 sublimation is the transition of a substance directly from the solid to the gas phase without passing through an intermediate liquid phase. sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram. the reverse process of sublimation is desublimation, or deposition. 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 Â°c). for some substances, such as carbon and arsenic, 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. ==examples== thumb|400px|comparison of phase diagrams of carbon dioxide (red) and water (blue) showing the carbon dioxide sublimation point (middle-left) at 1 atmosphere. as dry ice is heated, it crosses this point along the bold horizontal line from the solid phase directly into the gaseous phase. water, on the other hand, passes through a liquid phase. ===carbon dioxide===  solid carbon dioxide (dry ice) sublimes everywhere along the line below the triple point (e.g., at the temperature of â78.5 Â°c (194.65 k, ) at atmospheric pressure), whereas its melting into liquid co2 can occur only along the line at pressures and temperatures above the triple point (i.e., 5.2 atm, â56.4 Â°c). ===water=== snow and ice sublime, although more slowly, at temperatures below the freezing/melting point temperature line at 0 Â°c for most pressures; see line below triple point.[http://www.appstate.edu/~perrylb/research/snow/snow_density/faasnacht_2004.pdf s. r. fassnacht, estimating alter-shielded gauge snowfall undercatch, snowpack sublimation, and blowing snow transport at six sites in the coterminous usa, hydrol. process. 18, 3481â
Alchemical_processes Atmospheric_thermodynamics Chemical_processes Gases Laboratory_techniques Phase_transitions Separation_processes