You are not adding any hydrogen ions, so why does the pH of the solution decrease? Thank you in advance.
EDIT: I will add the context of my asking this question to clarify some details. This is not a homework question. I am preparing for an upcoming examination in Chemistry, and I found this statement in the mark scheme of a practice exam I was doing, and yet I don't understand why it is true.
I have learnt pH as simply being the negative log of the concentration of H+ ions. I fail to see how adding the above salt changes the pH, especially since HCl is a strong acid (hence it completely dissociates in solution).
Thought process: Is the pH change a result of Aluminium forming Aluminium Oxide?