Sublimation is the process of transformation
directly from the solid phase to the gas phase
without passing through an intermediate liquid
phase. Sublimation is an exothermic phase
transition that occurs at temperatures and
pressures below a substance's triple point in its
phase diagram.
At normal pressures, most chemical compounds and elements possess three different states at different
temperatures. In these cases, the transition from the solid to the gaseous state requires an intermediate
liquid state. Note, however, that the pressure referred to here is the partial pressure of the substance, not the
total (e.g., atmospheric) pressure of the entire system. So, all solids that possess an appreciable vapor
pressure at a certain temperature usually can sublime in air (e.g., water ice just below 0°C). For some
substances, such as carbon and arsenical, sublimation is much easier than evaporation from the melt,
because the pressure of their triple point is very high, and it is difficult to obtain them as liquids.
Sublimation requires additional energy and is an endothermic change. The enthalpy of sublimation (also
called heat of sublimation) can be calculated as the enthalpy of fusion plus the enthalpy of vaporization. The
reverse process of sublimation is deposition. The formation of frost is an example of meteorological
deposition.