Isn’t that easy? A substance, like sodium, that depletes hydrogen from water and forms hydroxyl ions is a base. A substance, like HCl, that adds hydrogen to water and forms hydronium ions is an acid. It’s that simple.

We can measure the concentration of hydrogen ions in a solution (and thereby measure the acidity). This measurement is called the pH. Technically, the pH is the inverse of the log of the hydrogen ion concentration—what this means, quite simply, is that the lower the pH, the more hydrogen ions there are in solution, and the more acid it is. The higher the pH, the less hydrogen ions there are—meaning more hydroxyl ions. A high pH indicates more hydroxyl ions and a basic solution.

Many biological molecules are acids or bases. Many are both acids and bases—as we shall see.