The solution contains hydrated aluminium ions and chloride ions:
AlCl3(s) + aq → [Al(H2O)6]3+(aq) + 3Cl -(aq)
The hexaqua complex ion behaves exactly like ions of similar type formed from transition metals; the small, highly charged metal ion polarises (withdraws electron density from) the water molecules that are attached to the aluminium ion through dative covalent bonds. This makes the hydrogen atoms d+ and susceptible to attack from solvent water, which is acting as a base. The complex ion is deprotonated, causing the solution to be acidic from the formation of hydroxonium ions H3O+:
[Al(H2O)6]3+(aq) + H2O(l) → [Al(H2O)5OH]2+(aq) + H3O+(aq)