posted by Alex
Aluminium sulphate when dissolved in water forms aluminium hydroxide and sulphuric acid.Since, aluminium hydroxide is insoluble in water, a clear solution is not obtained.
Al2(SO4)3 + 6H2O →2Al(OH)3 + 3H2SO4Aluminium Aluminiumsulphate Hydroxide
In anhydrous condition aluminium sulphate is a white powder. Hydrated, it forms colourless, acicular, sour-tasting crystals. Aluminium sulphate dissociates in water to form a colloidal solution of aluminium hydroxide molecules and positively charged aluminium hydroxide ions (Al (OH)2+, Al (OH)2+):
Al2(SO4)3 + 6 H2O <--> 2 Al(OH)3 + 3 SO42- + 6 H+
Through this process, such aqueous solutions are reacting strongly acidic. The hydroxide formation comes – after the achievement of equilibrium – to a halt, so that the solution also contains some free Al3+ ions. The cationic activity of aluminium sulphate solutions is dissolved by the positively charged aluminium hydroxide ions, which are aggregated with the aluminium hydroxide molecules.
Note from Bob Pursley's response, that Al2(SO4)3 IS soluble in water and it does NOT form a ppt; therefore, the solution of same is colorless.