It is water’s Ionization Property. Water Ionization also known as self-ionization or auto ionization of water creates a positive and Negative ion which is essential for chemical reactions. It is a reaction of pure water in where a water molecule and H2O combined to make a hydroxide ion. It usually occurs due to the electric field fluctuation between 2 neighbouring molecules. It is a property of pure water that mixed or dissociates to from a new substance which is a hydrogen ion (positive ion) and hydroxide ions (negative ion)