Water of Hydration

One of the terms you come across when studying compounds in chemistry (especially salts), is water of hydration. An alternative name for it is 'water of crystallization'.

Definition
Water is a versatile compound that participates in many chemical reactions on Earth. You will rarely find a biological reaction without water being a part of it. Water of crystallization is the water contained in crystals, that is bonded to the ions or molecules that form the crystal. A large number of compounds require a water medium to crystallize. The fact that crystallization does not occur without water, even though water creates no bond with the core crystal ions, is surprising. It affects the crystal structure, color and shape.

The nature of water as a solution somehow aids the formation of crystals. Upon completion of crystallization, a small fraction of water content still remains to be a part of the crystal structure and is known as water of crystallization. In inorganic chemistry, a crystalline compound or salt, that has discrete water molecules as part of its crystalline framework, is known as a 'Hydrate'. These crystallized compounds have water molecules associated with them in definite ratios. The water contained in the crystal structure of any inorganic compound is quantified in terms of the number of water molecules associated with each molecule of the compound.

Crystalline compounds with water contained in them, are mostly metal complexes in which the water is not directly associated with the main metal ion, which forms the center. Water is bonded with surrounding atoms obviously, as it cannot be associated with a crystal without some kind of covalent bonding. The association with water does affect some of the physical properties of a compound including its optical characteristics. An example of such a physical change in color is found in the hydration of cobalt (II) chloride, which changes color from blue to magenta red, as water is added to its crystal structure.

Examples
In the early years of inorganic chemistry, the nature of crystals and metal complexes, that incorporate water was not known properly. Today we know that water combines with these crystal structures through hydrogen bonding. The chemists of the early days didn't know the exact nature of water bonding, but determined the ratio of water molecules associated with each molecule of the crystalline compound. Due to absence of knowledge about the exact bonding, the formula of hydrates was written with the crystalline chemical formula, followed by the number of water molecules, separated by a dot. The actual number of discrete water molecules inside the crystal structure may be lesser. The general formula for representing hydrates is as follows:

Hydrate Molecular Formula: Hydrous Compound . n (H₂O)

The 'n' stands for number of water molecules and it may vary according to compound. Some examples of hydrates or compounds with water of crystallization incorporated are the following.

• Barium chloride (BaCl₂.2H₂O)
• Calcium Nitrate (Ca(NO₂)₂.4H₂O)
• Copper (II) Chloride (CuCl₂. 2H₂O)
• Zinc sulfate (ZnSO₄.7H₂O)
• Potassium Aluminum Sulfate (K₂SO₄.Al₂ (SO₄)₃.24H₂O)
• Magnesium Sulfate (MgSO₄.7H₂O)

There are many compounds that form crystals without association with water. Some examples of such compounds include the chlorides, nitrates and bromides of sodium, potassium, lead, sulfate of ammonia and iodides of lead and silver.