 |
 |
1 of 16 |  |
 |
 |
C-1
EXPERIMENT C: HYDRATES
OBJECTIVES
You will apply some basic chemical principles to the study of hydrated compounds. You will observe changes
in the structure of some hydrates caused by the loss of water molecules from their hydration shells. Simple
chemical tests will be used to identify the component ions of an unknown hydrate.
THEORETICAL CONSIDERATIONS
Most solid chemicals will contain some water if they have been exposed to the atmosphere. In some cases,
the water present is adsorbed only on the surface of the compound, and is easily removed by gentle heating.
In other cases, the water present is present as an integral part of the crystal structure of the compound and is
generally bound to the cation. The water present in such cases is called the water of hydration or water of
crystallization. It is usually present in stoichiometric amounts, for example, AlCl3•6 H2O.
Some hydrated compounds spontaneously lose or gain water from the atmosphere. Hydrates that
spontaneously lose water to the atmosphere are classified as efflorescent, and those that spontaneously
absorb water from the atmosphere are classified as hygroscopic. Some hygroscopic compounds absorb
water from the atmosphere so strongly that they dissolve in their own water of hydration. They are said to be
deliquescent.
Industrially, some hygroscopic compounds, such as CaCl2 or P2O
5
, are widely used to dry liquids or gases. As
a result they are also called desiccants. For these compounds, the vapor pressure of water exerted by the
solid is less than the vapor pressure of water in the surrounding atmosphere. Thus, if we know that a
compound is hygroscopic, we can conclude that the vapor pressure of water exerted by the solid is less than
the vapor pressure of water in the surrounding atmosphere. Efflorescent compounds lose water spontaneously
if they are open to the atmosphere. For these compounds, the vapor pressure of water exerted by the solid is
greater than the vapor pressure of water in the surrounding atmosphere. As before, if we know that a
compound effloresces, we can conclude that the vapor pressure of water exerted by the solid is greater than
the vapor pressure of water in the surrounding atmosphere.
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
