The Six Changes of State — States of Matter | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable

Low

Stable6%

Appears regularly as short-answer questions on particle diagrams and state changes.

Process	Direction	Energy
Melting	solid → liquid	absorbed
Boiling / evaporation	liquid → gas	absorbed
Sublimation	solid → gas	absorbed
Freezing	liquid → solid	released
Condensation	gas → liquid

Exam tip

Naming a change of state

Naming a change of state from its direction comes up on almost every paper, so you need to know all six: melting (s→l), freezing (l→s), boiling/evaporation (l→g), condensation (g→l), sublimation (s→g), deposition (g→s). Read the direction carefully — freezing and condensation are the easiest pair to swap.

Melting

Solid changes to liquid
Particles gain kinetic energy and escape their fixed lattice positions, starting to slide
Happens at one specific temperature, the point (m.p.)
A pure substance has a sharp m.p. (a standard purity test)

Freezing

Liquid changes to solid (the reverse of melting)
Particles lose kinetic energy and lock back into a fixed arrangement
For a pure substance, freezing point = melting point
- Water freezes / ice melts at 0 °C

Boiling

Liquid changes to gas at one specific temperature, the point (b.p.)
Vapour bubbles form throughout the bulk of the liquid
The temperature of the liquid stays at the b.p. while boiling continues
Continuous heating is needed to maintain the change

Evaporation

Liquid changes to gas at any temperature the b.p.

Exam tip

Explain why a liquid evaporates faster (e.g. hot water vs cold)

Write (two marks): (1) the particles gain more kinetic energy and move faster; (2) so a greater number of surface particles have enough energy to break free and overcome the forces of attraction.

Watch out: both points are needed — just saying "the particles move faster" without "more particles escape" usually scores only one mark. Evaporation happens only at the surface and at any temperature below the boiling point, unlike boiling.

Condensation

Gas changes to liquid on cooling
Particles lose kinetic energy, and attractive forces hold colliding particles together
Occurs over a range of temperatures, not at one fixed value

Sublimation and deposition

= solid → gas, skipping the liquid state
- Solid iodine (purple vapour on warming)
- Solid carbon dioxide ("dry ice")
(also "desublimation") = gas → solid, skipping the liquid state
- Frost forming on cold glass from water vapour