sublimation

Sublimation is the phase transition in which a solid changes directly into a gaseous state, bypassing the liquid phase. This process occurs when the vapor pressure of the solid becomes equal to or exceeds the surrounding pressure at a given temperature. Sublimation typically takes place at low ambient pressures or at sufficiently high temperatures relative to the material’s melting point, and it is endothermic, requiring energy input to overcome intermolecular forces.

Unlike evaporation, which occurs at the surface of a liquid, sublimation involves no liquid phase. The rate depends on factors such as temperature, pressure, surface area, and the presence of impurities. The enthalpy of sublimation ΔH_sub equals the sum of the enthalpies of fusion and vaporization (ΔH_sub = ΔH_fus + ΔH_vap). The pressure-temperature relationship follows the Clapeyron equation, describing how sublimation pressure changes with temperature.

Common examples include dry ice (solid carbon dioxide), which sublimes at room temperature, and iodine, which sublimes readily under moderate heating. Sublimation has practical applications in freeze-drying (lyophilization), where frozen water is removed by sublimation under reduced pressure, and in purification techniques that exploit volatile solids. It also underpins certain vacuum-deposition processes in materials science, where a solid material is converted to vapor and then condensed on a substrate to form thin films.

Natural occurrences include sublimation of frost and snow on sunlit surfaces, and the gradual loss of volatile ices on comets and extraterrestrial bodies. Safety considerations emphasize that sublimed vapors may be toxic or flammable and should be handled with appropriate ventilation and containment.