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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
In this experiment, you will perform a few simple tests on some selected hydrates to study their properties.
You will also be given an unknown hydrate, for which the number of molecules of water and the ions present,
are to be determined. You will obtain a known mass of the unknown hydrate in a test tube, and then drive off
the water of hydration by heating the hydrate to obtain the anhydrous salt. By weighing the dried salt in the
test tube, you will be able to determine the mass of water that is lost and, when you have determined which
ions are present, you will then be able to determine the empirical formula of the hydrate.
As there will be some time spent waiting for the test tube to cool, start immediately with Part I, the
determination of the percentage of water in the unknown hydrate. Cycles of heating, cooling, and weighing are
repeated in Part I until the final mass of the anhydrous residue is constant. During the waiting (cooling)
periods, do Part II and run the chemical tests to determine what ions are present
(Part III).
