Changing of solid to gaseous state (Sublimation)

Sublimation is a direct change from the solid state to gaseous state, without passing through the liquid state. If a mixture contains two solids, one sublimable, and the other non-sublimable, they can be separated by the process of sublimation.                    

The solid that sublimes, escapes in the form of vapours. When these vapours are collected on a cold surface, they condense to form the sublimable solid, known as the sublimate.

Some solid substances when heated get converted directly to the gaseous or vapour state without first passing through the liquid state. When this vapour or gas is cooled, it gets converted directly into a solid, without passing through the liquid state. Such change of state of a solid directly into gas or vapour and back to the solid state, without passing through the liquid state, is called as sublimation. When a sublimable solid substance is heated, it is said to ‘sublime’ into a gaseous state; and when sublimable substances are cooled from their vapour state, the solid obtained is called the ‘sublimate’. Some sublimable substances are: iodine, camphor, naphthalene, dry ice carbon dioxide) etc.

Importance

The technique of sublimation is used for separating such solid substances which sublime on heating from the non volatile substances. e.g. ammonium chloride can be separated from the sodium chloride.