Iron(III) chloride

Iron(III) chloride, generically called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron(III) chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red. Iron(III) chloride is deliquescent, fuming in moist air due to the evolution of HCl, which hydrates, giving a mist.

When dissolved in water, iron(III) chloride undergoes hydrolysis and gives off heat as the reaction is exothermic. The resulting brown, acidic, and corrosive solution is used as a coagulant in sewage treatment and drinking water production, and as an etchant for copper-based metals in printed circuit boards. Anhydrous iron(III) chloride is a fairly strong Lewis acid, and it is used as a catalyst in organic synthesis.