Crystallisation

Question

A solid organic product is contaminated with a soluble coloured impurity, and the chemist wants pure colourless crystals; which purification technique is most appropriate here?

RankGuru · Accepted Answer

Crystallisation is the most widely used method for purifying solid organic compounds and relies on the difference in solubility of the compound and the impurity in a chosen solvent. The compound is dissolved in a hot solvent in which it is sparingly soluble at room temperature but highly soluble when hot; on cooling, the pure compound separates as crystals while soluble impurities stay behind in the mother liquor. When a coloured impurity is present, the hot solution is treated with activated animal charcoal, which adsorbs the colouring matter, and after filtration colourless crystals are obtained on cooling. Simple distillation is wrong because it is meant for liquids that vaporise and recondense, not for separating a soluble impurity from a solid. Steam distillation applies only to substances that are steam-volatile and immiscible with water. Sublimation works only when the compound itself sublimes directly from solid to vapour, which is not stated here. This decolourising-by-charcoal step is the standard NCERT crystallisation procedure. Plausibility check: only the option that combines selective solubility with adsorptive decolourising can deliver pure colourless crystals, confirming the choice.

Crystallisation

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