Why would the pH of pure neutral water heated from 25 Celsius to boiling DECREASE?

Answer 1

H2O <=> H+ + OH-

Kw= [H+][OH-]
For pure water (with no CO2 dissolved in it) [H+]=[OH-] so

Kw=[H+]^2 =>pH =0.5pKw
where pKw=-logKw

At 25 Celsius Kw~10^-14, thus pH=0.5*14= 7

Kw is a function of temperature (and pressure). As you increase the temperature, Kw increases (the self-dissociation of water is endothermic thus increasing the temperature favours the products) thus pKw decreases and the pH also decreases.