Sublimation

Sublimation is the process in which a solid, on heating, passes directly into the vapour state without melting into a liquid, and on cooling the vapour condenses back to the solid. It is used to purify solids such as camphor, naphthalene, anthracene, and benzoic acid that sublime, provided the impurities present do not sublime. When the camphor-sand mixture is heated, camphor vaporises and recondenses as pure crystals on a cooler surface, while the non-volatile sand remains behind. The statement that camphor changes directly from solid to vapour is precisely the property exploited here. High solubility in cold water is incorrect and is unrelated to sublimation, which is a phase-change separation, not a solubility separation. A very high boiling point is not the relevant criterion; sublimation depends on direct solid-to-vapour transition, not on boiling. Camphor does not react chemically with sand, so chemical reaction is irrelevant. This separation of a sublimable solid from a non-sublimable impurity is the standard NCERT illustration of sublimation. Plausibility check: only a substance that bypasses the liquid phase can be separated cleanly by sublimation from non-volatile sand.