 When dissolved in water , iron ( III ) chloride undergoes hydrolysis and gives off heat in an exothermic reaction . The resulting brown , acidic , and corrosive solution is used as a flocculant in sewage treatment and drinking water production , and as an <unk> 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 . 
