JCE Classroom Activity: #50

Acid Raindrops Keep Fallin’ in My LakeIn 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 acidityof each solution over two days by periodically removing samples of each solution for testing with red cabbage indicator.

Background 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 fossilfuels. Rain with pH values as low as 3 has been observed (1). When rain this acidic enters a lake, it can lower the pHenough 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 sulfuricacid (a major component of acid rain), is H2SO4(aq) + CaCO3(s) Ca2+(aq) + SO4

2–(aq) + H2O(l) + CO2(g).

Integrating the Activity into Your CurriculumThis 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 oflocal rain and its effects on nearby lakes and water supplies. A Tested Demonstration in this issue of JCE illustrates thedry and wet deposition of sulfur dioxide into simulated lakes (2). The demonstration is a simulation that is much closerthan this Activity to what actually happens with acid rain, but it requires a fume hood for the generation of sulfurdioxide. Related Activities and experiments are available (3–4). A reference to a simulation similar to this Activity canbe found on the Student Activity (Web site 1).

About the ActivityRed 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 beblue-green throughout the Activity. Water and vinegar—samples will be pink throughout the Activ-ity. Water, vinegar, and low-dust chalk or lime—samples will be pink immediately after vinegar isadded; as time passes, samples become purple, then blue or blue-green. In testing, low-dust chalkneutralized the acid faster than lime. Water, vinegar, and other solids—samples will be pink immedi-ately 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 pur-chased in educational supply or office supply stores. Other chalk may be calcium sulfate (gypsum) rather than calciumcarbonate and will not neutralize the acid. You can test chalk by placing a small piece in a few mL of vinegar: if itcontains calcium carbonate, bubbles will form. Crushed lime, available at garden centers, has a coarsely-crushed gray-white appearance and is commonly added to soil to change its pH.

Answers to Questions1. 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 indicatorchanges. The reaction is CH3COOH(aq) + CaCO3(s) Ca2+(aq) + CH3COO–(aq) + H2O(l) + CO2(g).

2. Answers will vary. Possible answers include: It is like acid rain falling in a lake because it acidifies the water in thebowl, and limestone in lakes can neutralize the acid. It is unlike acid rain in that vinegar is not in acid rain, and lakesdo not mix as completely as the water in the simulation. The simulation could be made more like real acid rain bycreating 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 inlimestone, so adding a large quantity of limestone to a lake might reverse the acidification.

References, Additional Related Activities, and Demonstrations1. Charola, A. Elena. Acid Rain Effects on Stone Monuments. J. Chem. Educ. 1987, 64, 436.2. Goss, Lisa M. A Demonstration of Acid Rain and Lake Acidification: Wet Deposition of Sulfur Dioxide. J. Chem.

Educ. 2003, 80, 39–40.3. Halstead, Judith A. Rain, Lakes, and Streams—Investigating Acidity and Buffering Capacity in the Environ-

ment. J. Chem. Educ. 1997, 74, 1456A–1456B. Halstead, Judith A. Spring Shock!: Impact of Spring Snowmelton Lakes and Streams. J. Chem. Educ. 1998, 75, 400A–400B.

4. 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; ICEPubl. 93-007; Institute for Chemical Education: University of Wisconsin–Madison, Madison, WI, 1993.

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JChemEd.chem.wisc.edu • Vol. 80 No. 1 January 2003 • Journal of Chemical Education 40A

JCE Classroom Activity: #50 Student Activity

Acid Raindrops Keep Fallin’ in My LakeNormal 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 canlower the pH of the lake water, causing problems for the plants and animals that live there. Do some lakes have anatural protection against acid rain? Could we use chemistry to protect lakes from acid rain? In this Activity you willmake very simple models of lakes and see what happens when acid is added.

Try ThisYou 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 placinga stick of it in a plastic bag, setting it on a sturdy surface, and tapping it with a hammer. Additional samples mayinclude 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 itemsis 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 IndicatorIf 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 refrigeratethe 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 sameway as the bowls.

__2. To the bowl(s) labeled with the name of a solid sample, add enough of the sample tocover 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 bowlgets 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 eachof the bowls and place it in the test tube with the matching label. Add several drops of redcabbage indicator to each test tube. Observe the colors and record them. After recording thecolors, 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 eachbowl 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. EPA’s Clean Air Market Programs—Acid Rain. http://www.epa.gov/airmarkets/acidrain/2. The Green Lane: Acid Rain. http://www.ec.gc.ca/acidrain/3. Acid Rain. http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Environmental/acidrain/acidrain.html4. Acid Deposition and Precipitation. http://royal.okanagan.bc.ca/mpidwirn/atmosphereandclimate/acidprecip.html5. 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.

40B Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu