The purification method described by the transformation:

$$ \underset{(X)}{\text{Solid}} \xrightarrow[]{\text{Heat}} \underset{(X)}{\text{Vapour}} \xrightarrow[]{\text{Cool}} \underset{(X)}{\text{Solid}} $$

is known as Sublimation.

In sublimation, a solid substance directly transforms into its gaseous phase upon heating and reverts back to the solid phase upon cooling. This technique is commonly used to purify substances, particularly those that sublime more easily than others under the same conditions. Sublimation is effective for separating substances with different volatility, like separating ammonium chloride from a mixture with sodium chloride, as ammonium chloride sublimates while sodium chloride does not.