Sublimation is the process in which a gas without passing through the intermediate liquid state. Such materials are called sublime. This due to the fact that if a solid can at some temperature exert a vapour pressure equal to the external pressure, it will on heating to this point, pass direct to vapour, and reverse changes will occur on cooling the vapour.
This is called true sublimation (Type-1).
On the other hand, if a solid cannot at any temperature exert a pressure equal to the external pressure, it will, when heated, first liquefy and later, when vapour-pressure of the liquid equals the external pressure, the liquid will boil, and the reverse changes will occur on cooling the vapour. This is categorized as Type-2 sublimation.
The sublimation curve represents the vapour pressure curve of the solid phase, the distillation curve and the vapour pressure curve of the liquid phase, the melting point curve showing the temperature at which the solid melts under increasing pressure.
The point of intersection of these curves is called the triple-point, and the point where the vertical line from the triple-point cuts the abscissa-axis is called the triple-point pressure.
It is evident that if the pressure on a sold is lower than the triple-point pressure, the solid cannot exist as a liquid. Hence, upon heating, the sold must pass directly to vapour showing true sublimation of Type-1 represented by inner dotted lines.
Conversely, if the pressure on the solid is greater than the triple-point pressure, the solid can exist as a liquid. Thus upon heating, the solid will melt to form a liquid and at a higher temperature this liquid will pass into vapour, showing sublimation of type-2 .
The above discussion can very well be stated in two obvious facts:
a) When the vapour of the substance exerts a pressure less than the triple-point pressure, it must, on condensation, pass directly into solid by passing liquid phase.
b) When the vapour of the substance exerts a pressure greater than triple-point pressure, it must, on condensation pass to liquid phase which on further cooling solidifies.
The sublimation (Type-1) and the sublimation (Type-2) can be exemplified by the sublimation of arsenic trioxide and the iodine.
Arsenic trioxide undergoes true sublimation (Type-1). The triple-point pressure of arsenic trioxide is slightly greater than atmospheric pressure (760 mm of Hg), i.e. one atmosphere.
Hence, when heated, solid arsenic trioxide yields a vapour pressure equal to the atmospheric pressure and therefore passes directly to vapour. The vapour of the arsenic triode exerts a pressure less than the triple-point pressure, consequently upon cooling, the vapour passes direct to solid. Arsenic trioxide therefore undergoes true sublimation Type-1).
In case of iodine, the triple-point pressure of iodine is 91 mm of Hg. When it is heated, solid iodine cannot exert a vapour pressure equal to the atmospheric pressure. Consequently, iodine melts (at 114°C) at triple-point pressure, to form a liquid which (at 184°C), exerts a vapour pressure equal to atmospheric pressure (760 mm of Hg) and therefore boils. The phase changes on cooling are dependent on the pressure of the iodine vapour when condensation takes place. Thus, if the space is saturated, with iodine vapour, the latter will on cooling, condense to form a liquid which will solidify to form solid iodine. These phase changes will occur because under the conditions stated, the iodine vapours exerts a pressure greater than the triple-point pressure, and thus Type-2 'sublimation occurs.