Water is a truly unique molecule made up of two hydrogen atoms bound to one oxygen atom. It is highly polar because of the angle made by the the three molecules and is known as the "universal solvent" because it can dissolve just about anything other than nonpolar substances.
Water is intriguing because it acts as both an acid and a base (and in effect neutralizes itself). A small number of water molecules in a pure solution will dissociate into a proton and a hydroxyl ion.
The ionization constant of water is the concentration of protons multiplied by the concentration of hydroxyl ions, which always equals 10^ (-14) molar concentration squared.
Because water has an exact ionization constant, the pH of an aqueous solution can always be measured or calculated by determining the concentration of protons in the solution. This is possible because a rise in proton concentration will result in a drop in hydroxyl ion concentration, and vice versa.
If water molecules did not dissociate into protons and hydroxyl ions, the laws governing aqueous solutions would be radically different. In fact, life itself might not be possible.