Many simple compounds of elements such as , and are covalent in anhydrous form, but in aqueous solution, these are ionic in nature. In anhydrous condition, the (charge/radius) ratio i.e. polarisability of is high and hence, according to Fajans' rule, polarises ions to large extent, thus exhibits covalent character in the compound, i.e. behaves as a covalent compound in anhydrous conditions. In aqueous medium the ions get hydrated, because the amount of hydration enthalpy released exceeds, the sum total of ionisation enthalpy required.
Since, the (charge/radius ratio of hydrated aluminium ion is much smaller as compared to that of , the tendency of to polarise hydrated ion decreases and the resulting hydrated compound is ionic in nature.