Freezing – IGCSE Chemistry Definition

Freezing is the change of state from liquid to solid. On the 0620 syllabus it appears in questions on changes of state, cooling curves, and the identification of pure substances by their sharp freezing points. Understanding what happens to particles and energy during freezing is essential for Paper 2 and Paper 4 structured questions.

The 0620 definition

Freezing is the change of state from liquid to solid, occurring at a fixed temperature called the freezing point. For a pure substance, the freezing point equals the melting point.

Particle explanation

In a liquid, particles are close together but can move past one another. As the liquid is cooled:

1. Particles lose kinetic energy and slow down
2. Intermolecular forces of attraction hold particles in fixed positions
3. Particles can only vibrate about fixed positions — the substance is now a solid

During freezing, the temperature remains constant because the energy being removed is the energy released as intermolecular forces strengthen, not the kinetic energy of the particles.

Freezing on cooling curves

On a cooling curve for a pure substance, freezing appears as a horizontal plateau at the freezing point. During this section:

• Temperature is constant
• The substance exists as both liquid and solid
• Energy is released to the surroundings (exothermic process)
• All the liquid must freeze before the temperature drops further

For an impure substance or mixture, the plateau is replaced by a sloped region and the freezing point is lower than that of the pure substance. This is how you distinguish a pure substance from a mixture on a cooling curve.

Pure vs impure freezing behaviour

Feature	Pure substance	Mixture / impure substance
Freezing point	Sharp, fixed temperature	Range of temperatures (lower than pure)
Cooling curve shape	Flat horizontal plateau	Sloped, no clear plateau
Use in identification	Melting/freezing point matches data book value	Does not match — indicates impurity

Worked exam question

A student cools liquid naphthalene and records the temperature every minute. The cooling curve shows a flat section at 80 degrees C. (a) What is the freezing point of naphthalene? (1) (b) Explain why the temperature remains constant during freezing. (2) (c) What would the cooling curve look like if the naphthalene were impure? (1)

Mark scheme

(a) 80 degrees C [1]

(b) Energy is being released as intermolecular forces form / strengthen [1]; the kinetic energy of the particles is not changing, so temperature stays constant [1]

(c) The flat section would be replaced by a slope / the freezing would occur over a range of temperatures (below 80 degrees C) [1]

Common exam mistakes

• Saying “bonds form” during freezing instead of specifying that intermolecular forces strengthen. Covalent bonds within molecules are not affected.
• Forgetting that freezing is exothermic. Energy is released, not absorbed — the reverse of melting.
• Confusing the flat section of a cooling curve. The first plateau (higher temperature) is condensation; the second plateau (lower temperature) is freezing.