Chapter 11 Laboratory: Acid-Base Chemistry 191

Acids and bases are two of the most important classes of compounds in chemistry.

Surprisingly, it took chemists many years to agree on just what made an acid an acid and a

base a base.

From experience, we understand that acids and bases have certain characteristics. For

example, acid solutions taste sour and base solutions bitter. (Obviously, you should never

taste any laboratory chemical, but the sour taste of edible acids such as lemon juice and

vinegar is familiar to most people, as is the bitter taste of edible bases such as baking soda

and quinine water). Acid solutions generally have an astringent feel on the skin, and base

solutions feel slimy. Acid solutions turn blue litmus paper red, and base solutions turn red

litmus paper blue. Acids and bases combine to form salts. And so on.

But these characteristics are insufficient to define acids and bases. Around the turn of

the nineteenth century, French and European chemists believed that all acids must contain

oxygen. (They were wrong, but echoes of that error persist; for example, the German

word for oxygen is Sauerstoff, which translates literally as “sour material.”) Davy and

other English chemists were much closer to the mark. They believed that all acids must

contain hydrogen. And, although that statement is not absolutely true for all substances

considered to be acids by modern definitions, it is true that all acids known to be acids at

that time do contain hydrogen.

The first good working definition of acids and bases was proposed in the late nineteenth

century by the Swedish chemist Svante August Arrhenius, who defined an acid as a

substance that when dissolved in water increases the concentration of the hydronium

(H 3 O+) ion and a base as a substance that when dissolved in water increases the

concentration of the hydroxide (OH–) ion. Although that definition limits acids and bases to

compounds that are water-soluble, it was a pretty good definition for the time and remains

a useful definition even today.

Laboratory: 11

Acid-Base Chemistry