An acid is a compound that dissociates or ionizes in water to produce a positively charged hydrogen ion and a corresponding negative ion. A base is a compound that produces a hydroxide ion when it dissolves in water.
Over the centuries, scientists, such as Antoine Lavoisier, Justus von Liebig, Svante Arrhenius, Johannes Nicolaus Bronsted and Thomas Martin Lowry, have defined acids and bases in several ways. While these definitions do not contradict each other, some of the definitions are more comprehensive than others.
Arrhenius defined an acid as a substance that dissociates in water to form hydrogen ions. In other words, an acid raises the concentration of hydrogen ions in an aqueous solution. He defined a base as a substance that dissociates in water to form hydroxide ions. In other words, a base increases the concentration of hydroxide ions in an aqueous solution.
Arrhenius further described an acid-base reaction as the formation of a water molecule from a hydroxide ion and a proton. According to the Arrhenius acid-base theory, an acid-base reaction forms salt and water when an acid and a base react. This is called a neutralization reaction, where the acid and base properties of hydrogen ions and hydroxide ions are neutralized as they combine to form the water molecule.
However, Johannes Bronsted and Thomas Lowry had a slightly different way of defining acids and bases. According to the Bronsted-Lowry theory, their definition was founded on the idea of de-protonation of acids and the protonation of bases. That is the ability of acids to donate hydrogen ions or protons to bases, which accept them. In other words, the theory defines acids as proton donors and bases as proton acceptors.
According to the Bronsted-Lowry acid-base theory, an acid-base reaction results in the removal of a hydrogen ion from the acid and its addition to the base.