[4] The anhydrous form is a blue crystalline solid; the dihydrate is purple and the hexahydrate is pink.

Under atmospheric pressure, the mass concentration of a saturated solution of CoCl2 in water is about 54% at the boiling point, 120.2 °C; 48% at 51.25 °C; 35% at 25 °C; 33% at 0 °C; and 29% at −27.8 °C.

[4] Dehydration can also be effected with trimethylsilyl chloride:[11] The anhydrous compound can be purified by sublimation in vacuum.

[citation needed] The hexahydrate and the anhydrous salt are weak Lewis acids.

This 19-electron species is a good reducing agent, being readily oxidised to the yellow 18-electron cobaltocenium cation [Co(C5H5)2]+.

These reactions are often performed in the presence of charcoal as a catalyst, or with hydrogen peroxide H2O2 substituted for atmospheric oxygen.

Other highly basic ligands, including carbonate, acetylacetonate, and oxalate, induce the formation of Co(III) derivatives.

[17] Cobalt chloride is a common visual moisture indicator due to its distinct colour change when hydrated.

It is impregnated into paper to make test strips for detecting moisture in solutions, or more slowly, in air/gas.

Desiccants such as silica gel can incorporate cobalt chloride to indicate when it is "spent" (i.e.

[18] Cobalt is essential for most higher forms of life, but more than a few milligrams each day is harmful.