Self-ionization constant of water

Definition

Kw is the equilibrium constant for water self-ionization, equal to the product of hydrogen and hydroxide concentrations. At 25 degrees Celsius Kw equals 10 to the minus 14.

Kw forces hydrogen and hydroxide to be interdependent: push one concentration up and the other must fall to keep the product constant. Its value rises with temperature because heating makes self-ionization easier to pay for energetically. Because Kw governs both ions, knowing one concentration at fixed temperature determines the other.

Related concepts

Self-ionization of water · Temperature dependence of neutral pH · Neutral pH

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Part of The Blueprint of Water · 🧪 chemistry