The pH scale is used to describe the hydrogen-ion (H⁺) concentration of an aqueous solution. The concentration of hydrogen ions is measured on a logarithmic scale. If you use a calculator: pH = - log [H₃O⁺] (also seen as - log [H⁺]). The pH scale ranges between 0 and 14, with 7 being neutral. A solution with a pH below 7 is acidic, and a solution with a pH greater than 7 is basic (alkaline). Each one-unit change in pH is a tenfold increase (or decrease) in the strength of the acid or base. A change from pH 5 to pH 3, for example, would be a 100 times (10 x 10) increase in acidity.

It may be helpful to use the concept of money to explain relative acid/base strength. Assign a neutral pH (7) the value of 1 cent. A pH of 6 (or 8) is ten times stronger, so it is worth 10 cents. A pH of 5 (or 9) is ten times stronger than that, so it is worth 100 cents (or $1). A pH of 4 (or 10) is 10 times stronger than that, so it is worth $10.

See the pH Values of Common Substances attachment for more information.

Indicators are tools used to measure/indicate the pH of a substance. A color change in the indicator corresponds to different pH levels.

Litmus paper is frequently used to investigate the pH of different substances. Litmus paper is infused with litmus, a water-soluble blue powder derived from certain lichens (a type of fungus). The paper color changes depending on the pH of the substance with which it is in contact. The more basic a substance, the bluer the indicator turns. The more acid the substance, the redder the paper turns. There is wide-range pH litmus paper available as well as acid/base specific paper (pink litmus paper turns blue for bases, blue litmus paper turns pink for acids).

Another naturally occurring indicator is red cabbage juice. The pigment in the cabbage juice is what actually acts as the indicator. Red cabbage juice starts out purple and becomes more and more red as a solution becomes more acidic. It becomes blue, green or yellow in basic solutions.

Red cabbage contains two main types of plant dyes: anthocyanin and flavonol. Anthocyanin pigments are red in strongly acidic solution, blue in neutral and weakly basic solutions, and colorless in strongly basic solutions. Weakly acidic solutions contain some of the red form and some of the blue form and thus appear purple. Flavonol pigments are colorless in acidic and neutral solutions, and yellow in basic solutions. Weakly basic solutions thus contain both blue (anthocyanin) and yellow (flavonol) dyes, and appear to be green. The pH corresponding to various colors varies slightly with concentration, solvent, age and variety of cabbage. Most flowers and fruits contain anthocyanin as pigments.

When environmental and chemical engineers examine a pollutant, they find out whether the substance is an acid or base in order to know what kind of reactions it causes. Acid rain is an environmental problem that concerns many engineers. The effects of acid rain include damage to the limestone and marble in statues and buildings; weakening of the exposed metal on bridges and cars; damage to bodies of water, wildlife, plants, forests and crops; and the contamination of the drinking water supply. Researchers are investigating other possible effects of acid rain on human health.

Robert Angus Smith, an English chemist, first used the term "acid rain" in 1852 when he noted the connection between the acidity of London's rainfall and its polluted skies. Most scientists agree that "normal" rainfall has a pH of 5.6. Rain in the atmosphere reacts with carbon dioxide (CO₂) to form a weak carbonic acid, altering the rain pH to 5.6 (instead of a pH of 7).

Acid rain is defined as any form of wet precipitation (rain, snow, fog, dew or sleet) that has a pH less than 5.6 (on a scale of 0 to 14, with 7 being neutral). Large quantities can also be deposited in a dry form through dust. Acid rain is more acidic than normal rain and forms through a complex process of chemical reactions involving air pollution and water molecules in the air. The two most important pollutants that contribute to the formation of acid rain are nitrogen oxides (NO_x) and sulfur dioxide (SO₂), which react with moisture in the atmosphere to form nitric and sulfuric acid. See the Acid Rain Formation — Small Scale attachment.

The sulfur and nitrogen compounds that contribute to acid rain primarily come from combustion products (burning coal and oil) from large industrial and utility sites. Emissions also come from automobiles and other forms of transportation, and other industrial processes such as smelting. See the Acid Rain Formation — Large Scale attachment.

One way that we can help prevent acid rain is by burning less fossil fuel. We can also make laws that prevent large factories from burning fossil fuels or that require them to limit (minimize) their pollutant output. Engineers have developed many useful technologies for this purpose, but the companies must adhere to the laws. For example, emissions from cars have been reduced because cars now have catalytic converters that remove the poisonous gases from exhaust fumes.

Unfortunately, pollutants that contribute to acid rain may be carried hundreds of miles by wind before being deposited on the Earth. Because of this, it is sometimes difficult to determine the specific sources of these acid rain pollutants. For example, Canada has an acid rain problem because of manufacturing in the mid-western states of the U.S. Sulfur dioxides are produced in the industries in Ohio, Illinois and Pennsylvania and are carried over the land by the weather patterns. The acids then combine with rain over Canada and the Adirondack mountains in New York, making those lakes lifeless. Elsewhere, even though southern Norway has little heavy industry, 80% of its lakes are devoid of life or are on the critical list.