The crude product obtained directly from a chemical reaction is usually amorphous (not crystalline). To obtain the substance in uniform, well-defined crystals and to separate it from impurities it must be dissolved again with the aid of heat, filtered, and allowed to cool slowly. Then the dissolved substances will separate into large crystals or into very fine crystals termed "crystal meal," according to conditions. Since the large crystals are compact and offer a relatively small surface to the action of water, they dissolve slowly. Crystal meal, on the other hand, dissolves quite readily and is therefore more commonly used.
The theory of crystallization is based on the fact that every liquid has the power of dissolving substances. This power can usually be increased by raising the temperature of the liquid. There are a few substances, however, whose maximum strength of dissolving is reached at a temperature much lower than the boiling point. When a solution has dissolved all the solid that it can take up, it is said to be saturated; any decrease in the temperature will then result in the separation of a part from the main body of the substance - usually as crystals. While crystals are being formed, there is a tendency to exclude from the solution all matter not homogeneous with it, that is, all matter not of the same kind. If a concentrated solution which is impure is allowed to crystallize, the impurities may become enclosed or entangled among the forming crystals. This is undesirable and can be prevented by stirring the solution while crystallization is taking place. Thus the formation of the very fine crystals, called "crystal meal," is caused. These fine crystals may be washed free from the "mother liquor" (the liquor from which the impurities are obtained), and may be cleansed of all impurities.