Acid Raindrops Keep Fallin' In My Lake
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1
2Instructor 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.
Background
Because atmospheric CO
dissolves in water to form carbonic acid, rain is naturally acidic. The reaction is
2
O(l) → H
+
–
CO
(g) + H
CO
(aq)
H
(aq) + HCO
(aq) (1), resulting in a pH of approximately 5.6. Other gases in
2
2
2
3
3
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 (CaCO
) foundations,
3
have a natural buffering ability that helps neutralize acid rain. The reaction between calcium carbonate and sulfuric
2+
2–
acid (a major component of acid rain), is H
SO
(aq) + CaCO
(s)
Ca
(aq) + SO
(aq) + H
O(l) + CO
(g).
2
4
3
4
2
2
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 Activ-
ity. 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 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 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 gray-
white appearance and is commonly added to soil to change its pH.
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
2+
–
changes. The reaction is CH
COOH(aq) + CaCO
(s)
Ca
(aq) + CH
COO
(aq) + H
O(l) + CO
(g).
3
3
3
2
2
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 CaCO
in
3
limestone, so adding a large quantity of limestone to a lake might reverse the acidification.
References, Additional Related Activities, and Demonstrations
1.
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 Snowmelt
on 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; 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
40A
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