edited by:
cherniftry for kid/
RICHARD STEINER university of Vtah Salt Lake City, UT 84112
An "Indicating" Experience for Students Londa L. Borer Califomla State Unlverslty-Sacramento, Sacramento, CA 95819 Out of curiosity, the question arises, "How can we determine what something is made up of?" Of course, we chemists know that the answer lies in carrying out tests and examining the properties of substances. Children, however, need to experience such first-hand experiments. This article outlines such a presentation aimed at upper elementary students. lndlcatlng Tests for Carbon Dloxlde My presentation begins by challenging the student to test the air they exhale. As a volunteer blows into a test tube of limewater, the milky white precipitate that results helps to indicate the presence of Cot. The acidic nature of Con can he shown by asking the volunteers to blow into a test tube containing a bromthymol blue solution. The blue solution turns yellow as Con dissolves. This indicator works hecause of a change in the hydrogen ion concentration of the solution as the carbon dioxide reacts with the water. The solution becomes more acidic in nature. There still is the question of "How do I know these two indicators tell me that I breathe out carbon dioxide instead of some other gas?" This question can he answered bv using a known source of carbon dioxide such as the carbonated beverage, 7-Up, or dry ice and discuss the similarities of the results. With the correlation drawn, the statement can he made that the air we exhale contains carbon dioxide, a colorless, odorless gas that produces an acid solution in water. One of the most important ways that children learn a concept is by actually doing an experiment (hands-on approach). At the Miami ACS meeting during the "Chemistry for Kids" session, the students did three different experiments with indicators. They were each given a data sheet so that the results could be recorded and an analysis could he made of the results when everyone was finished. All experiments that they did involved food items or other materials familiar to the students so that some relationships could he ----made to other previously known facts. ~
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lndlcatlng Test for Acldlty The first experiments involved the use of purple cabbage water as an acid-base indicator. This indicator is easily prepared by hoilinga few leaves of purple cabbage' in a quart of water. The dark hlue-purple water is poured off and used in that form. The cabbage water was added to each of the following items separately: water, lemon juice, tea, baking soda, soda pop, aspirin, and Tums. From a chart of the Presented at the ACS Meeting, Miami, Florida, April 28, 1985. 'The juice does decompose with time and will discolor plastic containers brown. To prevent this, store in glass container in a cool place, and discard if mold begins to grow. 446
Journal of Chemical Education
appropriate color changes, the students were able to determine the relative acidity of the substances. Once they had determined the acid-base nature of each item, a discussion followed centered on the following questions: 1. Which of the substances would you take to make an "acid"
tummy feel better? (Somestudents answered tea which is actually an acid. Another explanation must be given for why students are given tea when they do not feel well.) 2. Lemon juice is an acid. What other properties do you know that lemons have? (Taste is sour.) Discuss other items that are acidic based on the fact that they have similar sour taste. 3. Can this indicator be used to determine the acid-base nature of orange juice? (The limitations of indicators can then he discussed. The children painted out several other items that could not be used in this test.)
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lndlcatlna Tests for Starch A second set of experiments involved the starch indicator, iodine. Iodine can be purchased as an alcoholic solution in a drugstore. Iodine sol;tion was placed on such items as cornstarch (the standard), potato, sugar, flour, bread, cheese, aspirin tablet, egg yolk, rice. The purple-black color that became immediately visible on some of the items indicated the presence of starch in those foods. A discussion of this topic centered around such questions as: Do cakes and cookies contain starch based on your experiments? (Flour contains starch.) 2. Why do some medicines contain starch? (Filler can be explained.) 3. Starches account for approximately 80% of the calories that we consume daily. They provide energy. Discuss some foods that you eat that are high in starch content. (The children can be made aware that a certain amount of starch is necessary in everyone's diet.) 1.
lndlcatlng.Test for Chlorlde A third set of experiments involved the use of silver nitrate, which I called thedsalty" indicator. Nodistinrtion was made the~ determination of sodium chloride or the ~ - between - ~ ~ ~ chloride ion. So that the students would know something about the indicator they were going to he using, two of its properties were discussed. Silver nitrate is used to wash a newborn infant's eyes, and it is also used in cauterizing. The students then added a couple of drops of 0.1 M AgNOa to each of the followinn samples: ocean water, swimming pool water, tap water, deionized water, Gatorade, and soda pop. The amount of the white precipitate (silver chloride) indicated the extent of "sa1tiness"~of the ocean water while i t pointed to the relative purity of the deionized water. Questions were brought out about mineral water, bottled water that people buy in the store, etc. There seemed to he an overall awareness of pollution and the students were con~
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cerned ahout the purity of their drinking water. I t was also apparent to many of the students that this indicator has limitations. For example, milk could not be used in this experiment nor could most of the heavily colored or murky liquids.
There are many other indicators that could he used to show how they are used in chemistry. I believe that these are representative and show both the usefulness and the limitations of indicators in pursuing the properties of substances to allow safe identification.
Volume 64
Number 5
May 1987
447
