Distilled Water

Pure water is about 7 at 25 °C; this value de­creas­es with in­creas­ing tem­per­a­tures. At 50 °C, it is 6.55. It is also worth to note, that water that has been exposed to air is mildly acidic, because it absorbs carbon dioxide from the air.

The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from water is an endothermic process.

$\ce{H2O_{(l)} \rightleftharpoons H+_{(aq)} + OH}^-_{\text{(aq)}}$

If we increase the temperature, according to Le Châtelier's Principle, the reaction will shift its equilibrium position to reduce the temperature by favoring the forward reaction which absorbs heat. The effect is that $K_w$ increases with temperature while $pH$ decreases ( more ).

Kw increases with the temperature, water ionization is an endothermic process, according to Le Châtelier's Principle, the position of equilibrium moves to counter the change,increasing the temperature will make it reduce the temperature going to the forward reaction which absorbs heat, more hydrogen and hydroxide ions will be formed