Acids and bases typically exist in a state of equilibrium. In a sample of an acid, some molecules will give up their hydrogen ions and others will accept them.

Even water is a mixture of an acid ion (H₃O⁺) and a basic ion (OH^-). A hydronium ion (H₃O⁺) will give up its proton to a hydroxide ion, forming two molecules of water, which is neutral. The chemical equation below shows this.

H₃O⁺ + OH^- 2H₂O

This process is also called the auto-ionization of water. The following animation shows this process.

This reaction happens continuously in water; therefore, the sample is neutral because there are equal amounts of hydronium and hydroxide in the sample. For most solutions, however, the hydronium and hydroxide ions are not present in equal amounts. When this imbalance occurs, a solution is not neutral.

The equilibrium, or ionization, constant for water (Kw) is 1×10^-14 (mol/L)².

Because water is neutral, and therefore must have equal concentrations of H₃O⁺ and OH^-, you can write an equation for the dissociation of water as follows:

Kw= [H⁺] [OH^-]
10^-14= [10^-7] [10^-7]

The [H⁺] is the concentration of hydrogen ions in a solution. The [OH^-] is the concentration of hydroxide ions in a solution. You will return to a discussion of this equation later on in this lesson.