Anhydrous
Describing a substance from which all water, including water of crystallisation, has been removed.
Unit
Dimensionless
In depth
Anhydrous forms are prepared by heating, vacuum drying or treatment with a desiccant, and many are strongly hygroscopic afterwards. Reactions involving Grignard reagents, acyl chlorides or alkali metals demand anhydrous solvents because even trace water destroys the reagent.
Examples in the real world
White anhydrous copper(II) sulfate turning blue on contact with water — the classic test for water.