of the sign regulates the screw clamp and catches the luminol solution as it comes through the tubing. After the performance, we flush the sign with water and then store it filled with ethanol to prevent the growth of any material that may clog the tubing. All spent materials may be disposed of down a drain with water. No attempt is made to describe the chemistry behind this demonstration during our performance. However, an excellent explanation of chemical luminescence may be found in the citation of Shakhashiri (2)if one desires to make this demonstration educational as well as entertaining. The luminescent sign sounds like a lot of work, and it was when we orieinallvassembled it. Now we can null the " materials out of storage and perform this enlightening demonstration, with musical accompaniment, in less than 5 minutes. ~~
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1. The use of Avogadro's number to calculate the number of molecules in a samole is demonstrated. 2. The balance used i s the snmr nr rhr balances the students drmonstraung that they will nepd uspln the I~hnr~tory 10"molecules to register a mass. 3. The means to weigh out smaU numbers of molecules is beyond current technology. Therefore, if one is asked to determine the numher of molecules in a weighed sample, answers of 1, 10,100,or any other small number of molecules are meaningless. Literature Cited 1. van Lubek. H . ' Chpm. Edue. 1989,66 762. 2. MeCullough,T.,CSC: J Chem. Edus. 1990,67,783.
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Rainbow Demonstration Using Acid-Base Indicators
Literature Cited ~~~
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1. Alyea, H.: Dunon, F,Eds. T e a Demonstmham in Chemktn. 6th d.Jovmal of Chemical Education: Easton, PA, 1965, p 186. 2. Shskhashiti, B. C h M 1 DemmtllVIom: A Handbmh far k h w s of Chemistry: University of Wismnsin Press: Madison, WI, 1985:Vol. 1, pp 12%204.
Demonstrating the Magnitude of Avogadro's Number Submitted by: Philip T. Johns University of Wisconsiwwhitewater Whitewater, WI 53190
Danette Crookston Malone Co lege Canton. OH 44709 Mel Mosher Missouri Southern State College Joplin, MO 64801
Mel Mosher Missouri Southern State College Joplin, MO 64801
Various methods have been suggested to show the magnitude of Avoeadro's numher (.1. 2). Most chemistm instmctors have seen answers to examination questions indicating that 1g of a substance may contain one molecule. This simple demonstration illustrates the great magnitude of Avoeadm's number and, in doing so. Droves that the capacity OFweighing one molecule well beyond the means of current technology.
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The Demonstration
Materials analytical balance (0.1mg) 30-mL beaker acetone medicine dropper Procedure With the assistance of a student to read the balance, weigh one drop of acetone. Write this value on the board. Almost immediately ask the student to reread the weight and write this new value on the board. Observations Approximately a 0.9-mg loss in weight is observed due to evaporation. The students are asked to determine the number of molecules of acetone that have evaporated. Using Avogadro's number they will determine this to be equivalent to 9 x 10'' molecules. Discussion From this simple demonstration presented early in the first semester of general chemistry a number of points can be made to the students. Journal of Chemical Education
Robert E. Loffredo University of North Carolina at Charlotte Charlotte, NC 28223
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When acid-base titrations are discussed in General Chemistry, the Rainbow Demonstration can be used to introduce the concept of indicators. The demonstration is done as chemical magic to spark student interest. A colorless solution is added to six "empty" test tubes producing different colors. When more of the "same" solution is added, the colors change to give the rainbow colors. Materials 1. Six large test tubes and a test tube rack with a white background 2. 2 x 103M HC1 in an aqueous solution that is 20% by volume ethanol 3. 0.01 M aqueous NaOH solution 4. Indicators
Mixture component ratios are on a weight basis. All indicators are ground to fine powders to accelerate dissolving. Procedure The test tube rack is marked incons~icuouslv " R..0. . Y.. G.. B, and V, and small amounts of indicators or mixtures listed above are added to the test tubes so the rack markings match the base colors of the indicators. Long stem funnels (or straws) are used to add the powdered indicators to insure that no solid clings to the sides of the test tubes. The test tubes should appear empty. Enough HCI solution is added to a clear squirt bottle so each test tube can be half filled. The tip of the squirt bottle can be cut off to allow for a rapid liquid flow. The NaOH solution is stored in a clear bottle. To begin the demonstration, a comment is made about the importance of solutions in chemistry, and as the HC1 solution is added to the first test tube, it is referred to as the "solution of many colors." Solution is then added to each test tube, and as it is squirted into the sixth test tube (each test tube now approximately halffull), the squirt bottle is empty. While the NaOH solution is added to the squirt bottle (not mentioningit is a different solution), students are asked what else in nature has many colors.
