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There are three theories that identify a singular characteristic which defines an acid and a base: the Arrhenius theory, for which the Swedish chemist Svante Arrhenius was awarded the 1903 Nobel Prize in chemistry; the BrOnsted-Lowry, or proton donor, theory, advanced in 1923; and the Lewis, or electron-pair, theory, which was also presented in 1923. Each of the three theories has its own advantages and disadvantages; each is useful under certain conditions.
The Arrhenius Theory
When an acid or base dissolves in water, a certain percentage of the acid or base particles will break up, or dissociate (see dissociation), into oppositely charged ions. The Arrhenius theory defines an acid as a compound that can dissociate in water to yield hydrogen ions, H+, and a base as a compound that can dissociate in water to yield hydroxide ions, OH+, and also chloride ions, Cl- . The base sodium hydroxide, NaOH, dissociates in water to yield the required hydroxide ions, OH-, and also sodium ions, Na+.
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