Sublimation is a process in which a substance transitions directly from the solid phase to the gas phase without passing through an intermediate liquid phase). This process is an endothermic phase transition that occurs at a temperature and pressure below the triple point of the substance. Sublimation is an important technique used by chemists to purify compounds. A solid is typically placed in a sublimation apparatus and heated under vacuum. Under this reduced pressure, the solid volatilizes and condenses as a purified compound on a cooled surface, leaving a non-volatile residue of impurities behind). Deposition is the reverse of sublimation, in which a gas is directly converted into a solid-state.