Final answer:
In the Brønsted-Lowry theory, an acid is a species that can donate a proton, expanding the definition of acids and bases beyond the Arrhenius theory to include a wider variety of chemical reactions.
Step-by-step explanation:
According to the Brønsted-Lowry theory of acids and bases, an acid is defined as a species that can donate a proton (hydrogen ion, symbolized as H+) to another molecule. This concept was introduced in 1923 when Johannes Brønsted and Thomas Lowry independently proposed that acids and bases should be defined by their ability to donate and accept protons, respectively. This definition broadened the understanding of acids and bases beyond the previous Arrhenius definition, which was limited to substances dissociating in water to provide hydronium and hydroxide ions.
For example, in the reaction between ammonia (NH3) and water (H2O), ammonia acts as a Brønsted-Lowry base by accepting a proton from water, which acts as a Brønsted-Lowry acid. As a result, the ammonium ion (NH4+) and hydroxide ion (OH−) are formed:
NH3(aq) + H2O(l) ⇒ NH4+(aq) + OH−(aq)