Calcium sulfate is soluble in water in percentages ranging from 0.2 to 0.88 grams per 100 milliliters of water, depending on the anhydrous or hydrated form of the substance. For example, insoluble anhydrite, a dehydrated form of calcium sulfate, dissolves slowly in water and does not absorb moisture from air.
In contrast, soluble anhydrite, a compound obtained by heating calcium sulfate dihydrate, has a high affinity for water and quickly absorbs it. The hydrated form, which is called calcium sulfate dihydrate, dissolves in water at a ratio of 0.21 grams per 100 milliliters of water. Other hydrated forms, such as the calcium sulfate hemihydrate alpha and beta forms, have solubilities of 0.67 and 0.88 grams per 100 milliliters of water.