Iron (II) hydroxide precipitate (Fe(OH)2) formed by adding few drops of ammonium hydroxide (NH4OH) to a solution of ferrous sulfate (FeSO4). The reaction is FeSO4 + NH3(aq) + H2O -> Fe(OH)2 + (NH4)2SO4. This is an example of a double replacement reaction. Pure iron (II) hydroxide is white, however even trace amounts of oxygen make it greenish.