Ionic Product of Water
The equilibrium constant for the self-ionisation of water, equal to the product of the hydrogen and hydroxide ion concentrations.
Formula / equation
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴
Unit
mol²/L²
In depth
Kw applies in every aqueous solution, whether acidic, neutral or alkaline, so knowing one ion concentration always gives the other. Because self-ionisation is endothermic, Kw rises with temperature: neutral water at 60 °C has a pH of about 6.5 while remaining exactly neutral.
Examples in the real world
In 0.01 M HCl, [H⁺] = 0.01 M forces [OH⁻] down to 1.0 × 10⁻¹² M.