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Coal: Classroom Activities

Acid Rain and the Environment


Goal

To understand how the burning of coal can increase the acidity of rainfall and to examine some of the impacts of acid rain.

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Background Information

Natural rainfall is slightly acidic. It has a pH between 5 and 6. Reactions between carbon dioxide and dissolved rainwater form a weak acid called carbonic acid. In the recent past, the acidity of rainwater has increased because of the great increase in carbon dioxide concentrations in the atmosphere as a result of burning of fossil fuels, such as coal. In addition, combustion of fossil fuels, together with industrial smelting of metal ores, releases sulfur and nitrogen oxide gases into the atmosphere. When mixed with rainwater, these form sulfuric acid and nitric acid, compounds that also increase the acidity of rainfall.

Acid rain has diverse effects on the environment. Acid rain falling on crystalline igneous and metamorphic rocks, such as granites and gneisses, can release aluminum. Uptake of aluminum by plants and trees can cause them to become sickly or even die. Also, toxic metals released from rock minerals into solution by acid rain are transported by groundwater flow and surface runoff into lakes, and can be harmful to fish.

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Activity Overview

Students analyze a map that shows the acidity of rainfall across the United States. They then correlate pH levels of rainwater to the locations of coal-producing regions and coal-fueled power plants. Next, students watch a demonstration that illustrates the buffering effects of different types of rocks on acid rain.

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Materials and Equipment

Acidity of Rainfall map and Coal Resources map - color copies or projected

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Activity

acid rainfall
Source: The American Geological Institute, EarthComm: Project-Based Space and Earth System Science, 2nd Edition, Armonk, New York: It's About Time, 2012, pg. 737.

 

coal resource map
Source: The American Geological Institute, EarthComm: Project-Based Space and Earth System Science, 2nd Edition, Armonk, New York: It's About Time, 2012, pg. 737.

 

  1. Ask students to examine the Acidity of Rainfall map. he map shows the acidity of rainfall across the United States. Explain to them that normal rainfall is slightly acidic (pH 5.6, where 7.0 is neutral). Explain to them as well that carbon dioxide in the atmosphere reacts with water to form a weak acid called carbonic acid. The lower the pH, the more acidic is the rainwater.
  2. Ask students to examine the Coal Resources map.
  3. Using the data represented in both maps, have students answer the following:
    1. What part of the United States has the most acidic rainwater?
      The areas around OH, PA, and NY have the most acidic rainwater.
    2. How does the pattern of rainwater pH correlate with the locations of coal-producing regions and coal-fired power plants?
      In general, more acidic rainwater corresponds to a larger number of coal-producing regions and coal-fired power plants.
    3. What parts of the United States have the least acidic rainwater? Why?
      The west coast and Midwestern region have the least acidic rainwater. There are not as many coal-fired power plants in these areas.
    4. What can you infer about the direction in which wind and weather move across the United States?
      Wind and weather generally move across the U.S. from west to east.
  4. Conduct a demonstration of the effects of acid rain. You can do this by pouring an "acid rain" solution (pH of about 4.5) through samples of crushed limestone and granite. Demonstrate according to the following steps:
    1. Prepare an "acid rain" solution by combining dilute hydrochloric or sulfuric acid with water to obtain a pH of about 4.5. You can also use vinegar which has a pH of about 4.
    2. Cut off the bottoms of two clean 1-L soda bottles. Turn them upside down and support them so that they are stable and there is adequate space to collect the "acid rain" solution.
    3. Place crushed granite in one container and the same weight of crushed limestone in the other container. Place a beaker beneath each of the inverted bottles.
    4. Slowly pour some of the acid rain solution into each container of crushed rock.
  5. Ask students to observe the solution infiltrating each of the crushed rock samples and then flowing into each collection beaker.
  6. Using pH paper, determine the acidity of the liquid in each beaker.
  7. Have students answer the following:
    1. What was the pH of the liquids after passing through the two types of crushed rock?
      Answers will vary. The solution in the granite beaker should still be close to 4.5, but the pH of the solution from the limestone container should increase.
    2. Which sample neutralized more of the acid?
      The limestone has a greater ability to neutralize the acid than the granite. The effective ingredient in limestone is calcium carbonate (CaCO3) which reacts with acid to form water, carbon dioxide, and calcium salts.
    3. Where would you expect a greater environmental impact from acid rain - in a region with granite bedrock or one with limestone bedrock? Explain your answer.
      You would expect that an area with granite bedrock would have a greater environmental impact from acid rain, because granite has no neutralizing effect on acid rainwater. In areas of granite bedrock, therefore, acid precipitation would accumulate in lakes and ponds over time.
    4. What does this investigation suggest about how areas may differ in their sensitivity to acid rainwater?
      Areas differ greatly in their sensitivity to acid rain, depending upon the composition of the soil and bedrock in the area.

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