sulfur dioxide, (SO2), inorganic compound, a heavy, colorless, poisonous gas. It is produced in huge quantities in intermediate steps of sulfuric acid manufacture.
Sulfur dioxide has a pungent, irritating odor, familiar as the smell of a just-struck match. Occurring in nature in volcanic gases and in solution in the waters of some warm springs, sulfur dioxide usually is prepared industrially by the burning in air or oxygen of sulfur or such compounds of sulfur as iron pyrite or copper pyrite. Large quantities of sulfur dioxide are formed in the combustion of sulfur-containing fuels.
Sulfur dioxide pollution carries serious health and environmental risks and is one of the six criteria air pollutants regulated by the U.S. Environmental Protection Agency and other regulatory agencies around the world. In the atmosphere sulfur dioxide can combine with water vapor to form sulfuric acid, a major component of acid rain; in the second half of the 20th century, measures to control acid rain were widely adopted. Most of the sulfur dioxide released into the environment comes from coal-fired power plants and petroleum refineries. Paper pulp manufacturing, cement manufacturing, and metal smelting and processing facilities are other important sources.
(Read Britannica’s essay “What Happened to Acid Rain?”)
Sulfur dioxide is a precursor of the trioxide (SO3) used to make sulfuric acid. In the laboratory the gas may be prepared by reducing sulfuric acid (H2SO4) to sulfurous acid (H2SO3), which decomposes into water and sulfur dioxide, or by treating sulfites (salts of sulfurous acid) with strong acids, such as hydrochloric acid, again forming sulfurous acid.
Sulfur dioxide can be liquefied under moderate pressures at room temperatures; the liquid freezes at −73° C (−99.4° F) and boils at −10° C (14° F) under atmospheric pressure. Although its chief uses are in the preparation of sulfuric acid, sulfur trioxide, and sulfites, sulfur dioxide also is used as a disinfectant, a refrigerant, a reducing agent, a bleach, and a food preservative, especially in dried fruits.