Water self-ionises – essentially it reacts with itself to very small extent. We know this because even pure water has a slight electrical conductivity.
H2O(l) + H2O(l) ⇌ H3O+(aq) + OH–(aq) or H2O(l) ⇌ H+(aq) + OH–(aq)
As with all equilibrium reactions, we can determine an equilibrium constant for the reaction:
Kw = [H+(aq)] [OH–(aq)]
Kw = 1.00 x 10-14 mol2 dm-6 at 298K
Ionisation of water is an endothermic process (ΔH⦵ = +6.5 kJ mol-1) and so is favoured by higher temperatures e.g. Kw = 9.5 x 10-14 mol2 dm-6 at 60°C.