Zinc chloride is the name of inorganic chemical compounds with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water. This salt is hygroscopic and even deliquescent. Zinc chloride finds wide application in textile processing, metallurgical fluxes, and chemical synthesis. No mineral with this chemical composition is known aside from the very rare mineral simonkolleite, Zn5(OH)8Cl2·H2O. Structure and properties Four crystalline forms (polymorphs) of ZnCl2 are known: α, β, γ, and δ. Each case features tetrahedral Zn2+ centers. Here a, b, and c are lattice constants, Z is the number of structure units per unit cell, and ρ is the density calculated from the structure parameters. The orthorhombic form (δ) rapidly changes to one of the other forms on exposure to the atmosphere. A possible explanation is that the OH− ions originating from the absorbed water facilitate the rearrangement. Rapid cooling of molten ZnCl2 gives a glass. Molten ZnCl2 has a high viscosity at its melting point and a comparatively low electrical conductivity, which increases markedly with temperature. As indicated by a Raman scattering study, the viscosity is explained by the presence of polymers,. Neutron scattering study indicated the presence of tetrahedral {ZnCl4} centers, which requires aggregation of ZnCl2 monomers as well.. In the gas phase, ZnCl2 molecules are linear with a bond length of 205 pm. Hydrates Five hydrates of zinc chloride are known: ZnCl2(H2O)n with n = 1, 1.5, 2.5, 3 and 4. The tetrahydrate ZnCl2(H2O)4 crystallizes from aqueous solutions of zinc chloride. Preparation and purification Anhydrous ZnCl2 can be prepared from zinc and hydrogen chloride: Zn + 2 HCl → ZnCl2 + H2 Hydrated forms and aqueous solutions may be readily prepared similarly by treating Zn metal, zinc carbonate, zinc oxide, and zinc sulfide with hydrochloric acid: ZnS + 2 HCl + 4 H2O → ZnCl2(H2O)4 + H2S Unlike many other elements, zinc essentially exists in only one oxidation state, 2+, which simplifies the purification of the chloride. Commercial samples of zinc chloride typically contain water and products from hydrolysis as impurities. Such samples may be purified by recrystallization from hot dioxane. Anhydrous samples can be purified by sublimation in a stream of hydrogen chloride gas, followed by heating the sublimate to 400 °C in a stream of dry nitrogen gas. Finally, the simplest method relies on treating the zinc chloride with