Sublimation

Definition:
It is conversion of a substance in solid state to a gaseous state, which is not accompanied by the formation of liquid phase.

It is similar in some aspects to the distillation process.

Process of sublimation:
At normal pressures, most of the chemical substances either compounds or elements have three different states of matter at various temperatures. In these circumstances, change from solid state to gaseous state requires a median state that is liquid.

On the other hand, some of the chemical substances such as compounds and elements at certain pressures go directly from solid state to the gaseous state. This occurs when the atmospheric pressure applied on the material is very less to inhibit the molecules from going out of the solid state i.e. a substance go through the process of sublimation only if the vapor pressure is less than that of the triple point for that substance.

Triple point:
The triple point is the point possessing a fixed pressure and temperature at which the solid, liquid and gaseous phases of a material are able to co-exist independently.

If the vapor pressure over the solid substance is more than that of the triple point, the solid will convert to vapor after passage through the liquid phase. The following phase diagram will help to understand the triple point.

Line OA:
In this diagram, line OA shows the melting points of the solid substance at different pressures. In this line, left side shows the existence of solid form while the right side shows the existence of liquid form. This line shows the points of co-existence of the solid and the liquid.

Line OB:
The line OB shows the vapor-pressure curve of the liquid at different temperatures. Above this line liquid phase exists and below this line vapor phase of the substance exists. This line shows the points of co-existence of the liquid and the vapors.

Line OC:
The line OC shows the sublimation curve of the solid. This line shows the points of co-existence of the solid and the vapors at different states of temperatures and pressure.

Point O:
In this diagram, the point O represents the triple point.

Enthalpy:
Enthalpy of sublimation is equal to the sum of enthalpy of fusion and enthalpy of vaporization.

Sublimation process represents an endothermic phase transition as shown by the phase diagram.

Examples:
Nearly all of the solids have some tendency of conversion from solid to gaseous state at a particular temperature and pressure.

Elements:
Cadmium, Zinc, Arsenic, Carbon

Compounds:
CO2 (Dry ice), NH4Cl

Uses:
Its most important use is in freeze-drying.