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.

In case of water, it can behave as proton acceptor as well as proton donor.

For example, when HCl is added to water the reaction shows HCl acting as a proton donor (Bronsted acid) and \({ H }_{ 2 }O\) acting as a proton acceptor (Bronsted base). $$ HCl + \underset { Base }{ { H }_{ 2 }O } \rightleftharpoons { H }_{ 3 }{ O }^{ + } + { Cl }^{ - } $$ when \(N{ H }_{ 3 }\) is added to water the reaction shows \({ H }_{ 2 }O\) acting as a proton donor (Bronsted acid) and \(N{ H }_{ 3 }\) acting as a proton acceptor (Bronsted base). $$ N{ H }_{ 3 } + \underset { Acid }{ { H }_{ 2 }O } \rightleftharpoons { NH }_{ 4 }^{ + } + { OH }^{ - } $$ Therefore water behave as acids in one instance and can function as bases in the other. Such substances which can serve in either capacity depending on the conditions are said to be amphiprotic.