Theories of Acids and Bases

Chemists have refined their picture of acids and bases several times, and each theory is broader than the last. The earliest useful model, proposed by Svante Arrhenius, defined an acid as a substance that releases hydrogen ions ($H^+$) in water and a base as one that releases hydroxide ions ($OH^-$). This model explains why hydrochloric acid ($HCl$) and sodium hydroxide ($NaOH$) behave as they do, but it is limited to aqueous solutions and cannot account for bases such as ammonia, which contains no hydroxide group of its own.

The Bronsted-Lowry theory, developed independently by Johannes Bronsted and Thomas Lowry in 1923, is the workhorse model for IB Chemistry. It defines an acid as a proton (hydrogen ion) donor and a base as a proton acceptor. This shifts the focus from what a substance