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C-2
Most hydrates are stable at room temperature. To remove the water of crystallization from such hydrates,
they must be heated until the vapor pressure of water within the solid hydrate exceeds the partial pressure of
water vapor in the atmosphere above the solid. At this higher temperature, the water of crystallization is driven
off and dehydration occurs. When all of the water has gone, the compound is said to be in its anhydrous
form. The temperature required to remove water from a hydrate varies considerably from hydrate to hydrate,
depending on the vapor pressure of water within the hydrate. For example, Na2CO3•10 H2O exerts a vapor
pressure of 24.2 mm Hg at 20°C. If the partial vapor pressure of water in the atmosphere is greater than
24.2 mm Hg at 20°C, this compound will need to be heated until the vapor pressure of water within the
compound exceeds the vapor pressure of the atmosphere in order to drive off its water of hydration.
However, if the partial pressure of water in the atmosphere is less than 24.2 mm Hg, Na2CO3•10 H2O will
spontaneously lose water slowly; that is, it effloresces.
As a hydrate is heated, and the water of hydration is driven off, the compound may undergo one or more of the
following changes:
(a)
It may undergo a color change; often the color of a hydrate depends on the number of water molecules
associated with the cation. For example, CoCl2•6 H2O is red, CoCl2•2 H2O is violet, and CoCl2 is blue.
(b)
The form of the crystal may change. If it does, a dry powdery solid usually results.
(c)
It may dissolve in its own water of crystallization as the crystal structure breaks down. By continued
heating, this water will evaporate, leaving the anhydrous solid behind.
EXPERIMENTAL METHOD
