An ionic crystal is said to be anhydrous if it contains no water.

An example of anhydration can be seen in copper (II) sulfate. If the water of crystallization is removed from blue crystals of copper (II) sulfate, a white powder (anhydrous copper sulfate) is formed.

The original formula for crystalline copper (II) sulfate is CuSO₄·5H₂O. The formula for anhydration is as follows:

CuSO₄·5H₂O + heat → CuSO₄ + 5H₂O

Another example is in the heating of magnesium sulfate heptahydrate, MgSO₄·7H₂O. On heating, it undergoes the following reaction:

MgSO₄·7H₂O + heat → MgSO₄ + 7H₂O

Other uses of the term

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Ammonia used commercially is called anhydrous ammonia to distinguish it from ammonium hydroxide solution, which is household ammonia. Pure ammonia is usually in gaseous form on Earth, not a crystal.

Chemical properties