant at 63 degrees C while the solid melts, then rises again. (a) What is the melting point of this substance? (1) (b) Explain why the temperature remains constant during melting. (2) (c) The data book value for this substance is 63 degrees C. What does this suggest about the purity of the sample? (1)

Mark scheme

(a) 63 degrees C [1]

(b) Energy is used to overcome/break intermolecular forces (between particles) [1]; not used to increase kinetic energy of the particles / so temperature does not change [1]

(c) The substance is pure (because the melting point is sharp and matches the data book value) [1]

Common exam mistakes

• Writing that “covalent bonds break” during melting. For molecular substances, only intermolecular forces are overcome — the covalent bonds within molecules remain intact.
• Forgetting that melting is endothermic. Energy is absorbed from the surroundings to overcome intermolecular forces.
• Not linking melting point to purity. If a question gives you a melting point range or a value different from the data book, comment on purity.