[antimatter101]

I have read that as the temperature of water increases, so does its PH. While water at 25 degrees is neutral, when it is 37 degrees its PH is 6.81, though websites say that the number of hydronium and hydroxide ions are the same. This is confusing me! Does that mean that at higher temperatures, the acidity of hydronium ions increase, while the basicity of hydroxide ions decrease? THanks for *delete me*

Google does not want to tell me anything about increasing acidity of hydronium ions and decreased basicity of hydroxide ions. Maybe it is something else.

[antimatter101]

Or perhaps is it that hydronium ions are less acidic than pure H⁺ ions, and at higher temperatures the H⁺ ions cannot form hydronium ions as easily, so the PH decreases?

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product

[Corribus]

Note that the sum of pH + pOH also do not equal 14 at all temperatures.

[Montegue]

You didn't say whether your water is distilled water. If the water is from you pond or tap water it won't be neutral. De-chlorinated drink water in not neutral either. The pH will depend on the minerals in the water.

You might think that distilled water in neutral. No necessarily true. If there is dissolved carbon dioxide in the water, the pH will drop.

[Big-Daddy]

There is no increased or decreased acidity. A pure water solution is completely neutral, by definition of "neutral" (I believe). This means that [H+]=[OH-]. (Call [H+] by [H3O+] if you want)

pH=-log₁₀([H+])

This is the true definition of pH. Not that pH=7 or anything else which is relatively arbitrary.

It so happens that water has an ionic concentration of H+ of very close to 10^-7 mol/dm3, at temperatures of 298.15 K, as governed by the auto-ionization constant of water, Kw. Now as you should know, equilibrium constants change with temperature. So if you change the temperature, Kw will be different. [H+] for pure water = Kw^(1/2) by definition of [H+]=[OH-] which is our tenet for saying water is truly neutral (if you don't understand any of this about Kw, look up the Kw expression). Thus pH=-log₁₀(Kw^(1/2)), and as we've noted Kw is temperature-dependent. So it should be obvious why pH, too, is temperature dependent.

So the acidity is not increasing. If you want to talk about acidity, you might go for [H⁺]/[OH^-] ratio but I'm just thinking on the spot here. But the argument certainly works for pure water, which would be neutral as [H+]=[OH-], thus the ratio is 1 regardless of temperature, thus water is not acidic. Or basic. It's just neutral.

Hope this helps