Concept preview

What you will be able to do

1. 01Explain the self-ionization of water
2. 02Use Kw to find one ion concentration from the other
3. 03Classify solutions as acidic, basic or neutral

Even perfectly pure water is very slightly ionized. A tiny fraction of water molecules transfer a proton to one another in a process called self-ionization (or autoionization): $\mathrm{H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-}$, often written more simply as $\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$. It is this handful of ions that allows even ultra-pure water to conduct a minute electric current. Water is behaving amphiprotically here — one molecule acts as an acid and the other as a base.