What are Hydrates and how are they Named

Hydrates can be either inorganic or organic in nature and all have one component in common and that is water.

When certain inorganic ionic solids crystallize from aqueous solutions, they combine with a definite amount of water, which becomes a part of the crystalline solid. Salts that contain water as part of their crystal structure are called hydrates (or hydrated salts) and the water in the crystal structure is called the water of hydration. When the water of hydration is expelled from the hydrate, the salt that remains is said to be anhydrous.

Inorganic hydrates contain water of crystallization combined in a definite ratio as an integral part of the crystal. For example, anhydrous copper sulphate is a white solid with the formula CuSO4. When crystallized from water, a blue crystalline solid which contains water molecules as part of the crystals is formed. Analysis shows that the water is present in a definite amount, and the hydrate may be given the formula CuSO4 5H2O. Four of the water molecules are attached to the copper ion as coordination complexes, and the fifth water molecule is related to the sulfate and presumably held by hydrogen bonding.

Conversely, water can also be present in definite proportions in the crystal without being necessarily associated directly with the anion or cation The water occupies a definite place in the Chrystal lattice Such a case can be cited in Alums that have twelve molecules of water of hydration.

Organic Hydrates are formed when a water molecule is added to a carbonyl group of an aldehyde or ketone. Methane (CH4) hydrate is the dominant natural organic hydrate on Earth. It exists as a gas hydrate. Gas hydrates are crystalline compounds in which an isometric (cubic) ice lattice contains cages that incorporate small guest gas molecules. They are stable at moderate to high pressures and low temperatures, above and below the ice point. These ice lattices are stable only when the cages contain a gas molecule.


The nomenclature of hydrates follows simple naming rules as will be discussed as follows;

Inorganic hydrated compounds:

Consider a hydrated Crystal of copper (II) Sulphate (CuSO4.5H2O). It contains five molecules of water and therefore a prefix penta is adopted and the name becomes; Copper (II) Sulphate pentahydrate. Similary, for LiNO3. 3HO2 it would be Lithium Nitrate trihydrate. Notice the prefix tri in the naming that denotes three molecules of water. Other prefixes used include; mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca, etc. signifying 1 through 10 molecules of water of hydration respectively.

Organic compounds:

For organic compounds follows same rules for naming of saturated carbon chains. For instance, CH=O (Form aldehyde) yields CH4 (Methane). The longest carbon chain suggests the name of the compound. Recall that organic hydrates are obtained upon addition of a water molecule to unsaturated C =C bond in aldehydes and ketones.