Sublimation

A small handful of solid substances -- benzoic acid, naphthalene and camphor are the standard examples -- pass DIRECTLY from the solid state to vapour when heated, without ever passing through a liquid phase, and give back the same solid when that vapour is cooled again. This phenomenon is sublimation, and it is a genuinely useful way to separate a volatile solid from any NON-volatile impurity mixed in with it, though its usefulness is inherently limited, because only a small number of substances actually sublime cleanly at all.

Practically, the impure substance is placed in a beaker, which is then covered with a watch glass and heated gently for a while; the vapours given off condense back to solid crystals on the cool underside of the watch glass, which is then simply lifted off and the pure crystals collected from it. This straightforward version of the technique works for substances that have a genuinely high vapour pressure at a temperature below their own melting point (naphthalene and benzoic acid sublime quite quickly this way). For a substance with only a very small vapour pressure -- one that would tend to decompose if simply heated harder to force it to sublime faster -- sublimation is instead carried out under …