How is recrystallisation used to purify salts?

Recrystallisation is used to purify salts by dissolving them in a solvent and then allowing them to crystallise out.

In more detail, recrystallisation is a purification technique that exploits the differences in solubility of a desired compound and its impurities. It is a common method used in chemistry to purify salts and other solid substances. The process begins by dissolving the impure salt in a suitable solvent, usually water or ethanol. The choice of solvent is crucial as it should be one in which the salt is soluble, but the impurities are not.

Once the salt is dissolved, the solution is heated to increase the solubility of the salt. This allows more of the salt to dissolve, leaving the impurities behind. The hot solution is then filtered to remove any undissolved impurities. The filtrate, which contains the dissolved salt, is then allowed to cool slowly. As it cools, the solubility of the salt decreases and it begins to crystallise out of the solution.

The rate of cooling is important in this process. Slow cooling allows the salt crystals to grow large and pure, as impurities are excluded from the growing crystal lattice. Rapid cooling, on the other hand, can trap impurities within the crystals, reducing their purity.

Once the crystals have formed, they are collected by filtration and then dried. The result is a purified salt, free from impurities. This process can be repeated if necessary to increase the purity of the salt.

In summary, recrystallisation is a simple and effective method for purifying salts. It relies on the differences in solubility between the salt and its impurities, and the ability of slow cooling to exclude impurities from the growing crystals.