Question

Explain in detail the process of sublimation.

Answer 1

Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the 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. (Shown below is the phase diagram of $C{O}_2$ and water, 1 bar is the pressure at room temperature)



Triple point is the point (specific pressure and temperature) where all three states of a substance occur. If a substance is subject to conditions which take it directly from solid to gaseous state, sublimation will occur. For example, for water if pressure is kept below 611 Pa and temperature is increased then ice will sublimate to gas.

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. The pressure referred to 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 sublimate in air (e.g. water ice just below $0^{\circ }C$). 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.