The purification method based on the following physical transformation is:
\[
{Solid} \xrightarrow{{Heat}} {Vapour} \xrightarrow{{Cool}} {Solid}
\]
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Sublimation is a useful technique for purifying solids that can transition directly from the solid phase to the gas phase without passing through the liquid phase.
The process shown in the question involves heating a solid to convert it into vapour, followed by cooling the vapour back into a solid. This is a typical example of sublimation, where a substance transitions directly from a solid to a gas and then back to a solid without passing through the liquid phase. Sublimation is commonly used to purify substances that can undergo this phase transition, such as iodine, dry ice, or camphor. The heating provides enough energy for the molecules to overcome intermolecular forces and directly convert to vapour. When the vapour is cooled, it condenses back into a solid, leaving impurities behind. Thus, the correct purification method based on the described transformation is sublimation.
