Water is an amphiprotic molecule.This ability helps water to act as both a proton donor and a proton acceptor, this property of water suggests that a proton transfer exists for pure water
2 H2O (l) <-------------------> OH- (aq) + H3O+ (aq)
the equilibrium constant for this reaction is water equilibrium constant (Kw).Its value is 1.0 x 10-14 at 25 c.
In pure water the hydronium and hydroxide ion concentrations are equal:We find these concentrations by taking the square root of Kw :
[H3O+] = [OH- ] = 1.0 x 10-7 M
Equal concentration of these two ions means that pure water is neither acidic nor basic.