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

Question

Caleb Adams · Accepted Answer

Because the waters of crystallization are along for the ride.

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.

Owen Ambrose · Answer

undefined The "waters of crystallization" refer to water molecules that are incorporated into the crystalline structure of certain inorganic salts. These water molecules are not chemically bonded to the salt ions but are held within the crystal lattice by weak forces of attraction, such as hydrogen bonds. The presence of these water molecules in the crystal structure is often essential for maintaining the stability and structure of the salt. When the salt crystals are heated, these water molecules are released as water vapor, leading to a change in the physical properties of the salt, such as color or texture. The number of water molecules associated with a particular salt is specific to the compound and is typically represented as part of the chemical formula.