Sublimation

Sublimation is the change of a solid directly into a gas without passing through the liquid state. Malay: pemejalwapan · Chinese: 升华.

Most substances melt into a liquid and then boil into a gas, but a few skip the liquid stage entirely and turn straight from solid to gas when heated. In terms of the kinetic theory, the particles gain enough energy to break away from the solid completely, without first forming the looser arrangement of a liquid.

Example. When iodine crystals are gently heated, the grey solid changes directly into a purple vapour without any liquid forming, a classic demonstration of sublimation. Solid carbon dioxide, known as dry ice, also sublimes at ordinary pressure, which is why it produces a cold “smoke” and leaves no puddle. Ammonium chloride is another substance that sublimes.

Confusion to avoid. The key point is that sublimation misses out the liquid state, so do not describe it as melting followed by boiling. The reverse change, in which a gas turns directly into a solid without becoming a liquid, is called deposition, and students sometimes mix the two directions up. Only certain substances sublime; most do not.

It helps to picture the changes of state as a set: melting and freezing between solid and liquid, boiling and condensation between liquid and gas, and sublimation and deposition directly between solid and gas. Sublimation and its reverse are the two that bypass the liquid altogether, which is exactly what makes them stand out and worth learning by name.

In the Matter and the Atomic Structure chapter, sublimation is one of the named changes of state, alongside melting, freezing, boiling and condensation. Being able to place it correctly on a diagram of the changes of state, and to give a clear example, is exactly the kind of precise recall the exam rewards.