Overview of Acids and Bases
The definition of an acid or a base depends on the system you are using. In the Arrhenius definition of acids and bases, an acid is a substance that releases hydrogen ions when dissolved in water, and a base is a substance that releases hydroxide ions. Although in chemistry today, the Brønsted-Lowry definition has largely superseded it, under the Arrhenius definition it is possible to predict the result of the neutralization of an acid with a base, which is a salt and water.

If a chemist doesn't specify which acid-base system they are using, it can be assumed they are working within the Brønsted-Lowry system. A Brønsted acid is any substance that can donate a hydrogen ion. A Brønsted base is defined as any substance that can accept a hydrogen ion. In other words, a Brønsted acid is a proton donor, and a Brønsted base is a proton receiver. The Brønsted-Lowry system implies the acid-base reaction to occur within a system of conjugate acid-base pairs governed by an equilibrium. Acids increase the concentration of hydrogen ions, while bases decrease the concentration of hydrogen ions. The acidity or basicity of a solution can therefore can be measured by its hydrogen ion concentration (or pH, the negative logarithm of concentration).

The Lewis definition of acids (as electron pair receivers) and bases (as electron pair donors) is actually broader than either the Arrhenius or Brønsted-Lowry. The Lewis system provides a productive framework for viewing many chemical events from nucleophilic approach to the formation of coordination complexes, although the Brønsted-Lowry system is the traditional framework for discussion and problem solving involving acid-base equilibria.

Acids and Bases on the MCAT
As with Solutions, the previous chapter, every MCAT will have a number of questions deriving from the material in Acids & Bases. Although big, multi-variable quantitative acid-base equilibrium questions are a standard of Chem 101, you are not likely to run into such questions on the MCAT. However, the MCAT definitely will call on you to demonstrate that you understand the basic concepts of acid-base equilibrium. Buffers & indicators are MCAT favorites, as well as titration curves.