Many of us are familiar with the group of chemicals called acids. But do you know what it takes for a compound to be an acid? Actually, there are several different definitions of acid that chemistry uses, and each definition is appropriate under different circumstances. Less familiar—but just as important to chemistry and ultimately to us—is the group of chemicals known as bases. Both acids and bases are important enough that we devote an entire chapter to them—their properties and their reactions. 15.1: Prelude to Acids and Bases One of the most concentrated acids in the body is stomach acid, which can be approximated as a 0.05 M hydrochloric acid solution. Special cells in the stomach wall secrete this acid, along with special enzymes, as part of the digestion process. In a laboratory, a 0.05 M solution of hydrochloric acid would dissolve some metals. How does the stomach survive the presence of such a reactive acid? 15.2: Arrhenius Definition of Acids and Bases Arrhenius acid: a compound that increases the concentration of hydrogen ion (H+) in aqueous solution; Arrhenius base: a compound that increases the concentration of hydroxide ion (OH−) in aqueous solution. the reaction of an acid and a base 15.3: Brønsted-Lowry Definition of Acids and Bases A Brønsted-Lowry acid is a proton donor, and a Brønsted-Lowry base is a proton acceptor. Brønsted-Lowry acid-base reactions are essentially proton transfer reactions. 15.4: Water - Both an Acid and a Base Water molecules can act as both an acid and a base, depending on the conditions. 15.5: The Strengths of Acids and Bases Acids and bases can be strong or weak depending on the extent of ionization in solution. Most chemical reactions reach equilibrium at which point there is no net change. The pH scale is used to succinctly communicate the acidity or basicity of a solution. 15.6: Buffers A buffer is a solution that resists sudden changes in pH. 15.E: Acids and Bases (Exercises) Problems and select solutions to the chapter. 15.S: Acids and Bases (Summary) To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.