applications an3 analoqies
edited by
RONDELORENZO Middle Georgia College Codran. GA31014
Pictorial Analogies I:States of Matter
Liquids (Reunion Party)
John J. Fortrnan Wright State University Dayton, OH 45435
Small Distances
The teaching of science, in general, and chemistry, in particular, involves a great deal of abstract reasoning in order to explain principles and phenomena in terms of atomic or molecular structures and dynamics. However, the majority of students up through high school and even beginning college still are making the transition from the concrete-operational to the formal-operational mode of thinking. The concrete-operational student thinks in terms of concrete obsewables and direct experience. The formal-operational student can reason with abstract wncepts and possibilities about things that cannot be observed directly but can be experienced only through their final large scale consequences. For example, a student can experience the difference in properties between solids, liquids, and gases. Solids have rigid fured shapes and volumes. Liquid samples have definite volumes but are fluid and can take on the shape of their containers. A sample of gas not only can take on the shape of its container but also can expand or contract into various size containers. This is observable and can be learned by direct experimentation. The explanation of these differences in terms of the structure and motion of individual molecules. however. reouires students to imaeine something they cannot see.'~hestudents can be helped to develop these non-observable pictures if thev can compare them to something observable with which they are familiar (1). I t is also true that "a picture is worth a thousand words" and viewing a picture will make a more lasting impression
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Solids (Military Unit)
C .
a
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Much Disorder Some Random Motion
Figure 2. Analogy of a molecular view of a liquid to a reunion patty. than simply hearing or reading - words. Auicture or activitv will giv; students-something remembered more easilk with which thev can associate the conce~tfor later recall (I). An analogy can be made between the submicroscopic structure in the solid state and a close-order military formation in which every soldier has a fixed position and a close, definite distance between each pair of soldiers as depicted in Figure 1.This is comparable to the fured positions and close regular spacing of units, such as molecules, in the solid state.. The liquid state can be compared to a crowded reunion party as shown in Figure 2. The individuals still are close together, but there is random motion and the orientations and positions are constantly changing. The analogy is made to molecules in canstant random motion but still a t relatively close spacing as exists between molecules in the liquid state. Finally an analogy can be made between the molecules in gases with their faster motion and much greater and more variable distances between them and a group of soccer players during a game a s seen in Figure 3.
Gases (Soccer Game) Large Distances Between Molecules
Small Distance Between Molecules
Rigid No Disorder No Random Motion
Figure 1. Analogy of a molecular view of a solid to a military unit.
56
Journal of Chemical Education
.Much Disorder Much Random Motion Figure 3. Analogy of a molecular view of a gas to a soccer game
As with all analogies there are limitations if the student carries the comparison too far or if something is read into the analogy that was not intended by the teacher, ( I ) . For the analogies to the liquid and solid state it should he pointed out that the fundamental particles actually are touchine and do not have soace between them as muld be inferredfrom the pictures. it illno should be noted that the ~icturesillustrate samoles of three different substances of three different amou&. For example, a sample of solid iron; a slightly larger sample of liquid water; and a much smaller sample of gaseous chlorine. If the student infers that the figures represent the same size sample of the same substance (as for instance ice, water, and water vapor), then the fact that there are different numbers of people in the three pictures muld lead them to an incorrect conclusion in violation of the law of consenration of matter. Since 1977 analogies that my students have found useful have been converted into cartoon drawings by the Media Services Department a t Wright State for use as slides and overhead transparencies. The collection now numbers more than 100, and they have been organized into sets for this series of papers. Future articles will feature pictorial analogies useful for the teaching of topics ranging from acids and bases to wave function plots. The level of the analogies covers a spectrum from very fundamental concepts suitable for students in junior high to advanced topics useful for college students. Anyone wanting copies of the illustrations in this article, and future articles as they are published, will be sent on request dry photocopies of the correct size for making overhead transparencies by photocopiers or thermofax.
Figure 1. Analogy for a molecular solid.
Acknowledgment I wish to thank Bruce Stiver of the graphics section of the Media Services Department at Wright State University, whose talent converted these ideas into cartwns. I also wish to acknowledge funds from the chemistry department that paid for these services. Presented as part of a paper a t the 195th National ACS Meeting and 3rd Chemical Congress of North America; Toronto, Canada, June 9,1988. Literature Cited 1. Thiele. R. B.: Tre~regust,D. E Ausfral. Sci lhoch. J 1981.37, No. 2, pp. 1&14.
Pictorial Analogies II: Types of Solids John J. Fortman Wright State University Dayton, OH 45435
In part I of this series of pietorial analogies, a solid was related to an army unit a t rigid attention with regular distances between soldiers standine with fixed orientation~. In this article analogies are given that relate to the four different tvpes of solids on a fiubmicrosco~icbasis. It should be noted to students that in the real submicroscopic model the individual units are actually touching their nearest neighbors, but to have the units &at close together in the pictures would make it difficult toobserve the differences in the units. In molecular solids, units are individual, small molecules with strong covalent bonding within the molecules, hut weak polar, induced polar, or hydrogen bonding forces between molecules. Examples are sugar, sulfur, ice, and
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PaR of a presentation at the 195th National ACS Meeting and 3rd Chemical Congress of North America; Toronto, Canada. June 9. 1988.
Figure 2. Analogy for an ionic solid dry ice. An analogy to this is a regular arrangement of muples that represent the covalent molecules. This is depicted in Figure 1. The second class of solids are ionic solids with a regular structure of alternating positive and negative ions that have strong forces of attraction, making these solids harder and higher melting than molecular solids. An example is sodium chloride. Figure 2 gives an analogy to this in the form of a regular array of single men (cations) and women (anions) with what one can describe as strong romantic attractive forces between them. The third type of solids are macromolecular with a continuous lattice of covalent bonds that makes the whole Volume 70 Number 1 January 1993
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