A unique property of water is its ability to ionize or undergo dissociation. The Hydrogen atom is very light and is held tightly by the oxygen atom. Because of its mobility and small mass it has a tendency to dissociate in the following manner:

This is the conventional method to show dissociation of water ,but in actual practice the hydrogen atom of one water molecule after dissociation hops over to the neighboring water molecule to form a positively charged  hydro-nium (H₃O) ion , leaving a negatively charged hydroxyl group (OH¯).

           
   
Electrical Conductivity measurement show that one litre of water at 25⁰C contains 1.0 x 10^-7 moles of water H₃O+  ions and the same amount of OH¯ ions. The reason why a Hydronium ion is formed is explained by the Fact that protons do not occur as free ions in water, while the hydronium ion has a high electrical mobility. It is possible to calculate the equilibrium constant of dissociation of water from the equation.




   

Where the ions in brackets [] represent the concentration in moles per litre. In pure water the H⁺ = OH¯ = 1.0 X 10^-7  and the ion product of water,K_W=[H⁺][OH¯] at 25⁰C is 1.0X 10^-14.