Iron(III) chloride describes the inorganic compounds with the formula FeCl₃(H₂O)_x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated forms, which are both hygroscopic. They feature iron in its +3 oxidation state. The anhydrous derivative is a Lewis acid, while all forms are mild oxidizing agents. It is used as a water cleaner and as an etchant for metals. The anhydrous compound is a hygroscopic crystalline solid with a melting point of 307.6 °C. The colour depends on the viewing angle: by reflected light, the crystals appear dark green, but by transmitted light, they appear purple-red. Anhydrous iron(III) chloride has the BiI₃ structure, with octahedral Fe(III) centres interconnected by two-coordinate chloride ligands.Iron(III) chloride has a relatively low melting point and boils at around 315 °C. The vapor consists of the dimer Fe₂Cl₆, much like aluminium chloride. This dimer dissociates into the monomeric FeCl₃ (with D_3h point group molecular symmetry) at higher temperatures, in competition with its reversible decomposition to give iron(II) chloride and chlorine gas