Water of Crystallization Definition

What Water of Crystallization Means in Chemistry

Blue crystals of copper sulfate or copper sulphate.
These are blue crystals of copper sulfate pentahydrate, which is known as copper sulphate pentahydrate in the UK. Anne Helmenstine

Water of crystallization is defined as water that is stoichiometrically bound into a crystal. Crystal salts containing water of crystallization are called hydrates. Water of crystallization is also known as water of hydration or crystallization water.

How Water of Crystallization Forms

Many compounds are purified by crystallization from an aqueous solution. The crystal excludes many contaminants, however, water can fit within the crystalline lattice without being chemically bonded to the cation of the compound.

Applying heat can drive off this water, but the process typically damages the crystalline structure. This is fine, if the goal is to obtain a pure compound. It may be undesirable when growing crystals for crystallography or other purposes.

Water of Crystallization Examples

  • Commercial root killers often contain copper sulfate pentahydrate (CuSO4·5H2O) cyrstals. The five water molecules are called water of crystallization.
  • Proteins typically contain even more water than inorganic salts. A protein may easily contain 50 percent water.

Water of Crystallization Nomenclature

The two methods to denote water of crystallization in molecular formulas are:

  • "hydrated compound·nH2O" - For example, CaCl2·2H2O
  • "hydrated compound(H2O)n" - For example,  ZnCl2(H2O)4

Sometimes the two forms are combined. For example, [Cu(H2O)4]SO4·H2O may be used to describe the water of crystallization of copper(II) sulfate.

Other Solvents in Crystals

Water is a small, polar molecule that is readily incorporated into crystal lattices, but it's not the only solvent found in crystals.

In fact, most solvents remain, to a greater or lesser extent, in the crystal. A common example is benzene. In order to minimize the effect of a solvent, chemists typically try to remove as much as possible using vacuum extraction and may heat a sample to drive off residual solvent. X-ray crystallography can often detect solvent within a crystal.

Sources

  • Baur, W.H. (1964) "On the crystal chemistry of salt hydrates. III. The determination of the crystal structure of FeSO4(H2O)7 (melanterite)" Acta Crystallographica, volume 17, p1167-p1174. doi:10.1107/S0365110X64003000
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  • Klewe, B.; Pedersen, B. (1974). "The crystal structure of sodium chloride dihydrate". Acta Crystallographica B30: 2363–2371. doi:10.1107/S0567740874007138