applications and analogies
edited by
RONDELORENZO Middle Georgia College
Cochran. GA31014
The Movement of Molecules and Heat Energy Two Demonstrative Experiments Mariko Suzuki Graduate School of Human Sciences Osaka University 1-2 Yamadaoka, Suita-shi,Osaka 565, Japan
Everyday experiences provide us with clues for scientific concepts. Scientists can fmd clues. For students, however, selecting and recognizing which feature of a phenomenon is the clue often can be a difficult task. Consequently, teachers should make certain that the experiments they present to students provide them with the clue. The "molecule" is used by scientists to explain many aspects of the behavior of matter. In Japan, students learn the concept ofmolecules in sewndary school. However, it is difficult for them to recognize the movement of molecules, especially the relationship between the movement of molecules and heat energy, because molecules are invisible. In this paper, I propose two experiments for students who have learned about the movement of molecules to show the relationship between the movement of molecules and heat energy The three notable features of the experiments are that the results of the experiments are simple and visible, the experiments are easy to carry out without any special apparatus, and, finally, they show how strikingly a difference in temperature affects the movement of molecules. Correspondence address: 36-2 Kaguraoka-cho, Yoshida, Sakyoku, Kyoto, 606 Japan
Figure 1. Structural formula of tartrazine (I). First Experiment Sugar Cubes in Cold Water and Hot Water
Pour mld water into one beaker and the same volume of hot water into another. Hot water of about 80 C is taken from a Thermos flask. Gently drop sugar cubes into each beaker, then compare what occurs in the beakers. Results
A sugar cube dissolves much faster in hot water than in cold water. Because all conditions except temperature are the same, it is easy for children to realize that the difference in the dissolution rate is due to the difference in temperature. Second Experiment Dialysis Tubes in Cold Water and Hot Water
Tartrazine (Fig. 1) can be purchased from commercial sources (Sigma Chemical Co., St. Louis, MO, ICN Biochemicals, Inc., Costa Mesa, CA, or Aldrich Chemical Co., Milwaukee, WI). Dialysis tubing (cellophane tube) can be
Volume 70 Number 10 October 1993
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//-dialysis
rate. Moreover, tartrazine is classified as FD & C No. 5 and the waste solution can he dumped into the sink.
tube
Results
fastenwith a rubber band
-'4
rL I /
yellow tartazine solution
cold water
fasten the other side with a rubber band
=hot
water
white paper white paper Figure 2. How to compare the diffusion rates of tartrazine in cold water and hot water. purchased from Viskase Sales Co., 6855 W. 65th St., Chicago, IL (Dialysis Membrane, Size 18). Dialysis tubing is supplied in a dried and flattened form. Cut off a piece of about 12 cm in length, soften it in water, and make it into a tube. One end of the tube is fastened with a rubber band. Make a 20 mg1mL tartrazine solution in a test tube and show the color of the solution (yellow) to the students. Put the solution into the dialysis tube. Fasten the other end of the tube with a rubber band (Fig. 2). Wash the outside of the tube with water and then wipe it. Prepare two dialysis tubes containing the solution. Put one tube into cold water and the other into hot water. Compare the phenomena that occur in the beakers. The author has examined other possible colored solutions. These often are inferior choices. In some cases proper disposal presents difficulties. Other substances are mutagenic and/or carcinogenic. In many cases diffusion rates are either too slow or too fast. Of 25 colored solutions tested, that of tartrazine diffused a t the most moderate
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Journal of Chemical Education
A tartrazine solution is yellow. Cellophane is a molecular sieve and particles with molecular masses of less than about 12,000-14,000 pass through cellophane. Because the molecular weight of tartrazine is about 534, it passes through the dialysis membrane. As tartrazine diffuses through the membrane, the outside water turns yellow. hec color change is strikingly fast in hot waterithough it takes some time in cold water. Because all conditions except temperature are the same, it is easy for students to realize that the difference in the diffusion rate is due to the differencein temperature. These results are explained by a temperature increase. ena different temComparison of the ~ u o ~ ~ h e n o m with-the peratures allows children easily to recognize the relationship between the movement of molecul& and heat energy In the first experiment, the collapse of the sugar cube is visible, and in the second experiment, the movement of the yellow solution is visible. Therefore, these experiments can support students' ideas about the movement of molecules. Vaughn et al. (2)described how to show the diffusion of molecules in liquids. they did not describe the relationship between the movement of molecules and temperatures, while I emphasize it. The experiment they showed takes several hours or more to demonstrate that diffusion occurred. while mv " experiments need onlv 5-10 min. Therefore, aschool teacher can show the res& to the students within one class hour. Vauehn et al. used harmful chemicals such as copper sulfate,-br0mine;and iodine, but in my experiments I use sugar and tartrazine that is accepted to use as a food additive. Therefore, a school teacher can ask the students to do the experiments that I Drowse safely by themselves. &
Literature Cited 1. Budavari S.. Ed.The Memk I&; Merek and Co.: Rahway, NJ,1989: p 8948. 2. Vaughn, W . E.; Schreiner, R.: ShaWlashiri, B. 2.; Shaw, D. B. "Molecules in Slow Motion:m a i m in LiquidsninChamiccri~ma~lmfloM:A Handboak for Ternhers ofChomlsfry; Shakha6hiri.B.2..Ed.: University ofWiamnsiwMadi~an, WI, 1989: Vol. 3, pp 317321.
