Many simple compounds of elements such as $\mathrm{AlCl}_3, \mathrm{GaCl}_3$ and $\mathrm{InCl}_3$ are covalent while anhydrous but in aqueous solution, these are ionic in nature. In anhydrous condition, the (charge/radius) ratio, i.e. polarisability of $\mathrm{Al}^{3+}$ is high and hence, according to Fajans' rule, $\mathrm{Al}^{3+}$ polarises $\mathrm{Cl}^{-}$ions to large extent, there introducing covalent character in the compound, i.e. $\mathrm{AlCl}_3$ 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 $\mathrm{Al}^{3+}$, the tendency of $\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}$ to polarise hydrated $\mathrm{Cl}^{\ominus}$ ion decreases and the resulting hydrated compound is ionic in nature.