In pure water at 25o C, the concentration of H⁺ is always equal to the concentration of OH^-. Both have a concentration of 1.0 x 10^-7 Molar. (Placing the symbol for a chemical in brackets [H⁺] is chemical shorthand for “concentration of.” Therefore, [H⁺] is read “the concentration of hydrogen ion.”) If we add a substance that results in an increase in [H⁺], we say that substance is an acid. If we add a substance that results in a decrease in [H⁺], we say that substance is a base. An acid is any substance that, when added to an aqueous solution, increases the [H⁺] of the solution, and a base is any substance that, when added to an aqueous solution, decreases the [H⁺] of the solution. A common acid, for example, is hydrochloric acid, HCl. When HCl reacts with water, it dissociates into an H+ and a chloride ion (Cl^-), thus increasing the [H⁺]. HCl is considered a strong acid because when placed in water, it completely dissociates into its two ions.

HCl --> H⁺ + Cl^-

A weak acid, such as acetic acid (CH₃COOH), dissociates into H⁺ and CH₃COO^-. However, most remain as acetic acid, and there is a chemical equilibrium between the CH₃COOH and the H⁺ + CH₃COO^-.

CH₃COOH <-- --> H⁺ + CH₃COO^-

An example of a base is ammonia (NH₃), which will combine with H⁺ to form an ammonium ion (NH₄⁺), thus removing H⁺ from the solution.

NH₃ + H⁺ --> NH₄⁺

Another common base is sodium hydroxide (NaOH). How is this a base? When it dissolves, it dissociates into a sodium ion (Na⁺) and OH^-, no change in [H⁺], right? However, the OH^- will combine with H⁺ to form water, thus removing H⁺ from the solution.

NaOH --> Na⁺ + OH^-

H⁺ + OH^- --> H₂O