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A salt with associated water of crystallization is known as a hydrated salt; salts without water of crystallization are called anhydrous salts. The structure of these hydrates can be quite elaborate, because of the existence of Hydrogen Bond s. Because the structures are sometimes complex or unknown, a dot is used in the formula of a hydrate to specify the composition without indicating how the water is bound.

Examples:

  • CuSO4·5H2O - copper (II) sulphate pentahydrate

  • FeCl3·6H2O - Iron (III) Chloride hexahydrate

  • BaCl2·2H2O - Barium Chloride dihydrate


However, if the structure is known, a more informative formula can be given. For example, the
above mentioned hydrated salts show different atomic arrangements. The copper sulphate is arranged as Cu(H2O)42+ and SO4(H2O)2- ions. The iron chloride occurs as Fe(H2O)63+ and 3Cl-, while the water in the barium chloride occurs at positions of the crystal lattice that are not specifically associated with anions or cations. Water of crystallization is bound by the electrostatic attraction of the Oxygen Atom s of the water Molecule s for the metallic Cation . The metals in hydrated salts have no partly occupied d-orbitals in their Valence Shell s. Because bonding interactions between water and a metal cation increase with increasing charge on the cation, hydrate formation is common for salts that contain +2 and +3 cations.

When hydrated salts are heated, the water of crystallisation is driven off. For example, when blue crystals of CuSO4·5H2O (copper sulfate pentahydrate) are heated above 350 degrees Celsius, anhydrous white copper sulfate (CuSO4) is formed. Some anhydrous compounds are so prone to heating that they easily pick up water on exposure to the atmosphere. Such compounds are said to be hygroscopic and are very useful as drying agents. A few examples are anhydrous magnesium perchlorate and anhydrous calcium chloride.

To find the amount of water of crystallisation in a hydrated salt, it is first heated and the amount of anhydrous salt obtained is weighed. The amount of water driven off is then divided by the molar mass of water to obtain the number of molecules of water bound to the salt.


REFERENCES


  • Chemistry, the central science, 5th Ed. Brown, T.L., LeMay, H.E. and Bursten, B.E., Prentice Hall, Englewood Cliffs, N.J.


  • Chemistry, 4th Ed. Mcmurry, Fay, Pearson Education, Patparganj, Delhi, India



Note: UK English spelling - ''crystallisation''.