The understanding of the interaction of aluminium chloride with water, and the properties of any resulting solution, seem to cause students a great deal of trouble. Very few are able to clearly explain, for example, why aqueous solutions of aluminium chloride are acidic.

In order to deal with this you have to be clear whether you are considering:

•  the interaction of a small amount of water with anhydrous aluminium chloride, or

• a solution of aluminium chloride in water.

Anhydrous aluminium chloride and water.

The structure of solid aluminium chloride is complex and temperature dependent ^{1, 2} ; however at room temperature with water it simply behaves like any other covalent metallic chloride and is hydrolysed.

If a small amount of water - a few drops - is added to anhydrous aluminium chloride there is a hissing sound and hydrogen chloride gas is given off as steamy acidic fumes. The reaction that occurs will depend on the amount of water added, but is probably

AlCl₃(s) + H₂O(l) → AlCl₂OH(s) + HCl(g)

 The products are a solid and a gas - no acidic solution is formed.

Aluminium chloride in aqueous solution.

If aluminium chloride is dissolved in a large amount of water the solution is acidic, but this has nothing to do with formation of hydrochloric acid. 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)

References:

1.   Greenwood NN and Earnshaw A, 'Chemistry of the Elements': Pergamon 1990.

2.   A page on the structure of aluminium chloride is here.