Copper chloride dihydrate is a form of copper chloride that has formed crystals around molecules of water. In this state, each crystal of copper chloride contains two molecules of water, hence the name "dihydrate."
Anhydrous copper chloride is brown, but copper chloride hydrate is light blue. Copper chloride converts easily from its hydrate to its anhydrous form by heating it to water's boiling point (212 degrees Fahrenheit); the water leaves the crystals as steam and the copper chloride is said to be decomposed. Both forms of copper chloride have a chemical formula of one copper atom attached ionically to two atoms of chlorine.