Sublimation

Sublimation is a chemical process similar to distillation, but differing from it in the nature of the substances to which it is applied. While in distillation liquids are converted by the agency of heat into vapour, which is condensed in the liquid form usually by the cooling action of water, in sublimation solid bodies are reduced by heat to the state of vapour, which reassumes the solid form on cooling. Sublimation is usually conducted in a single vessel of glass or iron, the product being deposited in the upper part of it in a solid state, while the impure residue remains at the bottom; but in the case of sulphur the vapour is condensed on the walls of a large chamber. Iodine affords a good example of sublimation. On gently heating the lower part of a Florence flask containing a little of this substance a purple vapour rises, which almost immediately condenses in small brilliant dark purple crystals in the upper parts of the flask, while any impurity that may be present remains at the bottom. Amongst the substances obtained by this process, and employed in the Pharmacopoeia, are arsenious acid, benzoic acid, corrosive sublimate, and sublimed sulphur.