The six states of matter

Between solid, liquid and gas, there are six possible transitions, organised into three pairs of transformations that are inverse to one another.

Table of the six names

Phase transition	Name	Reverse phase transition	Name
solid → liquid	melting	liquid → solid	solidification
liquid → gas	vapourisation	gas → liquid	liquefaction
solid → gas	sublimation	gas → solid	condensation

Each row forms a reversible pair: what happens in one direction can, in theory, be reversed in the other simply by changing the direction of heat transfer.

Examples for each case

• Melting / solidification: an ice cube melting on the table (melting); liquid water placed in the freezer turning back into ice (solidification).
• Vapourisation / liquefaction: water boiling in a saucepan (vapourisation); mist forming on a cold mirror, as water vapour in the air turns back into a liquid (liquefaction).
• Sublimation / condensation: dry ice (solid carbon dioxide) that vanishes directly into white smoke without ever becoming liquid (sublimation); frost that forms directly on a windscreen on a cold night, as water vapour in the air changes directly to a solid state (condensation).

Beware of the most common confusion

In everyday language, we often use the term ‘condensation’ to describe the mist that forms on a window — but this phenomenon is actually liquefaction (gas to liquid). In chemistry, the word ‘condensation’ specifically refers to the direct transition from gas to solid, without passing through a liquid stage, as with frost. Using precise terminology helps to avoid many misinterpretations in exercises.