The vaporization of a solid directly on heating without passing through the liquid phase and condensation of these vapors on cooling to solid without passing through a liquid phase is called sublimation. usually, substances with low intermolecular forces are sublime (i.e) liquid state is of a very short time, not easily observable, hence it is considered that solid directly converts into a gas without changing its liquid state.
The following are some important substances which can be sublimed.
- ammonium chloride,
- benzoic acid.
It is a separation and purification technique. Only those compounds show this property which has high vapor pressure at a temperature below their melting point.
The solid substance being sublimed is called sublimand.
The pure solid obtained after sublimation is called sublimate.
Process of sublimation:
in this sublimation process, a substance to be sublimed is taken in a watch glass which is covered with an inverted funnel. The stem of the funnel is closed with a cotton plug. Heat the substance slowly on the sand bath Fig (2 4). At the same time cool the funnel with wet cotton. The compound sublimes and the pure solid deposits on the inner side of the funnel. In a better method, the process is carried out in a cold finger.
A cold finger is a glass tube filled with ice which is fitted into the top opening of the flask with a rubber adaptor. The sample to be sublimed is put on the bottom of the flask. It is sublimed by heating and sublimate is deposited on a chilled piece of a cold finger which is just above the bottom of the flask. After completing the sublimation process, the cold finger is taken out of the flask and the sublimate is removed with the help of a spatula.