U.S. Environmental Protection Agency
Acids, Bases and pH
Acidic and basic are two ways of describing chemical compounds. Acidity is measured in terms of pH. A pH scale runs from zero (the most acidic) to 14 (the most basic or alkaline). A substance that is neither basic nor acidic is called "neutral” and has a pH of 7.
What is Acid Rain?
Acid rain is rain that has been made acidic by certain pollutants in the air. Acid rain can appear in many forms such as solid particles or actual liquid rain that has become more acidic than normal. It can be carried by the wind, sometimes for very long distances.
Acid rain is caused by compounds like SO2 (Sulfur Dioxide) and NOx (Nitrogen oxides)that are released into the air. These substances can react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain. Sulfur dioxide and nitrogen oxides can fall as solid particulates (called “dry deposition”) or dissolve easily in water to fall as acid rain (called “wet deposition”).
Sources of Acid Rain
Human activities are the main cause of acid rain. Power plants and factories release sulfur dioxide and nitrogen oxides when they burn fossil fuels, such as coal, to produce electricity. In addition, theexhaust from cars, trucks, and buses releases nitrogen oxides and sulfur dioxide into the air. These pollutants cause acid rain. Rain is naturally acidic due to CO2 which dissolves in water to form carbonic acid with a pH of around 5-6. However, human activities and pollution have made it more acidic in some areas.
The Chemistry of Acid Rain
Rain is naturally slightly acidic because of the presence of CO2 in the air that dissolves in the water to form cabonic acid. The reaction can be written in a simplistic way as:
Natural rain has a pH of around 5.6. In urban or highly industrialized areas, other gases of an acidic nature are produced from the combustion of fuels. Of these, the most common are the oxides of sulfur and nitrogen. In the case of sulfur, SO2 is the gas most commonly produced. Under the influence of UV light and through catalytic action of dust particles, building surfaces, and other agents, SO2 can be oxidized to SO3 by the oxygen or ozone in the air. The SO3 then dissolves in the water to produce sulfuric acid. The reaction can be written as follows:
The pH of this rain depends on the concentration of SO3; the rain will have a lower pH where this concentration is higher: urban areas, high traffic areas, and industrialized areas. The pH will change during the time the rain falls. At the beginning the of the rain, concentration of SO3 in the air will be highest, and rain will have its lowest pH value. Values as low as pH 3 or lower have been actually measured. This represents a thousandfold increase in H+ concentration with respect to normal rain! This is because every change of 1 on the pH scale is worth ten times a change in acid concentration. Similar reaction could be written for the case of nitrogen oxides, but this discussion will be limited to those of the sulfur oxides mentioned above.
The term “acid rain” was coined as a consequences of the observed lowering of pH. The problem, though, is not so simple, and the currently preferred term is “acid precipitation”. This term is more comprehensive and includes not only “wet deposition”, the deposition of pollutants in the presenc of water, as when it rains or snows; but also “dry deposition”, the settling of gaseous or solid pollutants, such as aerosols and dust in the absence of water.
Effects of Acid Rain
Acid rain can be extremely harmful to forests because it dissolves nutrients, such as magnesium and calcium that trees need to be healthy. This loss of nutrients makes it easier for infections, insects, and cold weather to damage trees and forests.Also, increased acidity levels can be deadly to organism which live in aquatic habitats and are sensitive to pH changes in their environment. Many species of fish, frogs, salamanders, and other organisms that live in lakes and rivers can be endangered by acid rain. Acid rain can also have damaging effects on buildings, statues, monuments, and cars. It can cause paint to peel and stone statues to appear old and worn down. The damage caused by acid rain can be costly or sometimes too difficult to repair.