SAT Subject Test Chemistry




Acids, Bases, and Salts


Acid rain is currently a subject of great concern in many countries around the world because of the widespread environmental damage it reportedly causes. It forms when the oxides of sulfur and nitrogen combine with atmospheric moisture to yield sulfuric and nitric acids—both known to be highly corrosive, especially to metals. Once formed in the atmosphere, these acids can be carried long distances from their source before being deposited by rain. The pollution may also take the form of snow or fog or be precipitated in dry form. This dry form is just as damaging to the environment as the liquid form.


Acid rain is the result of the formation of sulfuric acid from sulfur oxides reacting with water. Nitrogen oxides are also involved.

The problem of acid rain can be traced back to the beginning of the industrial revolution, and it has been growing ever since. The term “acid rain” has been in use for more than a century and is derived from atmospheric studies made in the region of Manchester, England.

In 1988, as part of the Long-Range Transboundary Air Pollution Agreement sponsored by the United Nations and the United States, along with 24 other countries, a protocol freezing the rate of nitrogen oxide emissions at 1987 levels was ratified. The 1990 amendments to the Clean Air Act of 1967 put in place regulations to reduce the release of sulfur dioxide from power plants to 10 million tons per year by 2000. That achieved a 20 percent decrease in sulfur dioxide. The attempts continue through international organizations to further clean the air.

These equations show the most common reactions of sulfur- and nitrogen containing gases with rainwater. The sulfur dioxide reacts with rainwater to form sulfuric acid solutions:

2SO2(g)+O2(g) → 2SO3(g)
SO3(g)+H2O() → H2SO4(aq)

The oxides of nitrogen react to form nitrous and nitric acid:

2NO2(g)+H2O() → HNO2(aq)+HNO3(aq)