Arrhenius concept: Arrhenius in 1884 defined acids as those compounds which give hydrogen ions in aqueous solution, and bases2 as those compounds which give hydroxide ions in aqueous solution.
\(HCl\underset{\text{acid}}{(aq.)} \rightleftharpoons H^+ (aq.) + Cl^- (aq.)\)
\(\underset{\text{base}} {NaOH}{(aq.)} \rightleftharpoons Na^+ (aq.) + OH^- (aq.)\)
