Up to this point our experimental evidence has pointed to pure water being a non-electrolyte. With better instruments it is found that a small number of water molecules actually dissociate:

H₂O(ℓ) H⁺(aq) + OH^-(aq)

or using Brønsted conventions:

2 H₂O(ℓ) H₃O⁺(aq) + OH^-(aq)

This equilibrium lies very far to the left at 25^oC. Following the pattern we have used to write K_c expressions for other reactions, the liquid water reactant would be excluded, giving:

K_c = [H⁺][OH^-] = 1.0 x 10^-14 = K_w