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In the Classroom

366 Journal of Chemical Education Vol. 76 No. 3 March 1999 JChemEd.chem.wisc.edu

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 andverbal communication including literature and science, andit serves quite different purposes in different applications. Thequestion of the suitability of using metaphor for teaching sci-ence has been considered, and strongly differing points of view have been taken. However, metaphor is much morecommon in teaching chemistry than might be imagined.Therefore, there is a need to examine the ways in which meta-phor is used in chemistry teaching, and it is appropriate toquestion the effectiveness of metaphor as a teaching tool. To

see how metaphor works I will begin by examining its rangeof uses before discussing its specific relevance to chemistry.

The Metaphor in General Use

Metaphors are commonly used in literature and in theteaching of science, but for very different purposes. In literaturethey create desired poetic effects by evoking new or discordantfeelings about a subject. In science teaching, particularly inchemistry, metaphors are used to help transfer knowledgeabout something that is abstract or unfamiliar by takingadvantage of the knowledge and experience that the learneralready has acquired ( 1 ). I will consider briefly how metaphorworks in literature as a basis for considering how it works inthe teaching of chemistry.Metaphor is often discussed along with analogy, simile,and allegory. An analogy is a statement that two things bearthe same relationship to each other ( 2 ). In general terms, ametaphor is a statement that two things are the same whenin fact they are not, a simile is a statement that one thing islike 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. E LIOT (3 )This metaphor implies that the yellow smoke is an

animal, presumably, a dog. Note that in this metaphor theconnection between the dog and the smoke is not statedspecifically, 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 thats 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 thatone subject has similarities to another, not it is the same.

An example of an allegory or extended metaphor is anarrative such as Animal Farm (5 ), in which the animals on thefarm are actually people living in a totalitarian state. Metaphorsor similes are used frequently for teaching science, whereasthe extended nature of the allegory makes it less appropriatein this setting.

Every metaphor or simile has a topic and a vehicle (6 ),and the topic is explained by likening the vehicle to it. Thussmoke is the topic and a dog is the vehicle in the metaphorexample above. Smoke and dogs, of course, actually have veryli tt le in common and the incompatibilities between the topicand the vehicle are known as tension (6 ). The tension is lessin a simile than in a metaphor, since the simile says only thatthe topic and vehicle are alike. However, the metaphor andsimile can be considered to be only slightly different formsof the same thing ( 7 ).

The various attributes of the vehicle are called entailments ,and the utility of a metaphor in teaching can be measured interms of how many of these entailments are appropriate tothe topic ( 8 ). In the example where the smoke is the topicand a dog is the vehicle, the entailments of being able torub against windowpanes and lick into corners fit the topicreasonably well. However, other entailments of the vehiclesuch as barking and chasing cars do not fit the topic andwould, in fact, confuse a person given this metaphor forlearning about smoke for the first time. For those of usfamiliar with the topic, the lack of accuracy of some of theentailments is taken for granted, since without this tensionthe topic and the vehicle would be indistinguishable, and thestatement would be uninteresting. In fact, the inappropriatenessof some of the entailments enhances the overall poetic effectof the piece. In the simile given above, Burns leaves it to thereader to decide if the thorniness and ephemeral nature of the rose are entailments that fit the topic or not. While suit-able for poetry, inappropriate entailments in a metaphor usedfor the transfer of knowledge are normally not useful andcan lead to confusion or mistaken ideas.

M etaphors and Teaching

There is a very distinct difference between using a meta-phor for poetic effect and using it for transferring knowledgeand understanding. Eliots metaphor of the smoke rubbingand licking like a dog gives us a wonderful poetical feeling,but doesnt really teach us anything about smoke because mostof the entailments do not apply to smoke. The high degreeof tension in the use of this metaphor improves it as a poeticdevice, since the poet is showing the reader how to see theworld with a sharper eye. But as an educational metaphor itwould fail, since the high degree of tension greatly limits theamount of knowledge that can be transferred.

*Email: hbeall@wpi.wpi.edu.

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Whether or not metaphor actually does help in the trans-fer of knowledge is a matter of debate, and views of pro ( 9 ),con ( 10 ), and sometimes ( 11 ) have all been expressed. Astudy on ninth and tenth grade biology students ( 12 ) showedno significant difference in performance between studentstaught with heavy use of analogies, metaphors, and similesand those taught more traditionally, except that those taughtwith analogies etc. incorporated such forms in their ownwritten descriptions more frequently.

A metaphor can be taken too literally and have more of its entailments accepted than should be. A classic case of thisis the solar system metaphor used to describe the Bohr modelof the atom, which was first presented in 1913 ( 13 ). Thismodel of relatively tiny spheres revolving in circular orbitsabout a massive central object connected the cosmic with theatomic in a manner that for many has been too seductive tobe resisted. Although the Bohr model never agreed with ob-served data for systems with more than one electron and wasreplaced in 12 years with the very different quantum me-chanical model, this metaphor has lived on for more than80 years, is an internationally recognized symbol of radioac-

tivity and, for many, is the premier emblem of the mysteri-ous world of science.For a metaphor to be effective as an instructional tool,

it is essential that the vehicle and its entailments be familiarto the student and understood correctly by him or her. Thusthe use of metaphor in teaching implies a constructivist view-point ( 14 ) where previously gained knowledge provides thebase upon which new knowledge is built, and it is essentialthat the previously gained knowledge is sound.

M etap hors in Chem istry Tea ching

Chemistry is a subject where what is happening at themicroscopic levelatoms, molecules, etc.is inferred, and isoften explained at the everyday level of things we can picture.This would appear to be a fertile field for the use of meta-phors, although there is clearly a problem, since the behav-ior of matter on the microscopic level, where the uncertaintyprinciple and waveparticle duality are important, introducescomplications not encountered in the macroscopic world.

Concepts in biochemistry are very commonly under-stood using language as a metaphor. For example, a letter isthe metaphor for a single amino acid residue in a protein; aword corresponds to the secondary protein structure; and soon, up to a complete book, which corresponds to the entirecell. This metaphor is so attractive that it colors thinkingabout these subjects and if carried too far can lead to erroneous

impressions ( 15 ).We recognize that metaphors are used to help studentslearn some of the many microscopic concepts of chemistrythat they often find difficult. In order that most of the studentscan use the metaphor for learning, we try to choose ones thatare relatively familiar and commonplace. In chemical metaphors,as in the literary example given above, we usually infer theactual nature of the vehicle. But here is a very direct metaphorbased on a common statement used to teach chemistry, andwe can analyze it in terms of topic, vehicle, entailments, andtension.

STATEMENT : An electron cloud surrounds the nucleusof an atom. (The metaphor very clearly states that the

electrons make up a cloud.)TOPIC : The electron distribution in an atomVEHICLE: A cloud in the skyENTAILMENTS:

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 i s sometimes black.4. A cloud drift s with the wind.5. A cloud can produce rain or snow.

TENSION : 35. The distribution of electrons does not havea color; it is not subject to macroscopic outside forcessuch as wind; it cannot condense. (Entailments 1 and 2do provide an appropriate image of an orbital.)

The number of metaphors with directly stated vehiclessuch as the one above is quite limited in chemistry teachingas in other areas.

There are certain metaphors or metaphorical models(16 ) that are easily recognized in chemistry teaching. Theseinclude the ball-and-spring model to describe molecularstructure, and a tiny bar magnet to describe the magneticproperties of a nucleus or an electron. But there are manymetaphors that we use in our teaching almost unconsciouslyand without considering how these will be taken by thestudents. We should evaluate whether the students will sortout the appropriate metaphorical entailments from those thatproduce tensions. An example of a metaphor in which thevehicle is not directly stated is,

STATEMENT : Heat flows from a hotter object to a colder one.TOPIC : H eatVEHICLE: A fluidIn this metaphor we do not directly say that heat is a

fluid. However, we use the word flow, and this is a conceptthat the students have never encountered, except involvingfluids, until taking up the study of heat. This metaphor isprobably 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 : 35. H eat is not a substance and has neithermass nor volume; heat cannot be collected, since it is onlyenergy in transition; the movement of heat can never becompletely stopped in the sense that it is possible to stopthe flow of water.We should be concerned about whether the tension

involved in this metaphor inhibits students understandingof heat.

Here are a number of examples of common statementsin chemistry education and their implied metaphors, whichcan be considered in terms of entailments, tension, and

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