By Sunil Bhardwaj

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According to Bronsteo-Lowry, an acid is a substance that is able to donate a proton \(({ H }^{ + })\) to some other substance and a base, is a substance that is able to accept a proton from an acid.

Acid \(\longrightarrow\) a proton donor Base \(\longrightarrow \) a proton acceptor.

For example, when HCl is added to water the reaction shows HCl acting as a proton donor (Bronsted acid) and \({ H }_{ 3 }{ O }^{ + }\) acting as a proton acceptor (Bronsted base). $$ HCl + { H }_{ 2 }O \rightleftharpoons { H }_{ 3 }{ O }^{ + } + { Cl }^{ - } $$ The reverse of the above reaction is also an acid base reaction, with hydronium ion serving as an acid by giving up its proton, and with chloride ion functioning as a base by accepting it. $$ \underset { Acid }{ HCl } + \underset { Base }{ { H }_{ 2 }O } \rightleftharpoons \underset { Acid }{ { H }_{ 3 }{ O }^{ + } } + \underset { Base }{ { Cl }^{ - } } $$ Thus, we might view our reaction as an equilibrium where we have two acids and two bases. Thus, every Bronsted acid is coupled with a related Bronsted base. When the acid HCl reacts it yields the base \({ Cl }^{ - }\). These two substances are related to one another by the loss or gain of a single proton. These two together constitute conjugate acid-base pair. ln general, we say that every acid has its conjugate base and every base has its conjugate acid.