Sublimation is the change of a substance from the solid state directly to a vapor. Sublimation can be used to purify solids that readily vaporize. An impure substance is heated so that it vaporizes, leaving nonvolatile impurities behind. When the flask containing the solid is heated, the solid initially sublimes. The vapor then condenses on the cold finger.
Condensation is the conversion of a vapor to a solid.
During the sublimation process it is important to use a vacuum so that pressure from the vapor does not build up inside the flask. Many substances contain various impurities, however, pure substances are usually preferred in order to better predict the outcome of reactions with accurate stoichiometry. Crystallization and sublimation are two methods of purifying a substance. They are especially important in determining an unknown substance because a pure sample is necessary in order to determine an accurate melting point to identify the unknown. If a solid substance does not readily vaporize, crystallization would be the favored method of purification.