In the Classroom
The Ubiquitous Metaphors of Chemistry Teaching Herbert Beall* Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, MA 01609-2280
Metaphor appears in essentially all forms of written and verbal communication including literature and science, and it serves quite different purposes in different applications. The question of the suitability of using metaphor for teaching science has been considered, and strongly differing points of view have been taken. However, metaphor is much more common in teaching chemistry than might be imagined. Therefore, there is a need to examine the ways in which metaphor is used in chemistry teaching, and it is appropriate to question the effectiveness of metaphor as a teaching tool. To see how metaphor works I will begin by examining its range of uses before discussing its specific relevance to chemistry. The Metaphor in General Use Metaphors are commonly used in literature and in the teaching of science, but for very different purposes. In literature they create desired poetic effects by evoking new or discordant feelings about a subject. In science teaching, particularly in chemistry, metaphors are used to help transfer knowledge about something that is abstract or unfamiliar by taking advantage of the knowledge and experience that the learner already has acquired (1). I will consider briefly how metaphor works in literature as a basis for considering how it works in the teaching of chemistry. Metaphor is often discussed along with analogy, simile, and allegory. An analogy is a statement that two things bear the same relationship to each other (2). In general terms, a metaphor is a statement that two things are the same when in fact they are not, a simile is a statement that one thing is like another, and an allegory is an extended metaphor. An example of a metaphor is, The yellow smoke that rubs its muzzle on the windowpanes, Licked its tongue into the corners of the evening. T. S. ELIOT (3)
This metaphor implies that the yellow smoke is an animal, presumably, a dog. Note that in this metaphor the connection between the dog and the smoke is not stated specifically, and the reader is left to complete the connection. This is the common situation when metaphor is used. A simile states that two things are alike; for example, My luve is like a red, red rose that’s newly sprung in June. R. BURNS (4)
Since the metaphor implies that one thing is another thing, it would appear to suggest a closer link than a simile, provided the reader makes the connection. A simile says that one subject has similarities to another, not it is the same.
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An example of an allegory or extended metaphor is a narrative such as Animal Farm (5), in which the animals on the farm are actually people living in a totalitarian state. Metaphors or similes are used frequently for teaching science, whereas the extended nature of the allegory makes it less appropriate in this setting. Every metaphor or simile has a topic and a vehicle (6 ), and the topic is explained by likening the vehicle to it. Thus smoke is the topic and a dog is the vehicle in the metaphor example above. Smoke and dogs, of course, actually have very little in common and the incompatibilities between the topic and the vehicle are known as tension (6 ). The tension is less in a simile than in a metaphor, since the simile says only that the topic and vehicle are alike. However, the metaphor and simile can be considered to be only slightly different forms of the same thing (7). The various attributes of the vehicle are called entailments, and the utility of a metaphor in teaching can be measured in terms of how many of these entailments are appropriate to the topic (8). In the example where the smoke is the topic and a dog is the vehicle, the entailments of being able to rub against windowpanes and lick into corners fit the topic reasonably well. However, other entailments of the vehicle such as barking and chasing cars do not fit the topic and would, in fact, confuse a person given this metaphor for learning about smoke for the first time. For those of us familiar with the topic, the lack of accuracy of some of the entailments is taken for granted, since without this tension the topic and the vehicle would be indistinguishable, and the statement would be uninteresting. In fact, the inappropriateness of some of the entailments enhances the overall poetic effect of the piece. In the simile given above, Burns leaves it to the reader to decide if the thorniness and ephemeral nature of the rose are entailments that fit the topic or not. While suitable for poetry, inappropriate entailments in a metaphor used for the transfer of knowledge are normally not useful and can lead to confusion or mistaken ideas. Metaphors and Teaching There is a very distinct difference between using a metaphor for poetic effect and using it for transferring knowledge and understanding. Eliot’s metaphor of the smoke rubbing and licking like a dog gives us a wonderful poetical feeling, but doesn’t really teach us anything about smoke because most of the entailments do not apply to smoke. The high degree of tension in the use of this metaphor improves it as a poetic device, since the poet is showing the reader how to see the world with a sharper eye. But as an educational metaphor it would fail, since the high degree of tension greatly limits the amount of knowledge that can be transferred.
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In the Classroom
Whether or not metaphor actually does help in the transfer of knowledge is a matter of debate, and views of pro (9), con (10), and sometimes (11) have all been expressed. A study on ninth and tenth grade biology students (12) showed no significant difference in performance between students taught with heavy use of analogies, metaphors, and similes and those taught more traditionally, except that those taught with analogies etc. incorporated such forms in their own written descriptions more frequently. A metaphor can be taken too literally and have more of its entailments accepted than should be. A classic case of this is the solar system metaphor used to describe the Bohr model of the atom, which was first presented in 1913 (13). This model of relatively tiny spheres revolving in circular orbits about a massive central object connected the cosmic with the atomic in a manner that for many has been too seductive to be resisted. Although the Bohr model never agreed with observed data for systems with more than one electron and was replaced in 12 years with the very different quantum mechanical model, this metaphor has lived on for more than 80 years, is an internationally recognized symbol of radioactivity and, for many, is the premier emblem of the mysterious world of science. For a metaphor to be effective as an instructional tool, it is essential that the vehicle and its entailments be familiar to the student and understood correctly by him or her. Thus the use of metaphor in teaching implies a constructivist viewpoint (14) where previously gained knowledge provides the base upon which new knowledge is built, and it is essential that the previously gained knowledge is sound. Metaphors in Chemistry Teaching Chemistry is a subject where what is happening at the microscopic level—atoms, molecules, etc.—is inferred, and is often explained at the everyday level of things we can picture. This would appear to be a fertile field for the use of metaphors, although there is clearly a problem, since the behavior of matter on the microscopic level, where the uncertainty principle and wave–particle duality are important, introduces complications not encountered in the macroscopic world. Concepts in biochemistry are very commonly understood using language as a metaphor. For example, a letter is the metaphor for a single amino acid residue in a protein; a word corresponds to the secondary protein structure; and so on, up to a complete book, which corresponds to the entire cell. This metaphor is so attractive that it colors thinking about these subjects and if carried too far can lead to erroneous impressions (15). We recognize that metaphors are used to help students learn some of the many microscopic concepts of chemistry that they often find difficult. In order that most of the students can use the metaphor for learning, we try to choose ones that are relatively familiar and commonplace. In chemical metaphors, as in the literary example given above, we usually infer the actual nature of the vehicle. But here is a very direct metaphor based on a common statement used to teach chemistry, and we can analyze it in terms of topic, vehicle, entailments, and tension. STATEMENT : An electron cloud surrounds the nucleus of an atom. (The metaphor very clearly states that the
electrons make up a cloud.) TOPIC: The electron distribution in an atom VEHICLE: A cloud in the sky ENTAILMENTS : 1. A cloud is diffuse and spread out thinly. 2. A cloud is hazy, with boundaries that are not well defined. 3. A cloud is usually white but is sometimes black. 4. A cloud drifts with the wind. 5. A cloud can produce rain or snow. TENSION: 3–5. The distribution of electrons does not have a color; it is not subject to macroscopic outside forces such as wind; it cannot condense. (Entailments 1 and 2 do provide an appropriate image of an orbital.)
The number of metaphors with directly stated vehicles such as the one above is quite limited in chemistry teaching as in other areas. There are certain metaphors or metaphorical models (16 ) that are easily recognized in chemistry teaching. These include the ball-and-spring model to describe molecular structure, and a tiny bar magnet to describe the magnetic properties of a nucleus or an electron. But there are many metaphors that we use in our teaching almost unconsciously and without considering how these will be taken by the students. We should evaluate whether the students will sort out the appropriate metaphorical entailments from those that produce tensions. An example of a metaphor in which the vehicle is not directly stated is, STATEMENT: Heat flows from a hotter object to a colder one. TOPIC: Heat VEHICLE: A fluid
In this metaphor we do not directly say that heat is a fluid. However, we use the word “flow”, and this is a concept that the students have never encountered, except involving fluids, until taking up the study of heat. This metaphor is probably rooted in the earlier notion that heat (or caloric) was, in fact, a material fluid. ENTAILMENTS : 1. A fluid can move freely and smoothly from place to place. 2. A fluid moves from a higher situation to a lower one. 3. A fluid is a substance that has mass and occupies volume. 4. A fluid can be collected in a suitable container until the container is full. 5. The movement of a fluid out of a container can be stopped completely as by turning a valve. TENSION: 3–5. Heat is not a substance and has neither mass nor volume; heat cannot be collected, since it is only energy in transition; the movement of heat can never be completely stopped in the sense that it is possible to stop the flow of water.
We should be concerned about whether the tension involved in this metaphor inhibits students’ understanding of heat. Here are a number of examples of common statements in chemistry education and their implied metaphors, which can be considered in terms of entailments, tension, and
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effectiveness.: STATEMENT: Gas molecules bounce off each other and the walls of their container. M ETAPHOR: Gas molecules are round balls. STATEMENT: A chemical reaction is depicted by an equation. M ETAPHOR: A reaction is a mathematical equality. STATEMENT: The reaction is reversible. M ETAPHOR: The reaction is a vehicle. STATEMENT: Metals have structures of closest-packed spheres. M ETAPHOR: Metal atoms are hard spheres. STATEMENT : The electron can spin around its axis in either direction. M ETAPHOR: The electron is a top. STATEMENT: A lead–acid battery is capable of storage. M ETAPHOR: A lead–acid battery is a container. STATEMENT : The activation energy is a barrier to the change of reactants into products. M ETAPHOR: Activation energy is a roadblock. STATEMENT: A radioactive nucleus decays to the nucleus of another atom. M ETAPHOR: The nucleus is a dead plant or animal.
Literature Cited 1. Ortony, A. In Metaphor and Thought; Ortony, A., Ed.; Cambridge University Press: Cambridge, 1979; pp 1-16. 2. Sternberg, R.; Rifkin, B. J. Exp. Child Psychol. 1979, 27, 195– 232. 3. Eliot, T. S. The Love Song of J. Alfred Prufrock (1917); In Collected Poems; Harcourt, Brace and World: New York, 1930. 4. Burns, R. My Luve is Like a Red, Red Rose (1796); In Life and Works of Robert Burns; Chalmers, R., Ed.; W. and R. Chalmers: Edinburgh and London, 1856. 5. Orwell, G. Animal Farm; New American Library: New York, 1946. 6. Richards, I. A. In The Philosophy of Rhetoric; Richards, I. A., Ed.; Oxford University Press: London, 1936; pp 98–112. 7. Black, M. In Metaphor and Thought; Ortony, A., Ed.; Cambridge University Press: Cambridge, 1979; pp 19–43. 8. Lakoff, G.; Johnson, M. Metaphors We Live By; University of Chicago Press: Chicago, 1980. 9. Petrie, H. G. In Metaphor and Thought; Ortony, A., Ed.; Cambridge University Press: Cambridge, 1979; pp 438–461. 10. Green, T. F. Ibid.; pp 462–473. 11. Boyd, R. Ibid.; pp 356–408. 12. Gilbert, S. W. J. Res. Sci. Teach. 1989, 26, 315–327. 13. Bohr, N. Philos. Mag. 1913, 26, 476–502, 857–875. 14. Ritchie, S. M.; Cook, J. Int. J. Sci. Educ. 1994, 16, 293–303. 15. Gribskov, M. Computers Chem. 1992, 16, 85–88. 16. Brushan, N.; Rosenfeld, S. J. Chem. Educ. 1995, 72, 579–582.
STATEMENT: Le Châtelier’s principle describes the effect of stress on a process. M ETAPHOR : A process is a physical object that can be subjected to a force.
In summary, (i) the metaphor is considered to be a powerful language tool, (ii) there is a risk of confusion in the use of a metaphor for teaching if it is taken too literally, and (iii) common expressions used in chemistry teaching employ far more metaphors than we might expect. We should remain conscious of our use of metaphors and consider how they will be interpreted by our students.
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