Anhydrous aluminium chloride is hygroscopic, having a very pronounced affinity for water. It fumes in moist air and hisses when mixed with liquid water as the Cl ligands are displaced with HO molecules to form the hexahydrate . The anhydrous phase cannot be regained on heating the hexahydrate. Instead HCl is lost leaving aluminium hydroxide or alumina (aluminium oxide):
Like metal aquo complexes, aqueous  is acidic owing to the ionization of the aquo ligands:
Aqueous solutions behave similarly to other aluminium salts containing hydrated  ions, giving a gelatinous precipitate of aluminium hydroxide upon reaction with dilute sodium hydroxide: