1. Auto-protolysis or self-ionization reaction of water molecule is defined as that chemical reaction in which two molecules of water react with each other to form hydroxyl ion and hydronium ion.

2. The reaction involved can be represented as:

3. The auto-protolysis reaction of water molecule exhibit the amphoteric nature of water molecule that is the molecule of water can acts as an acid as well as a base.

4. The acid-base reaction can be written as: