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Lesson 19:

Acid Rain

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Concepts conveyed | Materials | Procedure | Benefits | Resource

Concepts conveyed:

The purpose of this activity is to illustrate, aid in the discussion of, and to share facts concerning the environmental effects of acid rain. Several Internet sites on acid rain are also provided.

Materials:

One acid rain kit for every two to four students. Each kit contains the following:

  • half a piece of white chalk
  • 1 plastic test tube (disposable)
  • 1 small bottle (~5 oz. or less) containing 4% acidity vinegar
  • 1 small zip-closing storage bag (large enough to hold chalk, test tube, and vinegar)

Procedure:

Ask the students to get into small groups of two to four, and give each group an acid rain kit. Ask each group to place the chalk in the test tube, add the vinegar to the chalk, and observe the results. After the demonstration, share with the students various facts concerning the destructive effects of acid rain and explain to the students the chemical reactions involved.


Chemical Reactions

The main causes of acid rain are the large amounts of sulfur oxides (SO2 and SO3) and nitrogen oxides (NO and NO2) in the air. Although some of these compounds are produced by nature, most are a result of the burning of our natural resources in heavily-industrialized cities. Sulfur dioxide combines with oxygen from the air and becomes sulfur trioxide. The sulfur trioxide combines with water and becomes sulfuric acid.

2SO2(g) + O2(g) = 2 SO3(g)SO3(g) + H2O(l) = H2SO4(aq)

Nitrogen dioxide combines with water and oxygen to form nitric acid.

4NO2(g) + 2H2O(l) + O2(g) = 4HNO3(aq)

These two acids dissociate to yield H+ ions. These ions can react with calcium carbonate (which is the major component in marble, statues, monuments, eggshells, seashells, and chalk) and dissolve it as described in the following equations:

CaCO3(s) + 2H+(aq) = Ca2+(aq) + CO2(g) + H2O(l)

Sulfuric acid and nitric acids also corrode metals. Iron easily reacts with hydrogen ions from the acids and with oxygen from the air to form rust (a reddish brown substance with the formula of Fe2O3).

4Fe(s) + 2O2(g) + 8H+(aq) = 4Fe2+(aq) + 4H2O(l)
4Fe2+(aq) + O2(g) + H2O(l) = 2Fe2O3(s) + 8H+(aq)

Sample Acid Rain Facts

  • Healthy lakes have a pH of 6.5
  • Only few species survive below a pH of 5.0.
  • The average pH of lakes in Ontario. Canada is 5.0
  • The fog near Pasadena, CA was found to have a pH as low as 2.5 (January 1982). This is 500 times more acidic than unpolluted fog.
  • The fog at Corona del Mar, on the coast of Los Angeles, CA, has been recorded to be as low as a pH of 1.5!
  • Currently, in the United States, fog, dew, and the bottom layers of clouds often have a pH of 3.0 or lower.
  • In Chicago alone, there is nearly $300 million worth of damage to buildings caused by acid rain.
  • Many of the world’s monuments, such as the Taj Mahal in India and the United States Capitol, are being eroded by acidic atmospheric conditions.
  • Much more information can be obtained from other resources such as newspapers, magazines, books, encyclopedias, publications by groups such as the Environmental Protection Agency, Greenpeace, World Wildlife Federation, and the Internet.

There are several Websites for acid rain information. Here are some examples:

Benefits:

  • With this illustration of the relevance of acid rain in our everyday lives, students are more likely to want to find out more about the chemical reactions, the prevention, and the financial impact of acid rain.
  • This is also a useful demonstration with which to introduce to a class the concept of acid-base reactions.

Resource:

  • Chemistry in Context: Applying Chemistry to Society; Schwartz, A. T.; Bunce, D. M.; Silberman, R. G.; Stanitski, C. L; Stratton, W. J.; Zipp, A. P. (Eds); American Chemical Society: Washington, DC, 1994; pp 162-174.