THEORETICAL BACKGROUND
A hydrate is an ionic compound (salt) that contains a specific number of water molecules
attached to its crystal lattice structure. Some hydrates are familiar substances such as
gypsum, CaSO4 • 2H2O, Plaster of Paris, CaSO4 • ½ H2O, or borax, Na2B4O7 • 10H2O. The
formula of a hydrate is written as MX • nH2O, where MX is the formula of the ionic compound
(i.e. M represents the cation and X, the anion), n represents the number of moles of water
per mole of MX, and the centered dot (•) separates MX and nH2O. For example, Epsom salt,
MgSO4 • 7H2O, is a hydrate in which each unit or mole of magnesium sulfate (MgSO4) has
seven molecules or moles of water associated with it.
A hydrate is systematically named by writing the name of the ionic compound followed by a
numerical Greek prefix and the suffix “-hydrate.” The following Greek prefixes (their
meaning is given in parenthesis) are used: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5),
hexa- (6), hepta- (7), octa- 8), nona- (9), and deca- (10). For example, the systematic name of
Epsom salt, MgSO4 • 7H2O, is magnesium sulfate heptahydrate and that of CuSO4 • 5H2O is
copper(II) sulfate pentahydrate.
The water molecules of a hydrate are loosely held by the ions of the ionic compound and can
be driven off by heating. When the water molecules have been removed from the hydrate,
the remaining dehydrated ionic compound (salt) is referred as an anhydrate (or anhydrous
salt) and the water removed is referred as the water of hydration.
MX • nH2O (s) → MX (s) + nH2O (g)
Hydrate Anhydrate Water of Hydration
Upon dehydration by heating, the hydrate may undergo a physical change, such as color
change. For example, cobalt chloride, CoCl2, in its anhydrous form is a blue compound. As
CoCl2 is hydrated, its color changes from blue to purple to pink. CoCl2 • 6H2O has a pink
appearance. CoCl2 is thus used as a humidity indicator found in small packets stored with
electronic equipment to ensure no exposure to water or significant humidity1.
The chemical reaction illustrating the dehydration process of CoCl2 ∙ 6H2O is:
CoCl2 • 6 H2O (s) → CoCl2 (s) + 6 H2O (g)
Pink Blue
