Self-ionization of water

Definition

Self-ionization is the reversible reaction in which water molecules transfer a proton to one another, forming small amounts of hydronium and hydroxide. It is written H2O reversibly equals H+ plus OH-, or more honestly 2 H2O reversibly equals H3O+ plus OH-.

The process is rare, about one molecule in 555 million is split at any instant, but it never stops and is fully reversible. It explains why chemically pure water carries a small electrical conductivity near 0.055 microsiemens per centimeter. The forward and reverse rates are equal at equilibrium, leaving a constant tiny population of ions.

Related concepts

Neutral pH · Equilibrium · Hydrogen-Bond Network

How it connects

• this enables pH

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Part of The Blueprint of Water · 🧪 chemistry