Anhydrous copper II chloride (CuCl2) is a white or yellow-brown crystalline powder that is commonly used as a chemical reagent and desiccant. It is used to test for the presence of water because it readily absorbs moisture from the air, turning blue in the process. This change in color is due to the hydration of CuCl2, which forms a blue complex with water molecules.

The reaction between CuCl2 and water can be represented as follows:

CuCl2 + 2H2O → CuCl2·2H2O (blue)

The formation of the blue complex is a qualitative indication of the presence of water. The intensity of the blue color increases with the amount of water present.

Anhydrous CuCl2 is often used as a desiccant in laboratories and other environments where it is necessary to remove moisture from the air. It is also used as a reagent in a variety of chemical reactions, such as the preparation of other copper salts.