Self-Ionisation of Water
The reaction in which two water molecules transfer a proton to give a hydronium and a hydroxide ion.
Formula / equation
2H₂O ⇌ H₃O⁺ + OH⁻
Unit
mol/L
In depth
Also called autoprotolysis, this equilibrium lies far to the left: only about one molecule in 5.5 × 10⁸ is ionised at 25 °C. Nevertheless it is what makes pure water very slightly conducting and is the reason a pH scale exists at all.
Examples in the real world
Pure water containing 1.0 × 10⁻⁷ M of each ion at 25 °C, giving a pH of exactly 7.