Metal hydroxides, image 2 of 2, formed from the addition of sodium hydroxide (NaOH) to solutions containing metal ions. From left to right are precipitates of: calcium (Ca(OH2)), magnesium (Mg(OH)2), copper (Cu(OH)2), iron (II) (Fe(OH)2), iron (III) (Fe(OH)3), cobalt (Co(OH)2), nickel (Ni(OH)2), manganese (Mn(OH)2), chromium (Cr(H2O)3(OH)3), silver (AgOH), Zinc (Zn(OH)2), lead (Pb(OH)2) and aluminium (Al(OH)3) hydroxides. The addition of NaOH is used to discover the identity of metal ions in solution, as the resulting precipitates have unique characteristics, for example colour and degree of solubility. See A500/418 for start of reaction.