Acid and Base Salt Equation

[Nathaniel 2E]

A problem calls for writing out the equilibrium equation for AlCl3. The solution writes it out as:

Al(H2O)6 3+ (aq) + H2O (l) <---> H3O+ (aq) + Al(H2O)5OH 2+ (aq)

What does the H2O in parentheses attached to the aluminum ions mean? Why do we assume they exist and write the equation like that?

[Alyssa Wilson 2A]

This is a double replacement reaction, and we assume the aluminum ion is attached to a H2O molecule, because it is formed with a ligand complex that forms when AlCl3 reaches equilibrium. Al(H2O)5 OH2 is Pentaaqua-hydroxyaluminum(iii) ion.