Chemistry · General Principles Of Organic Chemistry · NEET
Only for SOLID organic compounds. If the compound is a liquid, you use distillation instead. NEET often gives a liquid mixture in the option list to trick you into picking crystallisation — do not. Crystallisation = solid purification, distillation = liquid purification.
After the pure compound crystallises out and you filter it, the leftover liquid is the mother liquor. It holds the impurities plus a small amount of your compound still dissolved. You keep the crystals and throw the mother liquor's impurities — this is how separation happens.
This solubility gap is the whole engine of the method. At high temperature the solid dissolves fully; on cooling, solubility drops sharply, so the compound is forced out as pure crystals. If it stayed fully soluble even when cold, nothing would crystallise. A good solvent is one where solubility rises strongly with temperature.
Both purify solids, but the principle differs. Sublimation is used when the solid changes directly from solid to vapour on heating (like camphor, naphthalene, benzoic acid). Crystallisation is used when the solid does NOT sublime — it relies on difference in solubility in a solvent instead of on vapour formation.
Coloured impurities that dissolve along with the compound are removed by adsorbing them onto activated charcoal. The charcoal is added to the hot solution, then filtered off, leaving a clear, colourless solution before crystals form. This step decolourises the product.
When the impurity has a solubility very close to that of the compound, a single crystallisation cannot separate them cleanly. You repeat the dissolve-cool-filter cycle several times to get a pure product. Also, if the compound is highly soluble in one solvent and poorly soluble in another, a mixture of the two solvents is used.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The compound is sparingly soluble at room temperature but appreciably soluble at higher temperature, so on cooling the pure compound crystallises out.
One in which the compound dissolves well when hot but poorly when cold, while the impurities behave oppositely (staying dissolved or being insoluble). Common examples are water, alcohol, and ether depending on the compound.
Not in one step. When solubilities are comparable, repeated crystallisation is required to remove the impurity, or a mixture of solvents is used.
Simple crystallisation purifies one main compound. Fractional crystallisation separates two dissolved solids that crystallise at different rates as the solution cools, collecting them in fractions.
By its melting point. A pure solid organic compound has a sharp, fixed melting point; an impure one melts over a range and at a lower temperature.
