Sometimes water molecules break apart, or dissociate, into ions. Each water molecule splits into a hydrogen ion (H plus) and a hydroxide ion (OH minus). In pure water, the concentrations of hydrogen ions and hydroxide ions are equal, as shown here, but the actual concentrations of these ions are drastically less than shown in this animation. In pure water, only one water molecule in every 554 million is dissociated. Living things are very sensitive to the concentrations of hydrogen ions (H plus) and hydroxide ions (OH minus). The pH scale is used to express the concentration of hydrogen ions in a solution. In a neutral solution like pure water, the pH is 7, which is the midpoint of the pH scale. In a neutral solution, the hydrogen ion and hydroxide ion concentrations are equal. An acid is any substance that increases the hydrogen ion (H plus) concentration of a solution. When the pH is less than 7, the solution is acidic and the concentration of hydrogen ions (H plus) is greater than the concentration of hydroxide ions (OH minus). The higher the pH, the less acidic the solution and the lower the concentration of hydrogen ions. A base is any substance that reduces the hydrogen ion concentration of a solution. When the pH is greater than 7, the solution is basic and the concentration of hydroxide ions is greater than the concentration of hydrogen ions. The higher the pH, the more basic the solution and the lower the concentration of hydrogen ions. Our animation is meant only to suggest the changes in concentration as the pH changes. The actual proportions of hydrogen ions and hydroxide ions are very small. The pHs of common solutions are shown next to the pH scale. Each change in a unit of pH represents a tenfold change in hydrogen ion concentration. For example, a milliliter of gastric juice at pH contains ten times as many hydrogen ions as a milliliter of vinegar at pH 3. And a milliliter of household bleach at pH contains one-tenth the hydrogen ions in a milliliter of household ammonia at pH 11.