Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state; this transformation occurs with heat acquisition and is, therefore, an endothermic process. Usually, in normal environmental conditions, to pass from solid to gaseous state it is necessary to go through the liquid state. Under certain pressure conditions, as the temperature changes, it is possible to pass directly from the solid to the gaseous state.
The temperature- and pressure-dependent phenomenon of sublimation is favored by low pressures and high temperatures. Sublimation occurs in all bodies, but only a few at ordinary temperatures and pressures. Common substances that exhibit sublimation at ordinary temperatures and pressures include iodine and solid carbon dioxide, the common dry ice. Sublimation finds use in the purification of various sublimatable substances: thus, pure commercial iodine is often referred to as bisublimated iodine because it is generally obtained through two successive sublimation operations. In particular, the heat of sublimation is said to be the amount of heat that the mass unit of a substance must absorb or give up in order to carry out this change of state.
When can a solid sublimate?
Even solids, like liquids, have a vapor pressure which is indicated as sublimation pressure and which varies with temperature according to the Clapeyron equation; in solids, the sublimation pressures values are in most cases extremely small since the binding energies are, with the same experimental conditions, greater than in the liquids.
Some solids, such as camphor, naphthalene, and iodine, at temperatures below their melting temperature, are characterized by a vapor pressure higher than atmospheric and tend to go directly from the solid to the gaseous/aeriform state.
Only a few molecular solids, in which the values of the binding energies between the molecules are modest, have an appreciable sublimation pressure even at low temperatures.
Sublimation occurs then in those substances in which the vapor pressure equals the value of atmospheric pressure before reaching the melting temperature. All solids can sublimate, but it is necessary to bring them under suitable conditions of pressure and temperature, since most of them, at atmospheric pressure, when heated above their melting temperature, passes into the liquid state.