Any chemical compound, one element of which is hydrogen, that dissociates in solution to produce free hydrogen ions. See [[ion]]. For example, hydrochloric acid, HCl, dissociates in water to produce hydrogen ions, H<sup>+</sup>, and chloride ions, Cl<sup>-</sup>. This reaction is expressed chemically as HCl{{:terms:p7arrows.gif}} H<sup>+</sup>Cl<sup>-</sup>. See [[dissociation]].