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tested demonstrations reflecting bulb) as uniformly as possible. A disk of suitable diameter with a dull black surface is placed on the bottom of each beaker. (We have used brass disks 0.5 mm thick Submined by and 63 mm diameter. The surface had been blackened beManfred Adelhelm and Ernst-Gerhard Hohn fore by placing the disks for about 2 h in 250 mL of a 10% Padagogische Hochschule Ludwigsburg NaOH solution containing ca. 2.5 g.. K2S208.) 7140 Ludwigsburg. Germany - Temperature in both beakers can be mensured by thermocouples ln the ccnter, about 2 cm above the bottom. We Checked by have uied NiICrNi thermocouples of the coaxial type, with David A. Franz one junction in each beaker, so that temperature differLycoming College Williamsport, PA 17701 ences between the two beakers are shown by the meter. After the light source has been switched on, radiation equilibrium and constant temperature in both beakers is One of the greatest threats humankind may face in the reached withina few minutes. The temperature difference future is the expected warming of the atmosphere within between the two beakers is 0 2 'C. This level of precision the next decades, caused by the release of infrared-absorbis sufficient for the following qualitative demonstration. ing gases especially carbon dioxide, into the atmosphere. Now one beaker is slowly filled with the "greenhouse For an increase of atmospheric COPconcentration to twice gas" (e.g., COz)through a glass tube. During this operation its present value, model calculations predict an increase in the other beaker is covered with a glass plate. When the temperature of the lower atmosphere of 1.5 to 4.5 'C, with beaker contains a sufficient amount of COz (aRer about 30 concomitant dramatic effects on vegetation, climate, and s), glass tube and glass plate are removed. After a short ocean levels. Much has been published about causes, eftime one can obsewe a pronounced temperature differfects, and possible strategies for abatement of this "greenence: the temperature in the beaker filled with COz rises house effect" (Id), and this is (or a t least should be) an by about 10 'C within 1min. Later on the temperature difimportant topic in science curricula. ference decreases again, due to convection and diffusion of Demonstrating the Principle of the Greenhouse Effect CO2out of the beaker. One can show that the presence of a black body convertWe have devised and tested in class a simple experiment ing visible light to infrared radiation that is then absorbed to demonstrate the principle of the greenhouse effect. One by the greenhouse gases is essential for the warming. also can show the different greenhouse potential of various Without the black disk in the beaker the demonstration gases. will not work! Two identical beakers (250 mL) are placed next to each Instead of the thermwouple, mercury thermometers can other on t h e benchtop. (See t h e figure). They a r e be used to measure the temperature in the beakers. Howilluminated from above by a strong light source (100-W, ever. because of their higher heat capacity, t h e reading of the temperature is more Lamp with reflecting bulb sluggish and the effect can be less well observed. In addi\ tion, one cannot show directly temperature ciifferences. In the same manCO,, or CF, CI, etc. ner t h e impressive greenhouse effect of chlorofluorocarbons can be demonstrated. I n a beaker filled R 12 (CF2ClZ)inwith TherrnoBeaker (250 mL) stead of COzthe temcouples perature rises within 2 min by 20 'C or more. (One may have Black brass disks to point out to t h e students, that actual concentrations of greenhouse gases in the atmosphere are much lower than in Experimental setup forthe demonstration of the greenhouse effectof differentgases. this experiment!) The
A Simple Demonstration of the Greenhouse Effect
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Volume 70 Number 1 January 1993
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effect of R 11(CFCld or R 113(C9F7Cla) is shown bv ~ l a c i n a 1-2 mL of the liquid into the be&er.'XRer a shokcooliG (due to the evaporation of the liauids) a temperature in~ ca. 18 X!, respectively, ;an be seen. crease of ca. 1 5 : and Similarlv. the meenhouse effect of other volatile substances Gg., pekane, diethylether, dichloromethane etc.) can be demonstrated. Gases that are lighter than air (ex., methane) are not suitable for a demoktration with &is setup. It should be emphasized, that similar to the situation in the real atmosphere, the obsewed temperature effects are caused not onli by radiation processes; but also are due to thermal convection and heat flow. One can show, however, by measuring the temperature simultaneously with a thermocouple in the beakers and an infrared-sensitive probe from above, that in our experiment radiation contribuks to a considerable extent to the measured temperature changes. Acknowledgment Technical assistance by Regina Hornstein is sincerely appreciated.
Figure 1. A common set-up for illustrating the rotation of polarized
Literature Cited 1. German Bundeatag, Ed. Plotecting *he Eorfh's Almosphero: A" InLe~notionol C h d k w . Bonn, 1989. (ISBN 3-924521-36-01 Bvndestsg (Hrsg.1 Schutl drr Erde: eine Beafondsoufnohm. mil V~mchJA#enzu P ~ M In e u n Energkpolitik. Bonn, 1990. (ISBN 3-92452161-11 3. Bolin, B.; Do&. B. R.;Jager,J.;W m i i k , R.ATheGmhou~ERpe1,ClimolleChangp and Eeos~slem;J.Wiley & Sons: Chiehester, 1986. 4. Schneider, S. H. Science 1989,243,771, 5. Peltie~WR.:lb8hingham.A. M. Seknea 1989,244,808.
2. Deutseher
Kaleidoscoptical Activity Submitted by Robert Becker Kirkwood High School Kirkwood, MO Checked by
Erwin Boschmann IU/PU at Indianapolis Indianapolis, IN 46202
Many chemical educators are familiar with the traditional demonstrations used for illustrating the rotation of polarized light bv a n opticallv active substance (1-5). The common set-up is shown in Figure 1. An empty beaker is placed on an overhead projector between two orthogonally oriented polarized filters. The image is completely dark, because all (or almost all) of t h e light t h a t makes it through the first filter gets screened out by the second. A solution of corn syrup is then poured into the beaker, causing a circular patch of light to-appear on the screen. Explanation The polarized light passing through the first fdter gets rotated by the optically-active sugar molecules, enough to allow it to pass through the second filter and project onto the screen. This demonstration may be effective for some students, but others might have trouble equating the appearance of a patch of light on the screen to the optical rotation occurring inside the beaker. If rotation is occurring, -~why . do we notsee something rotate? The following is a simple variation on the demonstration described above and may be used in place of it or in conjunction with it. The primary advantage of this variation is that it actually shows the rotation as it occurs. It also adds dramatically to the visual impact of the presentation.
74
Journal of Chemical Education
F gure 2 Modlfled demons!rat~on,nwrporat~ngone oratnary polarwed fdler an0 one thar IS radlal y polarma
The innovation is a simple one. Replace one of the two ordinary polarized fdters kith one d a t is radially polarized, having all lines oriented outward from the center of a circie, like
