ate the bell jar; the nitrogen liquid will boil. It takes from 4-8 min, and evaporation of up to one-third the liquid, for the freezing to occur, depending upon the efficiency of the vacFigure 2. Cryophorus. uum pump. During this time the cryophorus may be demonstrated and discussed. Displace all the water in the cryophoms to the f r s t bulb (without the indentation) and t h e n place liquid nitrogen or a dry-ice slush in the indentation of the second bulb (Fig. 2). In approximately one minute the water in the first bulb will freeze. 1 Some cryophori do not posFigure 3. Apparatus for freezing sess an indentation in the water using sulfuric acid. second bulb in which case the second bulb may be immersed or dipped in liquid nitrogen (or dry-ice slush) with the same result. While these demonstrations are in progress, discuss molecular motion, kinetic energy and temperature, the Maxwell-Boltzmann distribution and "hot" molecules. The demonstration of a second bell jar with 10-15 mL of water in a 50-mL beaker and 150 mL of concentrated sulfuric acid in a 250-mL beaker (Fig. 3) can be used to reinforce the same concepts. Under continuous evacuation, the water drops from room temperature (23 'C) a t a rate of C about 1"C every 10-15 s and supercools to as low as -10 O in 7-8 min. The water freezes suddenly and the temperature jumps back to 0 T.The temperature ofthe water can be monitored quite easily with an ordinary -10 to 110 'C thermometer. A video camera and projection screen or large TV set helps the presentation of the freezing process in all three experiments in large lecture halls. Results and Discussion I . Cryophorus:A q o p h o m s consists of two glass bulbs connected by a glass tube. One bulb often has an indentation to hold some cooling agent and the apparatus contains a small amount ofwater under vacuum. In the experiment, water in the first bulb is frozen, not by cooling the first hulb but by cooling the second hulb. The "hot" water vapor in the first bulb is removed as it is condensed in the cooled second bulb. The removal of these "hot" molecules causes the temperature of the water in the first bulb to fall until the water freezes. Often the freezing is quite sudden because of supercooling. We have noticed that a t the instant of freezing the liquid nitrogen in the indentation boils more rapidly demonstrating the exothermicity of the freezing process. 2. Bell Jar: Two beakers under vacuum in a bell jar, one containing water and the second containing concentrated sulfuric acid, exhibit the same phenomenon. The "hot" molecules of the water vapor are removed as the water vapor is absorbed by the concentrated acid. The temperature of the water remaining in the small beaker falls until it freezes. This demonstration has a pedagogical advantage over the cryophoms in that no external cooling agent is applied. The surface of the concentrated sulfuric acid gets hot and the acid shows an overall gain of about 1or 2 'C after mixing. A large surface area for the sulfuric acid is desirable. Continuous evacuation of the bell jar in the ab-
68
Journal of Chemical Education
sence of a beaker of sulfuric acid results in slow cooling, reaching -4 'C from 23 'C after 50 min with no freezing taking place. 3. Frozen Nitrogen: This is the most dramatic illustration of the three. Removal of the "hot" nitrogen molecules by evacuating the hell jar cools the liquid nitrogen sufficiently in 6 5 min to freeze it. The boiling point of nitrogen at 1 atm pressure is -195.8 T and the freezing point is -209.9 'C, not far below. Release of the vacuum causes a sparkling melting of the crystalline nitrogen. Literature Cited 1. Alyea, H. N.:Duhon, F. B. TpslpdfimonsVotions in Ckmis1ry;J. Chem.Educ.: 1965: pp 6M4.
An Inexpensive Flame Test Technique Submined by
Robert A. McRae Bonnyville Centralized High School Bag 1002 Bonnyville, Alberta, Canada T9N 2J7 Checked by
Richard F. Jones Sinclair Community College Dayton, OH 45402 Several designs, suggestions, and apparatuses have been suggested in this Journal for flame tests ( 1 4 . Of those cited, Barne's (2) technique can be modified slightly to include reviewing process variables (manipulatedlindependent, respondingldependent, and controlled) as well as attributingflame color to selected cations. By sandwiching a strip of paper towel between two metal scoopula (see figure) one is left with 2-3 an of paper extending from the scoopulas. Wetting the paper with distilled water keeps it from burning in the flame and causes the solid, dry compound (e.g., NaC1, NaBr, &SO4 etc.) to adhere to the paper. The need for a blank or control can be emphasized by asking students if the color is caused by the solid, the water, the paper towel, or a combination of them, and by asking how they would prove their idea. The flame colors produced are intense enough from a Bunsen burner to allow the demonstration to bedone with the classroom lights on. This technique does not require hvdrochloric acid nor spray bottles, is cheap to d;, and reasonably easy to clean up. Literature Cited 1.Agsr. D. J.: Eaat, M.B.;Miller,R.A.J. Chsm. Edue. 1888.65, 545 2. Barnes. Z . K J. Chem. Educ. 1991.68 246. 3. Boneher, J. H. J. Chem. Edvr 1986,63,158. 4. Gouge, E. M. J Chzm. Edur 1988.65.544. 5. Mattson, B. M.: Snipp, R. L.:Michels, G. D.J Chem. Edur 1990.67, 791 6. Pearson. R. S. J Chem.Edrrc. 1985.62, 622. 7Peyser. J. R.: Luoma, J . R & Chem. Educ 1988.65.432. 8. Rsgsdale, R. O.:Dtireoll,J.A. J Chm. Edue. 1992.69.828.
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Set-up for flame test.
paper 1 x l O ctowel rn stnp
Metal Scoopula
