Hydrangea is a shrub with clusters of showy flowers. The color of its flowers can change as the pH of the soil in which it grows changes. The flowers are red-pink under neutral to low soil acidity and blue under conditions of higher acidity. Hydrangeas contain a natural indicator, anthocyanin, which is responsive to the pH of its surroundings. Observations such as these about hydrangeas can constitute a central theme in understanding acids and bases.It is useful to think of an acid as a substance that releases hydrogen ions (H+) in water solution, as proposed by Arrhenius around 1885. Since bases have always been thought of as opposites of acids, Arrhenius also proposed that bases involve hydroxide ions (OH-). The positive hydrogen ion, which is a proton, and the negative hydroxide ion together constitute water. A more useful definition of acids and bases was proposed by Brønsted and Lowry in 1923. This definition, which gives a more complete picture of acids and bases, treated an acid as a proton donor and a base as a proton acceptor. An acid-base reaction then is essentially a transfer of protons. During the past several decades other useful definitions of acids and bases have built upon these early concepts. Some of these--the Lewis definition, for example--may be beyond the scope of the high school general chemistry classroom and less useful to high school teachers.
Neither hydrogen ions nor hydroxide ions exist alone. They are found combined with other ions of the opposite charge. Hydrochloric acid, for example, produces hydrogen ions and negative chloride ions when in solution. Sodium hydroxide, a base, produces negative hydroxide ions and positive sodium ions when in solution. When solutions of hydrochloric acid (HCl) and sodium hydroxide (NaOH) are mixed, a product resulting from the combination of sodium ions and chloride ions (NaCl) can be separated from the solution. This kind of compound is called a salt. Thus, the products of a typical acid-base reaction are a salt and water. This acid-base reaction, called neutralization, can be represented by the following equation.
HCl(aq) + NaOH(aq) -> NaCl(aq) + HOH(l)
Acids differ from one another in strength. Bases differ from one another in a similar fashion. Chemists have found that some acids produce more hydrogen ions in solution than similar amounts of others. These acids (such as hydrochloric acid, HCl, and sulfuric acid, H2SO4) are strong acids, whereas those that release only a few percent of hydrogen ions (hydrofluoric acid, HF, and acetic acid, HC2H3O2) are weak acids. Similarly, sodium hydroxide, NaOH, is a strong base and aqueous ammonia, NH3(aq), is a weak base. Strong and weak acids react with strong and weak bases in neutralization reactions to produce water and salts.
It is most helpful to the chemist to be able to express quantitatively the concentration of hydrogen ion in solution. We can do this with a term called pH. At room temperature, systems below pH 7 are acidic and systems above pH 7 are basic. A sample with a pH of seven is neutral. Special indicator solutions or indicator papers impregnated with certain dyes change colors at various pH values and allow us to estimate the pH of a solution. pH meters provide a more exact measurement of hydrogen ion concentration.
Acids and bases have real-life significance. The human body functions properly only when delicate acid-base balances are maintained; crops grow best in soil with the proper pH; substances released into the atmosphere as pollutants often form acid rain. In addition, many substances used in the home are acids or bases.
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