Avoiding Confusion—Use of “Acid” and “Base” and Related Terms

In our descriptions of the three theories of acids and bases above, we found that the terms “acid” and “base” can mean several different things depending on the context in which they are used. An acid can be (1) a substance that increases the concentration of protons (hydronium ions), [H₃O⁺], when dissolved in pure water—Arrhenius acid; (2) a proton donor—Brønsted–Lowry acid; (3) an electron pair acceptor—Lewis acid. Conversely, a base can be (1) a substance that increases the concentration of hydroxide ions, [OH⁻], when dissolved in pure water—Arrhenius base; (2) a proton acceptor—Brønsted base; (3) an electron pair donor—Lewis base.

Two additional pairs of terms used in discussions of acids and bases, acidic/basic and acidity/basicity, may also bring confusion as they are used in more than one context. The most common contexts for each pair include:

The terms “acidic” and “basic” are typically used in four contexts. The first is to describe the property of a solution. Any solution where [acid] > [base] could be described as “acidic,” and the converse situation where [base] > [acid] could be described as “basic.” Aqueous solutions of pH lower than 7, where [H₃O⁺] > [OH⁻], are acidic and those of pH higher than 7, where [OH⁻] > [H₃O⁺], are basic. The second context is to describe the property of a compound. For example, any acid can also be called an acidic molecule or an acidic compound, and conversely, any base can also be called a basic molecule or a basic compound. The third context is to describe a property of a functional group. Functional groups that can act as a proton donor are acidic functional groups, while those that can act as a proton acceptor are basic functional groups. For example, the carboxylic acid group, which has the general formula RCOOH, is an acidic functional group, and an amine, which has the general formula R₃N: is a basic functional group. Finally, the fourth context is to describe the property of a specific X–H bond in a Brønsted–Lowry acid or a specific lone pair of electrons in a Brønsted–Lowry base. Using the same examples, the specific bond that is acidic in a carboxylic acid is the O–H bond and the specific pair of electrons that is basic in an amine is the N: lone pair.

The terms “acidity” and “basicity” are typically used in two contexts. The first is to describe how far the pH of an aqueous solution deviates from pH 7. The acidity of a solution varies from weak to strong as the pH decreases from 7 to 0, while the basicity of a solution varies from weak to strong as the pH increases from 7 to 14. The second context is to describe the relative strength of an acid or base, respectively. The more readily a Brønsted–Lowry acid gives up its proton, the greater is its acidity. The more attracted a Brønsted–Lowry base is to a proton, the stronger is its basicity.