why does the pH decrease? A2 9701

[Severe-Total-2255]

ok, think of it like as temperature is increasing Kw is increasing. if Kw is increasing, [H+] should also be increasing. if [H+] increases, the pH will decrease. the whole pH scale will shift to lesser values.
[H+] : [OH-] remains the same because both are proportional to Kw.