Question

Many heat-sensitive and oxygen-sensitive solids, such as camphor, are purified by warming under a vacuum. The solid vaporizes directly, and the vapour crystallizes on a cool surface. What phase changes are involved in this method?

Short answer

The phase change which is involved for the vaporization of these heat-sensitive and oxygen-sensitive solids is from a solid phase to the liquid phase. When the vapour crystallizes on a cool surface, the gaseous form changes to a solid form.

Step-by-step solution

Sublimation and deposition

Sublimation is the process when the solid changes to the gaseous form directly whereas deposition is the process in which the gaseous form converts to the solid form.

Phase transformation

For many heat-sensitive and oxygen-sensitive solids, these solids are purified by converting these solids directly to vapour as they have a very low value of$\mathbf{∆}{\mathbf{H}}_{\mathbf{sub}}$ which converts these low b.p. solids to vapour form.

The opposite of this process is called deposition or desublimation in which the vapour form directly gets converted to the solid form by the cooling method.