Autoionization of Water and Kw

Pure water should be the ultimate neutral liquid, yet it conducts a faint current. That's because water molecules react with each other. One water molecule grabs a proton (H+) off another: H_2O + H_2O → H_3O^+ + OH^−. This is autoionization, and it happens constantly, in both directions, even in water with nothing else dissolved in it. At any instant a tiny fraction of molecules exist as hydronium and hydroxide instead of neutral H_2O.

At 25 C this equilibrium settles at a fixed product: Kw = [H_3O^+][OH^-] = 1.0 x 10^−14. Because autoionization makes one hydronium for every hydroxide, pure water has [H_3O^+] = [OH^-] = 1.0 x 10⁻⁷ M, which is where pH 7 comes from. But Kw is an equilibrium constant, and like all equilibrium constants it depends on temperature. Autoionization is endothermic, so heating water pushes the reaction forward, raising both ion concentrations and making Kw bigger. At 50 C, Kw climbs above 1.0 x 10⁻¹⁴, so neutral water now has [H_3O^+] slightly greater than 1.0 x 10⁻⁷ M, and neutral pH drops below 7, even though the water is still perfectly neutral since the two ions stay equal to each other.

This idea anchors everything else in the unit. Kw is the tool you use to flip between [H_3O^+] and [OH^-] in any aqueous solution, acidic, basic, or neutral, and it is what makes pH and pOH always add to 14 at 25 C. Buffers, titration curves, and the relationship between Ka and Kb all lean on this same constant relationship.