Sublimation

Sublimation

Some substances when heated, pass directly from the solid to the vapour state without melting. The vapours when cooled give back the solid substance. This process known as sublimation is very helpful in separating volatile from non-volatile solid. It is, however, of limited application as only a few substances like naphthalene, camphor, and benzoic acid can be purified by this process.

The impure substance is placed in a large beaker (Fig 2.3). The beaker is covered with a watch glass. Heat is applied gently to the beaker, and the resulting vapours condense as crystals on the bottom surface of the watch glass. The watch glass can be removed after sometime and the crystals collected.

Organic substances such as benzoic acid, naphthalene etc., which have high vapour pressure at temperatures below their melting points, can be sublimed relatively quickly. These can be conveniently purified by the method described above. Substances which have very small vapour pressure or tend to decompose upon heating, are purified by sublimation under reduced pressure.

The glass apparatus used for sublimation under reduced pressure is shown in figure 2.4. The chief features of this apparatus are a large heating and a large cooling surface with a small distance in between this is necessary because the amount of the substance in the vapour phase is too small in case of a substance with low vapour pressure.