Does pH change with dilution?

pH is generally calculated the negative log of the hydrogen ion concentration in solution.

If you dilute an acidic solution (with water), you lower the [H+] concentration, and

if you dilute a basic solution you increase the [H+].

 

You could also think of it as the [H+] concentration in both cases approaching the [H+] concentration of pure water (pH ~7).

 

So the pH of an acid will increase and the pH of a base will decrease.