water dissociation constant

Meaning: Water acts as both a weak acid and a weak base, so a tiny fraction of molecules transfer a proton to one another: 2 H₂O ⇌ H₃O⁺ + OH⁻. Liquid water's concentration is essentially constant, so it is absorbed into the constant, which gives Kw. In pure water at 25 °C, [H₃O⁺] = [OH⁻] = 1.0 × 10⁻⁷ M. In a dilute aqueous solution at that temperature the product stays 1.0 × 10⁻¹⁴, so raising one ion lowers the other. Autoionization absorbs heat (it is endothermic), so Kw increases as temperature rises.

Equation: Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, where concentrations are in mol/L (M) and Kw is written without units. Taking −log gives pKw = pH + pOH = 14. For a conjugate acid-base pair, Ka × Kb = Kw.

On the MCAT: You are given [OH⁻] from a strong base and asked for [H₃O⁺] or pH. You may need Kb of a conjugate base from the Ka of its acid. A harder question gives a temperature above 25 °C and asks you to predict that Kw is larger, so neutral water has a pH below 7.

Common confusion: Students equate neutral with pH 7. Neutral means [H₃O⁺] = [OH⁻]; pH 7 is neutral specifically at 25 °C. At higher temperature, water with a pH below 7 is still neutral, not acidic. Adding acid or base shifts the ion concentrations but leaves Kw unchanged, because only temperature changes it.