background of the stars and sprayed with 0.1 M HCl. To develop the blue color, spray the square with Congo red; the stars can be attached to a plastic food wrap and placed on the blue background. As with any demonstration, the success and usefulness of its presentation are hindered only by the imagination of the demonstrator. I generally try to keep a lively dialogue going while I am attaching the strioes to the flae. ~ u r i the n ~same time frame, I also was reminded of the 1970 Rov Lichenstein work "Peace throueh Chemistrv" which features the colors red, white, blaFk, and yellow. When a local third grade student was asked to draw a picture about the War, he drew what he thought was an appropriate picture and then wrote "Peace is a big, fluffy cloud of very bright yellow with red, white, and blue streamers." I have used this third-grader's idea as a demonstration. The big, fluffy yellow cloud is simulated by placing on a lighted magnetic stirrer a 4-L beaker containing dilute sodium iodide solution. A few crystals of lead(I1) nitrate are added t o produce a bright yellow cloud of lead(I1) iodide. Streamers developed using Congo red are hung between the beaker and the stirrer. Chen's ( I )''American Flag" does not produce flag-like colors. However, he does offera good flagpolesuggestion,i. e., treat paper with tannic acid (2%)and spray with imn(II1) chloride (1%)to produce gray-black iron(II1)tannate. Literature Cited
S.
E"*rtai"ing ~d Edumfional ChDmirol I * m o M t m t i o ~ ;Chemical Ekments: Csmsdlllo, CA, 1974: p
1. Chen, P.
20.
Simple Soda Bottle Solubility and Equilibria Cheryl A. Snyder
Penn Manor High School E. Cottage Ave. Miiiersvilie. PA 17551 Dudley C. Snyder
Penn State University Wilke-Barre Campus Lehman, PA 18627
pour out approximately one-third of the contents. Add 2-3 drops of bromocresol green indicator, and reseal the bottle. The solution should he clear-yellow. Allow the bottle to warm to room temperature. Green. Then shake the bottle vigorously, and carefully open the cap to vent the pressure of the C02. Repeat the process several times until little pressure remains. At this point, the solution should be clear-green. Blue. Place the open bottle in a bath of almost boiling water and wait. Within a few minutes the solution should turn completely blue. Discussion
Although the students may recognize that gaseous C02 can be dissolved in water (eq I), they may not know the chemical reaction that generates a small equilibrium concentration of carbonic acid (eq 2) in solution. Yellow. This equilibrium concentration is shown by the initial yellow color, which is the color of the bromocresol green indicator' at a pH below 3.8 (eq 3). When the bottle warms, the solubility of the C02 decreases, but the resulting pressure in the sealed container prevents any significant change. Green. Only when the bottle is opened and the C02 is allowed to escape do the three coupled equilibria shift. The rise in the pH is seen as the indicator moves into the transition range of 3.8 to 5.4. Blue. Since some COz remains in solution even at ambient pressure, heating the solution to a higher temperature is required to drive out the residual gas. The pH then rises beyond 5.4, where the indicator is blue. Comparlson with Other Demonstrations
This process clearly demonstrates the effect of both temperature and pressure on gas solubility, as well as Le Chatlier's Principle in all three equations. It has some similarity to Shakhashiri's2 pressure-variation demonstration, but it has several advantages. It is simpler to perform, and it requires no special equipment. Also, a larger volume of solution is used, and this provides better visihility.
Checked by:
Ronald E. DiStefano
Northampton Area Community College Bethlehem, PA 18017 Recently while searching for a simple visual demonstration of gadiquid solubility, we found success using a simple bottle of seltzer water and an acid-base indicator. The chemistry of the demonstration is summarized in the following equations.
Demonstration
Use a 296-mL clear-glass bottle of plain seltzer water, which is available in most supermarkets. Only seltzer water with no colorings, flavorings, or other additives should be used. Colors could mask the indicator change, and other materials such as phosphates might buffer the system. Yellow. Remove the label, and wol the bottle in an ice bath. Then open the bottle, and--as quickly as possible--
The Use of Potassium Alum in Demonstrating Amphoterism Kris F. Tesh Vanderbiit University Nashville. TN 37235
The property of amphoterism is surprisingly difficult to demonstrate. Almost all introductory chemistry texts state that A1203 is an amphoteric compound that undergoes the following reactions: NzO8(s) + 6HCl (aq) + 2AlC4 (aq)+ 3H20 Commercially available aluminum oxide does not always read as these equations suggest. Only the y-form of A1203 displays reactivity toward acids and bases. The more common a-form is made by heat treatment of the other forms of the oxides and hydroxides of aluminum(l), and is stable 'CRC Handbook of Chemistry and Physics, 51st ed.: The Chemical Rubber Co., 1971;pD-106. 'Shakhashiri. B. 2. Chemical Demonsiraiions-A Handbook for Teachers of Chemistry;University of Wisconsin Press, 1985;Vol. 2, p117. Volume 69 Number 7 July 1992
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