overheaJ projector demonstrations
edited by DORIS KOLB Bradley University Peoria, lL61625
Acid Rain Investigations Dianne N. Epp East High School 1000 South 70th Street Lincoln, NE 68510
dH20 +
bromathymal
blue
Robert Curtright Northeast High School 2635 North 63rd Street Lincoln, NE 68507
dH,O+
methyl orange dH,O+
Acid rain is defined a s precipitation in which the pH is lower than 5.6 and is formed ~rimarilvwhen sulfur dioxide and various oxides of nitrogen, emitted into the air from vehicle emissions and industrial pollution, undergo a series of chemical reactions in the presence of sunlight to form sulfate and nitrate ions. With water, or water vapor, these ions form dilute solutions of sulfuric and nitric acids. Unpolluted precipitation is always slightly acidic (pH 5.6) due to normal distribution of carbon dioxide in the air that dissolves in water to produce carbonic acid.'This overhead projector demonstration is designed to illustrate (a) the reaction of several nonmetallic oxides with water to produce acidic solutions and (b)the effects of several different bedrcck on acid rain.
cabbage juice
I
I
I
bromo- methyl cabbage thymol orange juice blue Figure 2. Experimental layout of the 24-well plate to show acidity of the generated gases.
-
Corange
6
IV
bromophenol blue
Cmelhyl orange
Sand, marble chips, and chalk have been
Size 0 rubber stopper
the abovce indicators.
Figure 3. initial set-up of the 96-well plate for examination of acid rain on rock samples. When 6 M HN03 is added to a small amount of sodium sulfite, slurried with a few drops ofwater to control the rate of reaction, sulfurous acid is formed which decomposes into sulfur dioxide and water. Figure 1. Design of the gnerator that supplies acid anhydrides. Formation of Acid Rain The following reactions are camed out using a 24-well plastic tissue culture plate with three wells used to generate the acid anhydrides: SO2, NOz, and CO2. The generator well is fitted with a modified rubber stopper and delivery tube and the gases are bubbled through distilled water in adjacent wells that contain a series of acidhase indicators (Fig. 1). The color changes observed are compared with reference solutions of the same indicators prepared in the first three rows of the plate (Fig. 2). NO2 is generated by reading 6 M HN03 with a small amount of 20 mesh copper.
1034
Journal of Chemical Education
Na2S03+ 2 HN03 -t 2 NaN03 + H20 + SO, Carbon dioxide is generated by the action of 6 M HC1 on calcium carbonate in the form of a marble chip. 2 HC1+ CaCO,
+ CaC12+ H20 + CO,
Avariety of indicators may be used provided they have a color shiR in the acid region. Methyl orange, bromothymol blue, orange N, bromophenol blue, and cabbage juice have all been employed successfully. Effect of Various Bedrock Samples on Acid Rain The purpose of this demonstration is to illustrate the result of acid rain falling on various types of soil. An area
'Acid Rain, 3rd ed.; ACS Committee on Environmental lmprovement: Washington, DC.,1991.
that is high in limestone (CaCOd, dolomite (MgC03, CaC03) or magnesite (MgC03)will have a different effect on acid rain from an area with primarily granite bedrock.' Using a 96-well plastic tissue culture plate, a series of reference pH solutions, 1-5 is set up in the first three rows. Orange N is added to the solutions in row A, bromophenol blue to wells in row B, and methyl orange to the wells in row C (Fig. 3). -
~p
'Mohnen, Volker A. Sci. Amer. 1988, 259,30-38
Prior to the demonstration, the effect of marble chips (metamorphic form of calcium carbonate), limestone-chalk (sedimentary form of calcium carbonate), and sand (silicon dioxide which is similar to a granite bedrock)on "acid rain" is simulated by heating samples of each with 0.1 M HC1 in a test tube in a hot water bath for 10 min. The pH effect of the "acid rain" thus treated is compared to the reference solutions on the overhead plate by placing these solutions in wrresponding rows in columns 7 , 8 , and 9 and treating with the same indicators used for the reference solutions.
Volume 68 Number 12 December 1991
1035
