In a container of water a small number of water molecules separate (dissociate) into hydrogen ions (H⁺) and hydroxyl ions (OH^-). When certain compounds are placed in water they increase the concentration of hydrogen ions (acids) or decrease it (bases). When an acid, such as hydrochloric acid, is added to water it dissociates increasing the number of hydrogen ions. When a base, such as sodium hydroxide, is added to water it dissociates. The resulting hydroxyl ions join with some of the hydrogen ions in solution to form water and decrease the number of hydrogen ions.