Sublimation – IGCSE Chemistry Definition

Sublimation is the direct change of state from solid to gas without passing through the liquid state. The 0620 syllabus requires you to name and define this process, give examples, and explain it in terms of particles. Sublimation questions often appear on Paper 1 multiple-choice and occasionally as part of a separation or purification context on Paper 4.

The 0620 definition

Sublimation is the change of state directly from solid to gas without becoming a liquid first. The reverse process (gas directly to solid) is called deposition.

Particle explanation

In a solid, particles are held in fixed positions by intermolecular forces. During sublimation:

1. Particles at the surface gain enough kinetic energy to completely overcome all intermolecular forces
2. They escape directly into the gas phase without the substance ever existing as a liquid
3. This happens because, for certain substances at normal atmospheric pressure, the energy needed to break free is reached before the structure can exist as a liquid

Sublimation is endothermic — energy is absorbed to overcome the intermolecular forces.

Key examples

Substance	Observation on heating
Iodine, I₂	Purple/grey solid produces purple vapour directly; vapour deposits as shiny crystals on a cold surface
Carbon dioxide (dry ice)	White solid turns directly to colourless gas at -78 degrees C; no liquid at atmospheric pressure
Ammonium chloride, NH₄Cl	White solid appears to sublime (actually decomposes into NH₃ and HCl which recombine on a cool surface)

The iodine example is the most commonly tested. Examiners expect you to describe the purple vapour and the crystals forming on a cold surface placed above the solid.

Sublimation in separation

Sublimation can be used to separate a mixture when one component sublimes and the other does not. For example, a mixture of iodine and sand can be separated by gently heating — the iodine sublimes and can be collected on a cold surface, leaving the sand behind.

Worked exam question

Iodine crystals are gently heated in a beaker covered with a watch glass containing cold water. (a) Describe what you would see. (2) (b) Name the change of state taking place. (1) (c) Explain why this method could separate iodine from a mixture with sand. (2)

Mark scheme

(a) Purple vapour/gas is produced [1]; purple/grey/shiny crystals form on the (cold) watch glass [1]

(b) Sublimation [1]

(c) Iodine sublimes / changes directly from solid to gas (but sand does not) [1]; iodine vapour deposits on the cold watch glass and can be collected separately [1]

Common exam mistakes

• Calling the purple vapour “smoke”. Smoke is solid particles suspended in gas. Iodine produces a vapour — a gas.
• Forgetting to mention the cold surface in the collection step. Without a cold surface, the vapour simply disperses and is not recovered.
• Confusing sublimation with evaporation. Sublimation is solid directly to gas. Evaporation is liquid to gas at the surface.