The original conception of an acid was the Arrhenius definition: any substance that increases concentration in an aqueous solution. A base was then any substance that increases concentration in aqueous solution. This definition was broadened by the Bronsted-Lowry definition: an acid is any substance that donates a proton, while a base is any substance that accepts a proton. The definition was finally expanded to the Lewis definition: an acid is any substance that accepts an electron pair, and a base is any substance that donates an electron pair. Conceptually, the Bronsted acids and the Lewis acids can be related by considering positive and negative charges. An acid can be thought of as a substance with "too much" positive charge. It can therefore give away some of this positive charge (Bronsted definition), or it can accept additional negative charge (Lewis definition). The advantage of the Lewis definition is that it is more general and describes the acidic behavior of more substances.
Example:
Classify the following as Bronsted acids or bases (all in aqueous solution): HCl,
HCl ionizes in water and donates its hydrogen ion (a proton) and is therefore a Bronsted acid. accepts a hydrogen ion to form ammonium ions and is thus a
