autoionization of water
Even the purest glass of water is never completely quiet. Now and then, two water molecules bump and one hands a proton to the other, splitting into a hydronium ion (H3O+) and a hydroxide ion (OH-). This faint, constant self-splitting is the autoionization of water — water quietly acting as both acid and base toward itself.
It is an equilibrium: H2O + H2O ⇌ H3O+ + OH-, governed by the constant Kw = [H3O+][OH-], which at 25 °C equals 1.0 × 10⁻¹⁴. The split is tiny — only about one water molecule in half a billion is ionized at any moment — but it is always there, and it ties hydronium and hydroxide together so that whenever one rises, the other must fall to keep their product fixed.
This quiet reaction matters because it sets the baseline of the whole pH scale. Neutral water has equal H3O+ and OH-, each 10⁻⁷ at 25 °C, which is why neutral pH is 7. It also means no acid can drive hydroxide all the way to zero, nor any base drive hydronium to zero. One honest caveat: Kw grows with temperature, so the neutral point drifts below pH 7 in hot water.
In pure water at 25 °C, exactly enough molecules split apart to give [H3O+] = [OH-] = 1.0 × 10⁻⁷ mol/L, so the water reads neutral at pH 7.
Water faintly splits into H3O+ and OH-, anchoring the neutral point of the pH scale.
Because [H3O+][OH-] = Kw is fixed, in any aqueous solution pH + pOH = 14 at 25 °C. Knowing how acidic a solution is automatically tells you how basic it is, and vice versa.