reactions are essential in both and industrial . Moreover, many of the substances we encounter in our homes, the supermarket, and the pharmacy are acids or . For example, aspirin is an acid (acetylsalicylic acid), and antacids are . In fact, every amateur chef who has prepared mayonnaise or squeezed a wedge of lemon to marinate a piece of fish has carried out an acid– reaction. Before we discuss the characteristics of such reactions, let’s first describe some of the properties of acids and . , whereas are defined as substances that dissolve in water to produce OH . In fact, this is only one possible set of definitions. Although the general properties of acids and have been known for more than a thousand years, the definitions of and have changed dramatically as scientists have learned more about them. In ancient times, an acid was any substance that had a sour taste (e.g., vinegar or lemon juice), caused consistent color changes in dyes derived from plants (e.g., turning blue litmus paper red), reacted with certain metals to produce hydrogen gas and a solution of a containing a metal cation, and dissolved carbonate salts such as limestone (CaCO) with the evolution of carbon dioxide. In contrast, a was any substance that had a bitter taste, felt slippery to the touch, and caused color changes in plant dyes that differed diametrically from the changes caused by acids (e.g., turning red litmus paper blue). Although these definitions were useful, they were entirely descriptive. in detail was the Swedish chemist Svante Arrhenius (1859–1927; Nobel Prize in , 1903). According to the , an acid is a substance like hydrochloric acid that dissolves in water to produce H (protons; Equation $$\ref{4.3.1}$$), and a is a substance like sodium hydroxide that dissolves in water to produce hydroxide (OH) (Equation $$\ref{4.3.2}$$): }{NaOH_{(s)}} \xrightarrow {H_2O_{(l)}} Na^+_{(aq)} + OH^-_{(aq)} \label{4.3.2} \] are due exclusively to the presence of H and OH , respectively, in solution. Although Arrhenius’s ideas were widely accepted, his definition of acids and had two major limitations: was needed. One was proposed independently in 1923 by the Danish chemist J. N. Brønsted (1879–1947) and the British chemist T. M. Lowry (1874–1936), who defined acid– reactions in terms of the transfer of a proton (H ion) from one substance to another. , thereby forming an acidic solution) is any substance that can donate a proton, and a is any substance that can accept a proton. The Brønsted–Lowry definition of an acid is essentially the same as the Arrhenius definition, except that it is not restricted to aqueous . The Brønsted–Lowry definition of a , however, is far more general because the hydroxide ion is just one of many substances that can accept a proton. Ammonia, for example, reacts with a proton to form $$NH_4^+$$, so in Equation $$\ref{4.3.3}$$, $$NH_3$$ is a Brønsted–Lowry and $$HCl$$ is a Brønsted–Lowry acid. Because of its more general nature, the Brønsted–Lowry definition is used throughout this text unless otherwise specified. that contain a single −COH group, such as acetic acid (CHCOH), are monoprotic acids, dissociating to form RCO and H. . For example, HSO can donate two H in separate steps, so it is a diprotic acid (a and HPO, which is capable of donating three protons in successive steps, is a (Equation $$\ref{4.3.4}$$, Equation $$\ref{4.3.5}$$, and Equation $$\ref{4.3.6}$$): of each species in solution remains constant. The reaction is then said to be in quantitatively until next semester. Qualitatively, however, we can state that react essentially completely with water to give $$H^+$$ and the corresponding anion. Similarly, dissociate essentially completely in water to give $$OH^−$$ and the corresponding cation. Strong acids and strong are both . In contrast, only a fraction of the molecules of weak acids and react with water to produce , so weak acids and are also . Typically less than 5% of a weak electrolyte dissociates into in solution, whereas more than 95% is present in undissociated form. are ionic compounds that contain the hydroxide ion as the anion; three examples are NaOH, KOH, and Ca(OH). Common weak acids include HCN, HS, HF, such as HNO and HClO, and such as acetic acid. The ionization reaction of acetic acid is as follows: of sulfuric acid contains $$H^+_{(aq)}$$ and a mixture of $$HSO^-_{4\;(aq)}$$ and $$SO^{2−}_{4\;(aq)}$$ , but no $$H_2SO_4$$ molecules. All other polyprotic acids, such as HPO, are weak acids. is ammonia, which reacts with water to form small amounts of hydroxide ion: , as are ionic compounds that contain derived from weak acids (such as S). : . Acids other than the six common strong acids are almost invariably weak acids. The only common strong are the hydroxides of the alkali metals and the heavier alkaline earths (Ca, Sr, and Ba); any other you encounter are most likely weak. Remember that Many weak acids and are extremely soluble in water.