How does crystallization purify a solid chemical compound?

Crystallization purifies a solid compound by exploiting differences in solubility between the desired substance and its impurities within a specific solvent.1

The process typically involves creating a saturated solution at a higher temperature. As the solution cools, the solubility of the components decreases, causing the less soluble substance to crystallize out of the solution while more soluble impurities remain dissolved. Because crystal growth is a selective process where molecules deposit onto a solid surface (or seed crystal), the resulting solid can achieve high purity if the precipitation occurs slowly.12

Rapid solidification can lead to the entrapment of foreign material within the crystal matrix, which reduces purity. To further enhance results, chemists may use techniques such as:1
* Controlled cooling: Ensuring slow growth to minimize impurity entrapment.13
* Reprecipitation: Redissolving an impure solid and crystallizing it a second time to remove remaining contaminants.3
* Seeding: Adding a small crystal to initiate and guide the growth process.1

This method relies on the organized structural formation of the solid to exclude unwanted constituents.3