# What are the "waters of crystallization" with regard to inorganic salts?

If we recrystallize an organic or inorganic compound from water, typically we get solvent molecules, i.e. water, included in the material. These are termed waters of crystallization. An inorganic salt may occlude several water molecules when it deposits from solution; these are included in the crystalline lattice. We might represent this as, for instance, $M g S {O}_{4} \cdot 7 {H}_{2} O$ (so-called $\text{Epsom salts}$). Such hydrates are often useful when we wish to add limited quantities of water to a very reactive material such as $L i A l {H}_{4}$, which generates volumes of dihydrogen gas upon water workup.