Question

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

Answer 1

Because the waters of crystallization are along for the ride.

Explanation:

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, `MgSO_4*7H_2O` (so-called `"Epsom salts"`). Such hydrates are often useful when we wish to add limited quantities of water to a very reactive material such as `LiAlH_4`, which generates volumes of dihydrogen gas upon water workup.

Why does the salt occlude water on crystallization? Well, clearly, the structure is more stable if several water molecules are included in the lattice; typically the water molecules solvate the cation. When we wish to do inorganic chemistry with anhydrous (i.e. water free) salts, we have to take a bit of time and effort to dry the salts thoroughly, and remove the water.