ANS:B - crystallisation

Crystallization is a separation and purification process used to produce solid crystals from a solution, melt, or gas. It is widely employed in various industries, including pharmaceuticals, chemicals, food processing, and materials science, to obtain pure substances or to recover valuable products from solutions. Here's how crystallization typically works:

1. Solution Preparation: Crystallization begins with the preparation of a solution containing the substance to be crystallized (the solute) dissolved in a suitable solvent. The solute may be dissolved in the solvent by heating or agitation until a clear solution is formed.
2. Saturation: The solution is then cooled or evaporated under controlled conditions to reach a state of supersaturation. Supersaturation occurs when the solution contains more dissolved solute than it would be able to dissolve under normal conditions. At this point, the solution is unstable, and the excess solute tends to precipitate out of the solution in the form of crystals.
3. Nucleation: Once the solution becomes supersaturated, nucleation occurs, where individual solute molecules or ions come together to form small clusters or nuclei. Nucleation can be spontaneous or induced by seeding the solution with tiny crystals of the solute or by agitation.
4. Crystal Growth: The nuclei act as sites for further crystal growth. As more solute molecules or ions are deposited onto the nuclei, the crystals grow in size. The growth of crystals occurs by the addition of solute molecules or ions from the surrounding solution onto the crystal surfaces.
5. Separation: Once the desired crystal size is achieved, the crystals are separated from the remaining solution. This can be done by filtration, centrifugation, or other solid-liquid separation techniques. The separated crystals are then washed with a solvent to remove impurities adhering to their surfaces.
6. Drying: Finally, the purified crystals are dried to remove any remaining solvent. This can be done by air drying, vacuum drying, or by heating in an oven.

Crystallization can be performed using various techniques, including cooling crystallization, evaporative crystallization, and antisolvent crystallization, depending on the properties of the solute and solvent and the desired outcome of the process. Overall, crystallization is a versatile and widely used method for purifying substances and producing high-quality crystals with defined properties for a wide range of applications.