Instructor Information
JCE Classroom Activity: #50
Acid Raindrops Keep Fallin’ in My Lake In this Activity, students simulate acid rain falling on lakes by adding vinegar to bowls of water. Several of the bowls contain solids such as crushed, low-dust chalk, sand, and lime. Students determine whether the solids affect the acidity of each solution over two days by periodically removing samples of each solution for testing with red cabbage indicator.
Because atmospheric CO2 dissolves in water to form carbonic acid, rain is naturally acidic. The reaction is CO2(g) + H2O(l) → H2CO3(aq) H+(aq) + HCO3–(aq) (1), resulting in a pH of approximately 5.6. Other gases in the atmosphere can acidify rain further; these include oxides of sulfur and nitrogen that are a product of burning fossil fuels. Rain with pH values as low as 3 has been observed (1). When rain this acidic enters a lake, it can lower the pH enough to be detrimental to plants and animals. Some lakes, particularly those with limestone (CaCO3) foundations, have a natural buffering ability that helps neutralize acid rain. The reaction between calcium carbonate and sulfuric acid (a major component of acid rain), is H2SO4(aq) + CaCO3(s) Ca2+(aq) + SO42–(aq) + H2O(l) + CO2(g).
Integrating the Activity into Your Curriculum This Activity is an easy simulation that could be performed at home in conjunction with a classroom discussion on acid rain. It includes the ideas of acidity, indicators, and neutralization reactions. Students could research the acidity of local rain and its effects on nearby lakes and water supplies. A Tested Demonstration in this issue of JCE illustrates the dry and wet deposition of sulfur dioxide into simulated lakes (2). The demonstration is a simulation that is much closer than this Activity to what actually happens with acid rain, but it requires a fume hood for the generation of sulfur dioxide. Related Activities and experiments are available (3–4). A reference to a simulation similar to this Activity can be found on the Student Activity (Web site 1).
About the Activity Red cabbage juice indicator gives a wide range of colors and gradations within a color. In neutral solutions, the indicator is blue-green; in the acidic solution created by adding vinegar, it is pink. Approximately these colors will be seen at the testing points of this Activity: Water—samples will be blue-green throughout the Activity. Water and vinegar—samples will be pink throughout the Activity. Water, vinegar, and low-dust chalk or lime—samples will be pink immediately after vinegar is added; as time passes, samples become purple, then blue or blue-green. In testing, low-dust chalk neutralized the acid faster than lime. Water, vinegar, and other solids—samples will be pink immediately after vinegar is added, but the color may change with time if the solid is basic. Red cabbage is available in grocery stores. Low-dust white chalk may be available in the classroom or can be purchased in educational supply or office supply stores. Other chalk may be calcium sulfate (gypsum) rather than calcium carbonate and will not neutralize the acid. You can test chalk by placing a small piece in a few mL of vinegar: if it contains calcium carbonate, bubbles will form. Crushed lime, available at garden centers, has a coarsely-crushed graywhite appearance and is commonly added to soil to change its pH.
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Answers to Questions 1. See About the Activity, above. In bowls that contain calcium carbonate in the form of low-dust chalk or lime, a chemical reaction occurs. You can tell that a neutralization reaction occurred because the color of the indicator Ca2+(aq) + CH3COO–(aq) + H2O(l) + CO2(g). changes. The reaction is CH3COOH(aq) + CaCO3(s) 2. Answers will vary. Possible answers include: It is like acid rain falling in a lake because it acidifies the water in the bowl, and limestone in lakes can neutralize the acid. It is unlike acid rain in that vinegar is not in acid rain, and lakes do not mix as completely as the water in the simulation. The simulation could be made more like real acid rain by creating the acidic oxides that are actually present in acid rain. 3. Acid rain falling into a limestone-lined lake would be neutralized. Acid rain would react with the CaCO3 in limestone, so adding a large quantity of limestone to a lake might reverse the acidification.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Background
References, Additional Related Activities, and Demonstrations 1. 2. 3.
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Charola, A. Elena. Acid Rain Effects on Stone Monuments. J. Chem. Educ. 1987, 64, 436. Goss, Lisa M. A Demonstration of Acid Rain and Lake Acidification: Wet Deposition of Sulfur Dioxide. J. Chem. Educ. 2003, 80, 39–40. Halstead, Judith A. Rain, Lakes, and Streams—Investigating Acidity and Buffering Capacity in the Environment. J. Chem. Educ. 1997, 74, 1456A–1456B. Halstead, Judith A. Spring Shock!: Impact of Spring Snowmelt on Lakes and Streams. J. Chem. Educ. 1998, 75, 400A–400B. Jacob, Anthony T. Acid Rain; ICE Publ. 91-009; Institute for Chemical Education: University of Wisconsin– Madison, Madison, WI, 1991. Aristov, Natasha; Cargille, Christine L.; Acid Rain: Experimental Supplement; ICE Publ. 93-007; Institute for Chemical Education: University of Wisconsin–Madison, Madison, WI, 1993. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 80 No. 1 January 2003 • Journal of Chemical Education
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JCE Classroom Activity: #50
Student Activity
Acid Raindrops Keep Fallin’ in My Lake Normal rain has a pH of about 5.6. If the air is polluted by oxides of sulfur and nitrogen that result from the burning of fossil fuels, these pollutants can cause the rain to become more acidic. When acid rain falls in a lake, it can lower the pH of the lake water, causing problems for the plants and animals that live there. Do some lakes have a natural protection against acid rain? Could we use chemistry to protect lakes from acid rain? In this Activity you will make very simple models of lakes and see what happens when acid is added.
Try This You need to prepare some solid samples for testing; your instructor will tell you how many to use. One solid sample must be crushed chalk (low-dust, white) or lime, both of which are mostly calcium carbonate. Crush chalk by placing a stick of it in a plastic bag, setting it on a sturdy surface, and tapping it with a hammer. Additional samples may include clean sand, marble chips, local soil, gravel, or small stones. You will need: small bowls, test tubes, and spoons or stirring rods (the number required of each of these three items is the number of solid samples you will use plus two); marker; test tube rack; solid samples described above; vinegar; distilled water; measuring spoons and cups or graduated cylinders; dropper; plastic wrap; and red cabbage indicator. Red Cabbage Indicator If your instructor has not prepared red cabbage indicator, you can make it using either of the methods below. When you are not using it, store the indicator solution in a closed container in a refrigerator. If you cannot refrigerate the indicator solution, you should make and use a new batch each day. 1. Place two or three torn red cabbage leaves in a blender. Add 1/2 cup (125 mL) of distilled water. Blend until the leaves are well-chopped. Strain the juice through a coffee filter. 2. Place two or three torn red cabbage leaves into a heat-resistant container. Add distilled water until the leaves are covered. Heat the mixture to boiling and boil until the liquid is dark blue or purple. Remove the leaves. Activity Procedure __1. Set several small bowls on a surface where they will not be disturbed. Label one bowl “water”, another “water and vinegar”, and the remaining bowl(s) with the name of the sample(s) you are using. Label the test tubes the same way as the bowls. __2. To the bowl(s) labeled with the name of a solid sample, add enough of the sample to cover the bottom of the bowl with a thin layer of solid. __3. Add 1/2 cup (125 mL) of distilled water to each bowl. If the solid material in a bowl gets disturbed, shake the bowl gently until the solid is evenly distributed. __4. Stir each bowl with a clean spoon or stirring rod. Remove 1 teaspoon (5 mL) of liquid from each of the bowls and place it in the test tube with the matching label. Add several drops of red cabbage indicator to each test tube. Observe the colors and record them. After recording the colors, empty the test tubes into a sink and rinse them with distilled water. __5. Add 1/2 teaspoon (2.5 mL) of vinegar to each bowl, except the one labeled “water”. Stir each bowl with a clean spoon or stirring rod. __6. Repeat step 4 immediately. Repeat it again three more times: after 30 minutes, after 24 hours, and after 48 hours. Observe and record the colors for each repetition. Between testing times, cover the bowls with plastic wrap.
Questions __1. Over time, what happens to the colors observed in each bowl? In which bowl(s) is a chemical reaction occurring? How can/could you tell? Write the reaction(s) that are occurring in the bowl(s). __2. How is this simulation like acid rain falling into a lake? How is it unlike acid rain falling into a lake? How could the simulation be made more like acid rain falling into a lake? __3. Low-dust chalk, limestone, and marble consist mostly of calcium carbonate. What happens when acid rain falls into a lake with a limestone lakebed? How could limestone be used to reverse the detrimental effects of acid rain?
Information from the World Wide Web (accessed Nov 2002) 1. 2. 3. 4. 5.
EPA’s Clean Air Market Programs—Acid Rain. http://www.epa.gov/airmarkets/acidrain/ The Green Lane: Acid Rain. http://www.ec.gc.ca/acidrain/ Acid Rain. http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Environmental/acidrain/acidrain.html Acid Deposition and Precipitation. http://royal.okanagan.bc.ca/mpidwirn/atmosphereandclimate/acidprecip.html What is Acid Rain and What Causes It? http://www.policyalmanac.org/environment/archive/acid_rain.shtml This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu
