The soluble salts are, when in the hydrated condition, also red, but in the anhydrous condition are blue.
As heat is supplied, the hydrate is transformed gradually into the anhydrous salt and water.
Cobalt chloride, CoC1 2, in the anhydrous state, is formed by burning the metal in chlorine or by heating the sulphide in a current of the same gas.
The most common of these sulphides is cobaltous sulphide, CoS, which occurs naturally as syepoorite, and can be artificially prepared by heating cobaltous oxide with sulphur, or by fusing anhydrous cobalt sulphate with barium sulphide and common salt.
By dissolving it in concentrated sulphuric acid and warming the solution, the anhydrous salt is obtained.