Hydration isomerism
In coordination chemistry, hydration isomerism is a kind of isomerism that is observed in some solids. Hydration isomers have identical formula but differ with respect to the numbers of water ligands.
Examples
One example is the pair. The former has one water of crystallization but the latter does not.Another example is the pair of titanium chlorides,. The former is violet and the latter, with two molecules of water of crystallization, is green.