Copper(I) chloride, commonly called cuprous chloride, is the lower chloride of copper, with the formula CuCl. The substance is a white solid sparingly soluble in water, but very soluble in concentrated hydrochloric acid. Impure samples appear green due to the presence of copper(II) chloride (CuCl2). History Copper(I) chloride was first prepared by Robert Boyle in the mid-seventeenth century from mercury(II) chloride ("Venetian sublimate") and copper metal: HgCl2 + 2 Cu → 2 CuCl + Hg In 1799, J.L. Proust characterized the two different chlorides of copper. He prepared CuCl by heating CuCl2 at red heat in the absence of air, causing it to lose half of its combined chlorine followed by removing residual CuCl2 by washing with water. An acidic solution of CuCl was formerly used for analysis of carbon monoxide content in gases, for example in Hempel's gas apparatus. This application was significant during the nineteenth and early twentieth centuries when coal gas was widely used for heating and lighting. Synthesis Copper(I) chloride is produced industrially by the direct combination of copper metal and chlorine at 450–900 °C: 2 Cu + Cl2 -> 2 CuCl Copper(I) chloride can also be prepared by reducing copper(II) chloride with sulfur dioxide, or with ascorbic acid (vitamin C) that acts as a reducing sugar: 2 CuCl2 + SO2 + 2 H2O -> 2 CuCl + H2SO4 + 2 HCl 2 CuCl2 + C6H8O6 -> 2CuCl + 2HCl + C6H6O6 Many other reducing agents can be used. Properties Copper(I) chloride has the cubic zincblende crystal structure at ambient conditions. Upon heating to 408 °C the structure changes to hexagonal. Several other crystalline forms of CuCl appear at high pressures (several GPa). Copper(I) chloride is a Lewis acid, which is classified as soft according to the Hard-Soft Acid-Base concept. Thus, it forms a series of complexes with soft Lewis bases such as triphenylphosphine: CuCl + 1 P(C6H5)3 → 1/4 {CuCl[P(C6H5)3]}4 CuCl + 2 P(C6H5)3 → CuCl[P(C6H5)3)]2 CuCl + 3 P(C6H5)3 → CuCl[P(C6H5)3)]3 Although CuCl is insoluble in water, it dissolves in aqueous solutions containing suitable donor molecules. It forms complexes with halide ions, for example forming H3O+ CuCl2− in concentrated hydrochloric acid. Chloride is displaced by CN− and S2O32−. Solutions of CuCl in HCl or NH3 absorb carbon monoxide to form colourless complexes such as the chloride-bridged dimer [CuCl(CO)]2. The same hydrochloric acid solutions also react with acetylene gas to form [CuCl(C2H2)]. Ammoniacal solutions of