An Anhydrate (from the Greek word án(h)ydros meaning "without water") is a chemical compound that does not contain water molecules. These molecules may be present implicitly in the compound itself or they may be present in the outer co-ordination sphere in the case of complexes or ionic salts. The anhydrous form of sulfuric acid, H₂SO₄ is sulfur trioxide, SO₃. Example: When the water from MgSO₄·7H₂O is driven off it becomes MgSO₄, an anhydrous compound.

Anhydrous compounds may differ radically, both physically and chemically, from their hydrated forms. For example, the hydrated form of copper sulfate (CuSO₄·5H₂O) is blue, whereas the anhydrous form (CuSO₄) is white. The colour is due to the water present in the outer sphere of the complex.

NaCl (or table salt) is a common anhydrate. The mineral anhydrite is the anhydrous form of the hydrous mineral gypsum.

Compare to a hydrate.

Chemical compounds | Anhydrid