Acids and bases are compounds with opposing sets of characteristics. In 1884, Arrhenius developed a concept in which he defined an acid as a substances that releases protons in aqueous solution , and a base as one that release OH ions. Acid base reaction in water are shown below:

Substances which yield H⁺ and OH¯ in aqueous solutions are called amphoteric. An extension of Arrhenius’s concept is the solvent–system concept in which the solvent is capable of producing cations and anions corresponding to acid and base. Thus H₂O is capable of producing H₃O+ and OH. In 1923, Bronsted –Lowry modified the Arrhenius concept and defined an acid as a proton donor and a base as a proton acceptor. In the same year, G.N. Lewis proposed yet another definition of acids and bases. According to him an acid can accept an electron pair from another molecules, while a base can share its electron pair with an acid.

This is a completion for protons between two bases, and an acid –base combination represents a conjugate pair.

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