Also known as copper (II) chloride, cupric chloride is a light brown solid chemical compound which slowly oxidizes to a dihydrate that is blue-green in color. Second only to copper sulfate in terms of common copper based compounds, both the anhydrous and dihydrate forms of cupric chloride do occur naturally in the exceptionally rare minerals tobachite and erochalcite. The process of commercial synthesis of cupric chloride involves the direct chlorination of copper metal. Pure copper does not oxidize when reacted with hydrochloric acid, but copper-containing bases such as copper carbonate, oxide, and hydroxide all react to hydrochloric acid. The resulting oxidized product is then purified by crystallization, a process initiated by placing hot dilute hydrochloric acid in a calcium chloride ice bath to force crystal formation. The most common uses for cupric chloride are as a catalyst in various commercial chemical production processes such as acetaldehyde and chlorine, as well as in various narrow applications in organic chemistry.