Ionic Product of Water (Kw at 25 °C)

Water is faintly self-ionising: about one molecule in 5.5 × 10⁸ has handed a proton to a neighbour at any instant, giving H₃O⁺ and OH⁻ at 1.0 × 10⁻⁷ mol/L each. The product of those two concentrations is fixed at any given temperature, and at 25 °C it is 1.0 × 10⁻¹⁴ — which is the only reason pH + pOH = 14 and the only reason pH 7 means neutral. Careful conductance work puts the measured value slightly higher, pK_w = 13.995 or K_w = 1.01 × 10⁻¹⁴; the round 1.0 × 10⁻¹⁴ is a convention that survives because it is within a rounding error of the truth and makes the arithmetic clean.

The temperature caveat is not academic. Autoionisation is endothermic, so heating drives it forward: at 0 °C, K_w = 1.14 × 10⁻¹⁵ and neutral water sits at pH 7.47; at 100 °C, K_w = 5.1 × 10⁻¹³ and neutral is pH 6.14. Inside a boiler drum at 300 °C, pK_w has fallen to roughly 11.3, so neutral is near pH 5.7 — a hot condensate line reading pH 6 is not acidic, it is neutral, and a plant chemist who forgets that will dose caustic into water that never needed it. Note also that the water has not become acidic in any of these cases: [H⁺] and [OH⁻] rise together and stay equal.