The pH of a solution is a measure of the hydronium ion (H₃O⁺) concentration in that solution. The pH is measured on a scale from 0-14. The hydronium ion in a solution results from the selfionization of water.

A drop of pure water is not composed entirely of H₂O molecules. Water always contains small quantities of two ions, hydronium ions and hydroxyl ions (OH^-). The presence of both of these ions makes water an amphoteric substance. An amphoteric substance can act either as an acid or as a base. An acid is a substance which can donate H⁺ ions. When a water molecule acts as an acid, it becomes an OH^- ion. A base accepts H⁺ ions. When a water molecule acts as a base, it becomes an H₃O⁺ ion. The self-ionization of water is a reaction that occurs between two water molecules...