Wichita State University Wichita. KS 67208
Gas Cans and Gas Cubes: Visualizing Avogadro's Law J. Bourna
Free Unlverslty Amsterdam. The Nemeriands I t is remarkable that only relatively seldom is attention paid to the psychological difficulties encountered when teaching Avogadro's Law. This law, as every chemistry teacher knows, states that an equal number of gas molecules is contained in an equal gas volume of the same temperature and pressure, regardless of the kind of gas molecules. Although J. Dudley Herron does not explicitly mention Avogadro's Law in his fundamental article "Piaget for Chemists" ( I ) , he has pointed out in a later article that the teacher should try to help his students get as clear a picture as possihle of the workings of concepts like these, should make i t concrete (2). According to Shayer and Adey, the reasoning involved in grasping Avogadro's hypothesis and its application to formulae from volumes of reacting gases, belongs to Stage 3B in the Piagetian development, Late Formal Stage (3).In such a situation, introduction of formal concepts can be facilitated hy making use of models (vide, e.g., 4). Of course, with the help of experiments, a sound basis should be laid for the assumption that gases react in volume proportions that can be expressed in simple integers (Gay-Lussac's Chemical Law) and for the explanation by means of Avogadro's Law. The reactions of hydrogen and chlorine and of hydrogen and oxygen are well known. Very fine demonstrations have been worked out by the German couple Johannes and Renate Zitt.' They have developed special and easy-to-handle apparatus for the synthesis of hydrogen chloride and of water, both in gas and in liquid form. T o explain what happens with a gas experiment, they work with little squares filled up with colored halls, e.g., Figure 1. If symbols like these are shown on an overhead projector, the demonstration will gain in effectiveness. In an attempt to make the gas laws "visible," I hit on the thought of using empty l - L cans to show what I meant. I reasoned this might help those students who areon the brink of Stage 3B or those who prefer an alternative approach. Recently Rowel1 (5) has pointed out that even if there were only one accepted theory of learning, e.g., Piaget's, there would still be different ways of applying it toactual teaching. In imitation of Toulmin (6), Rowell compares this with a map and an itinerary. My gas cans are a first itinerary. They were put a t my disposal by the Dutch paint manufacturers, Sigma Coatings. Height 13 cm, diameter 11cm, volume 1L. I glued colored cards on them and wrote "1L oxygen" on the front and, as a consequence of Avogadro's Law, "p molecules oxygen" on the back. I built up quite a collection of differently colored cans and a variety of gases. Using these gas cans, i t is relatively simple to show that when two cans (= 2 L) of hydrogen and one can (= 1L) of oxygen react, two cans (= 2 L) of water vapor are formed. You just put them down on the demonstration table (Fig. 2). After turning the cans This article is an enlarged translation of one that appeared in the Dutch science teachers' jownal Fwaday 1983, 5Z1). 8. 'Obtainable from Renate Zitt, Jacobstrasse 9. 7800 Freiburg im Breisgau. Federal Republic of Germany. 586
Journal of Chemical Education
over 180 degrees, one reads (Fig. 3): "2p molecules of hydrogen p molecules of oxygen yield 2p molecules of water". Of course, cylindrical beakers or other cylindrical objects will also make good teaching aids. I t worked, but I was not really satisfied. The final stage in this development was the gas cubes. I
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Figure 1. Blackboerd representation of two gas reactions, viz.. ti2 + CIS 2HCI and 0.
+ 2H,
2H,O.
Figure 2. Gas cans,front side.
Figure 3. Gas cans, back side.
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1 liter HYDROGEN
CHLORIDE Figure 4. Ges cubes, experimentaldata.
Figure 7. Gas cubes. molecular equation.
Figure 5. Gas cubes, alter application of Avogadro's Law.
Figure 8. Oas cubes used facaiculations.
CHLORIDE Figure 6. Gas cubes, alter dividing by p.
Figure 9. Oas cubes used for calculations
sawed wooden cubes of 1dm3. I pasted colored cards on all sides of these cubes, so there were six possibilities. If the right order of the inscriptions is followed, it works in the manner shown in Figure 4, "1L of hydrogen 1L of chloride 2 L of hydrogen chloride". Now we turn the cubes over 90" and we read (Fig. 5): "p molecules of hydrogen + p molecules of chlorine p molecules of hydrogen chloride". We divide by p, which is done by turning the cube over another 90" (Fig. 6); "1 molecule of hydrogen + molecule of chlorine molecules of hydrogen chloride". Finally, the last turn gives the molecular formula (Fig. 7), Hz + Clz 2HC1.
The inscriptions on the two other sides are, in connection with related purposes, "1 mol of Hz" and "22.4 L of Hz" (Figs. 8 and 9). Suggestions for colors are: Hz, red; 0 2 , blue; HzO, white or cream; Clz, green; HCI, brown; Nz, gray; NH3, orange; CH4, yellow; CO, purple; COa cyanine.
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Literature Cited (11 Henon,J.D.,J.ChemEduc. 1975,52,146. (21 Cmtu.L. L.:Hmon.J.D.J.Res.Sei. Teorh. 1978.15.135. (3) Shsyer, M.; Adey, Ph. "Towards a Science of Science Teaching": Heinemann Educational: h n d o n , 1977. ( I ) Ksuanaugh, R. D.;Maomaw, W. R.J. Chorn.Edue. 1981.58.263. d,,i,Educ, 1986, (5, (6) T O U I ~ S. ~ ~-me , philosophy of seienee":~ a m e & r RW: N ~ ~Wm k1977. .
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Volume 63
Number 7
July 1986
587
