If you add AlCl3 to water, why does the pH decrease?
A more clarified reaction in addition to Berry Holmes' answer:

$$\ce{AlCl3 + 4H2O &lt;=&gt; [Al(OH)4]- + 3Cl- + 4H+}$$

The reason this happens is the Lewis-Acidity of $\ce{Al^3+}$ as well as the higher affinity for $\ce{OH-}$ ions when compared with $\ce{Cl-}$.