Question

Which mixture is separated by using the technique of sublimation?

Answer 1

Hint: Physical combination of two or more substances in which the identity of each substance is retained after mixing are known as mixtures. These can be present in the form of solutions, suspensions and colloids. There is a very little change in energy when mixture is formed so, this indicates that the enthalpy of mixing for mixture is negligible.

Complete answer:
The conversion of a substance directly from its solid phase to its gaseous phase, without entering the liquid state is known as sublimation. It is an endothermic process i.e.; heat is absorbed within the process to provide enough energy to some molecules to overcome the attractive forces applied by the neighbouring atoms and escape to form a vapour phase.
$\text{Solid }\overset{\text{Sublimation}}{\leftrightarrows} \text{Gas}$
The mixtures which consist of sublimable and non-sublimable components can be separated using the sublimation process. The example of such mixtures are as follows:
Mixture of camphor and sodium chloride:
In camphor, the molecules are held by weak van der Waal forces which are even weaker than ice molecules. This means even at moderate temperature; the solid molecules of camphor convert into a gaseous phase and escape into the atmosphere. The major reason for sublimation is that the total atmospheric pressure of molecules of camphor is less than their vapour pressure and the temperature at which sublimation occurs is much less than the melting point of camphor whereas the atoms of sodium chloride are held together by strong electrostatic forces forming an ionic bond.
Hence, when a mixture of camphor and sodium chloride is heated to a moderate temperature, then the molecules of camphor sublimes and sodium chloride get separated from it.

Note:
A noteworthy point here is that the term sublimation refers to the physical change of state of a solid into gas but not a chemical transformation of a molecule. For example, conversion of snow into water vapour is a sublimation process whereas the dissociation of solid ammonium chloride into gaseous ammonia and hydrogen chloride is not a sublimation process instead it is a chemical reaction.