When aluminium salts are added to water, Al^(3+) ions are immediately attracted to the negative end of polar water molecules.They form hexaaqualuminum(III) ions, [Al(H_2O)_5]^(2+)+H_3O^+ This is often written simply as Al^(+3)(aq).However the electric field associated with small, highly charged Al^(+3) ion is so intense that it draws electrons in the O-H bonds of water towards itself.This enables the water molecules to become donors.In aqueous solution, free water molecules act as bases and the following equilibrium is established : [Al(H_2O)_(6)]^(3+)+H_2OhArr [Al(OH)(H_(2)O)_(5)]^(2+)+H_3O^(+) Thus, solution of Al^(+3) salts are acidic, in fact as acidic as vinegar. When a base stronger than H_2O, e.g. S^(2-) is added to aqueous aluminium salts further H^+ ions are removed from [Al(H_2O)_6]^(3+) and insoluble aluminium hydroxide precipitates. 2[Al(H_2O)_6]^(3+)(aq)+3S^(2-)(aq)to2[Al(OH)_3(H_2O)_3(s)+3H_2S(g) More stronger base can remove 4H^+ ions are follows : [Al(H_2O)_6]^(3+)+4OH^(-)hArr[Al(OH)_4(H_2O)]^(-)(aq)+4H_2O(l) Which of [Al(H_2O)_6]^(3+),H_2S or H_2O is the strongest acid ?