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An acid is a substance that donates a proton (H+ ion) when dissolved in water, while a base is a substance that accepts a proton (H+ ion) when dissolved in water. In other words, acids increase the concentration of H+ ions in a solution, while bases decrease the concentration of H+ ions by accepting them. The most common definition of acids and bases is given by the Brønsted-Lowry theory, which categorizes a substance as an acid if it donates a proton and as a base if it accepts a proton in the presence of another substance.

During an acid-base reaction, a proton is transferred from the acid to the base. As a result, the acid becomes a conjugate base (having lost the H+ ion) and the base becomes a conjugate acid (having accepted the H+ ion). This process, in which a proton is exchanged between the reactants, is the reason why acid-base reactions are sometimes called proton-transfer reactions. For example, when hydrochloric acid (HCl) reacts with ammonia (NH3), a proton is transferred from HCl to NH3, resulting in the formation of the ammonium ion (NH4+) and the chloride ion (Cl-): \[ HCl + NH3 \rightarrow NH_4^+ + Cl^- \] In this reaction, HCl acts as the acid, while NH3 acts as the base. The proton is transferred from HCl to NH3, thus classifying this reaction as a proton-transfer reaction.