- A New Road to Reactions Part 4. The Substance and Its Molecules Wobbe d e Vos and Adri H. Verdonk Utrecht University, P.O. Box 80.083. Utrecht, The Netherlands A student in elementary chemistry is confronted not only with several new and remarkable substances hut also with a new way of looking a t substances. In everyday life, substances are normally considered from the point of view of their significance to man and society, either in a positive or in a neeative sense: when we mention detergents. dyes. fuels, " insecticides, poisons, pollutants, or vitamins, we are generally thinking about what these suhstances mean or do to us. In the chemicalnomenclature on the other hand, the name of a substance, such as cyclohexane or carbon dioxide, refers to its composition in terms of elements or atoms. This does not mean that chemists do not care what a substance can do to man or to the environment, but i t shows that they are able to discuss it in a detached and disciplined manner. It is typical in everyday life that oily one or two properties of a substance are particularly important, for example, aspirin cures a headache, petrol is combustible, and chili pepper is hot. We tend to ignore other properties of these substances. In chemistrv. each substance is described in terms of a list of standard Goperties: density, melting and boiling noints. refraction index. solubilitv. " . etc. There are standard brocednres, founded on these properties, for identifying and comparing substances. Chemists have divided their substances into groups and subgroups, using properties and structure as criteria, thereby creating a hierarchy based on the periodic table and on the carbon chain. On the basis of these categories they are able to predict and prepare or discover new substances. Thus in a chemical hierarchy, each substance is determined by its properties and defined by its formula. This chemical substance concept lets a chemical reaction be defined as a process in which substances are converted into other substances. A similar, well-defined, substance concept does not exist in the language of everyday life. Words like diamond, glass, stone, and tin in everyday life refer to objects just as much as to substances. In a list of ingredients in a soft drink, orange juice, or other mixtures, are mentioned along with a single substance like glucose. Salt is distinguished from sugar not onlv bv the taste but also bv the size of the grains. Iron is recognized by the shape of nails, even if the nails are coated with zinc and no iron is visible. Such examples show that the substance concept used in everyday life isvery diffuse. Students in elementary chemistry, after heating a piece of copper sheet in a Bunsen flame, do not say that a new, black substance has formed on the surface of the copper. They simply say that the copper has turned black. Teaching the chemical reaction concept therefore implies teaching a more disciplined way of using substance names. There is, of course, no such thing as black copper, but there is copper with a thin layer of a newly formed black substance on its surface. Student Logic While attempting to develop a strategy for teaching the chemical substance concept, we werestruck bythe dominant role played by corpuscles (molecules, atoms, etc.) in the language of chemists. A sodium chloride crystal is described 692
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by a chemist almost automatically as an arrangement of Naf and C1- ions. Systematic substance names, especially in organic chemistry, reflect numbers and arrangement of atoms in molecules. Even a trivial name like acetone is associated by chemists not only with the colorless liquid but also with the corresponding structural formula CH3COCH3. It is the chemist's knowledge of the corpuscular world and his frequent and routine use of this knowledge that contribute much to the creation of acommunication gap between chemists and nonchemists. Familiarity with the world of atoms and molecules. which is so indis~ensahlein ~rofessional chemistry, becomes an enormous obstacle as soon as a chemist tries to communicate his subject with a layman. This is exactly what happens in an elementary chemistry course. One wav of solvina this problem would seem to be to teach molecules and atoms at i n early stage in the course and exuect students to use this knowledge in the same way as chkmists do. This approach is adopted in many chemistry courses but the results are often felt to be disappointing. While students may he able to reproduce and answer direct questions about what they have learned about the structure i f atoms. manv are unable to inteerate this information with " the picture they have of chemical and physical processes. The information presented by the teacher is not experienced as a clarifying, structuring, and explanatory tool for understandine the behavior of substances. Instead, i t is regarded by students as just another burden on the brain. ~ n every d teacher knows how deceptive such isolated "knowledge" may he; a student, after correctly relating the valency of magnesium to its atomic structure mag add a spontaneous remark about the presence of air between the electrons and the nucleus. Correctly teaching corpuscular models does not guarantee that theywill be c&re&ly learned and utilized; somehow many students seem unable to go beyond the stage of rote learning with regard to corpuscular models. Another,possible solution is for the teacher to try to forget what he knows about the cornuscular world for a moment and teach substances and chemical reactions without referrine to molecules or atoms. But this means that the teacher is ohcged to choose his words very carefully. Thinking along corpuscular lines is deeply rooted in a chemist's mind, and one's first impression is that there is nothing wrong in saying that water is "broken down" into oxygen and hydrogen or that water is a simpler substance than alcohol. Although we do not reject this noncorpuscular approach to teaching the concepts of substance and chemical reaction, we have explored a third possibility: we tried to make use of the inevitably rather primitive and unscientific ideas that students themselves have about the corpuscular nature of matter. Of course, this approach requires knowledge of the student's own corpuscular ideas. We therefore collected the information we needed bv carefullv listening to 14- and 15yar-old >tudents in their firit experimental n u r s e on elementarv chemistrv. The lis~eninowas done ~ a r t .i vwith thr use of audiotape equipment, but notes were also made, and answers written bv the students were collected. During most of the lessons, students-working in small gronps-were engaged in practical work and in answering open-ended
questions. Many of the questions were designed to stimulate careful observation and consistent interpretation of results obtained in practical work. T h e purpose of the questions was also to encourage students to put their own personal views into words. The socially safe situation of small groups was one of the reasons whv we preferred group. work to class teaching. Our analvsis of a large amount of collected material yielded several eases in which students expressed views on the behavior of molecules that appeared to differ considerably from views normally held by scientists. Whereas results of previous education, such as in physics, could sometimes he recognized, other passages seemed to indicate a strong influence of students' own imagination and logic. From a much longer list we selected the following examples to illustrate this: (1) Several groups, after learning that in hot water the molecules move faster than in cold water, continued to talk about hot molecules and cold molecules as if these molecules were not only in motion but also had a temoerature of their own. One moun, . after a d~scuiaionwirh the tenrher,corrertrd thla YIP%\ by !%riling!ha1 w e n in hut water I he molecules a r t cold inside. (2) After an experiment on heat conduction by metals, a student explained this phenomenon by stating that eaeh metal molecule is a good beat conductor. (3) Several students expressed the view that in liquids like water and alcohol, the molecules cannot be solid objects hut must be tiny little dronlets. (4) Students know that objects expand on heating. A number of them ascribed this to the expansion of individual molecules. (5) In an experiment with candle wax some students concluded that molecules of a soft substance must themselves he soft: "A soft substance cannot be made up of hard molecules." (6) The transparency of glass, water, and air was explained by one group to be the result of the transparency of each individual molecule of these substances. Opaque substances were said to have opaque molecules. (7) In one of the experiments a small piece of camphor was weighed twice on accurate scales. The interval between the weighi n g ~was about 10 minutes. Students were asked whether they thought the observed loss in weight had something to do with the smell they had noticed. Students seemed to believe that each camphor molecule was surrounded by a spherical "smell area". One of them expressed the view that she could smell a molecule from a distance, when it simply passed under her nose. (8) In trying to explain the action of glue, at least one group agreed on the idea that eaeh glue molecule was covered with a very thin sticky layer. (9) After they had investigated rust formation an iron, students were asked to describe a rust molecule. Some students made the rust molecule consist of an iron particle and an oxygen particle, others drew a brown circle, labeling its interior "iron molecule" and its circumference "rust". (10) Several students agreed with the statement that in living creatures eaeh molecule is alive. Others adjusted this statement by excluding molecules of hairs and teeth from the set. ~~~
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The dev(4opment of the ideas described aht~vemay be a natural and una\,oidablr stare in the cognitive develqxnent of the child.'However, it ma; also have been encouraged and reinforced hv the introduction of molecules in some physics and general science textbooks where a molecule is described as the final result of a lengthy cleavage procedure. If a drop of water is split into two, one half is split again, etc., the end result, according to this argument, will be an invisibly small
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Piaget. J.: Inhelder, 6. The Child's Construction of Ouantities: Conservation and Atomism; Routledge and Kegan Paul: London, 1974..
De Vos, W.: Verdonk, A. H. J. Chem. Educ. 1985, 62, 232.
particle called a water molecule. Following this line of thought, a student can only conclude that such a molecule is nothing but an extremely small amount of water with all the characteristics of water except divisibility (and this exception may well he due to technical limitations). Such a molecule would of course have a freezing point, a boiling point, and a refraction index. Sugar would even be able to dissolve in it. The existence of the molecule would not provide any exnlanations for these and other . oroverties of water. I n a . scientific theory, a (corpuscular) model derives its value lareelv " " from both the explanations it helps to offer and from its relative simplicity. "The Same" but Dlflerent After collecting the student ideas on molecules we decided not to reject them straightaway as being unscientific or false hut to accept them as an authentic element of the learner's situation and to make use of them in teaching the substance and reaction concepts. This would of course require the simultaneous development of these primitive ideas into scientifically more acceptable models. As a first sten we chose to introduce the term molecular rprcicv as the corpuscular cuuntcrpurt of the substance concent. \Ve detlared that all mdeculrc of one suh,tnnw I~clonr to the same species while molecules of different substances belong to different species. Self-evident as this may seem, it deviated considerably from views held bv students. I t is true, most of them did classify ice molecules and water molecules as one species, and they also accepted that a sugar solution does not consist of so-called solution molecules but of sugar and water molecules. However, in the case of chemical reactions they tended to assign a reaction product molecule to the same species as a molecule of a reactant. Magnesium oxide, obtained by hurning some magnesium ribbon, was said to consist of magnesium molecules "in a burnt state". After the experiment in which two white powders react to form a yellow one2, one student remarked: "They are still the same molecules but thev are not the same." and this statement met with the approval of several of his classmates. Closer analvsis of such data showed that most students attribute a particular identity t u a mdecule and wppost. the molecule k e w i this itlt.ntits thrmghout chrmical rc.uctions. This identit; resembles the identity of a person: he or she may change on growing older hut he always remains the same person. One student said: "If I paint my bicycle red, it is not the same any more, but it is still the same bicycle, isn't it?" According to this view, which is common in everyday life, a molecule can go through many radical changes and yet retain its identity and belong to the original species. In the language of chemists, on the other hand, atoms maintain their identitv in a chemical reaction (thoueh in principle even that is doubtful statement), h u t mole>ules generally do not. This made us realize that either we had to come to an agreement with our students about the exact meaning of the phrase "the same" or we must introduce a new word. In a first attempt to solve thts problem we declared all molecules of one substance to he exactly alike in all respects and we used the word "identical" to refer to this equality. If all magnesium molecules are assumed to be identical, the slightest change in any one of them, such as caused by burninn, would turn it into a molecule of a different substance. T G white ~ powder obtained by burning a magnesium ribbon would then no longer consist of magnesium molecules. Students appeared to have difficulties with the idea of objects being absolutely identical. After some discussion they refused to believe that two coins or two other objects could really be identical. The students felt there must be differences, caused for instance by little scratches, "even if you cannot see them". How then could two molecules be identical? We realized that we had introduced a theoretical
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August 1987
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concept that had no direct empirical basis. Like the ideal gas concept it is something that has been invented, not somethinn that has been discovered, and it should not be taught as aself-evident consequence of experience. After all, two apples belonging to the same species, say golden delicious, may differ considerably in color, weight and shape, albeit within certain limits. Insisting that all molecules of one species are ahnolutelv alike seems to be carrvine thines a bit too far. (This made i s aware of the fact thaite&hoo!& describe in detail how Millikan determined the charee of some electrons without giving reasons for the bold assumption that all other electrons in the world have the same charge.) A second problem arose when some students refused to accept even the theoretical ~ossibilitvthat two ohiects are idetkical. The gist of their bbjection was that two objects always differ in one respect, thaf ii, their position in spare. Also theobjects might be rno~,ingindiiierrnta.a.vs. U'erould counter fhrst. obiections by defining irlrntical as "not differing from each other in anyway except position and motion." This being agreed upon, the intuitive notion of students that molecules-of hotwater and cold water belong to the same species can now be confirmed. When a crystal of potassium permanganate is dropped into a glass of cold water, it leaves behind a slightly winding trail of purple solution, which remains visible for a long time. In hot water a potassium permanganate crystal produces a much wider trail which fades away much sooner. The interpretation presented by some students and easily accepted by others is that molecules of hot water move faster. The crystal is said to "wear away" more in hot water than in cold water. As mentioned before, several students seemed to assign to each molecule a particular temperature, but this did not prevent them from classifying molecules of hot and cold water as one species. ~ x t e n d i n gthis line of thought, students did not find it difficult to interpret phase transitions as results of changes in molecular motion. They were then able to reserve the name chemical reaction for those processes that cannot be interpreted simply as changes in the position and motion of molecules. Later in the course, a third prohlem emerged. Students had accepted the fact that the black suhstance formed on a sheet of copper in a gas flame did not consist of copper molecules, and therefore they agreed that this substance deserved a new name. The name copper oxide was suggested and, after it had been explained, agreed upon. In a chapter on decomposition, students heated a pale green powder called malachite, basic copper carbonate, Cu(OH)*. CuC03, and noted the formation of a black substance that, on further investigation, turned out to resemble copper oxide in all its known properties. Nevertheless, students refused to call this substance copper oxide or its molecules copper oxide molecules, since, they said, these molecules differed from cowoer oxide molecules in the wav thev had been prepared. Ol;;iously these students had included the prepaiation method in the substance's characteristic orooerties. While they were quite willing to accept the fact thatthe new hlack substance behaved like copper oxide in every way, they insisted that copper oxide could only be prepared from copper and oxygen. In discussions, some students put forward the name copper oxide to support this view. In the following edition of our experimental course we replaced the name copper oxide from the outset by the mineralogical name of the suhstance, tenorite. We hoped that this name would not he associated with one specific
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preparation method. Our hope was not realized: our students almost unanimously refused to use the name tenorite to refer to the black decomposition product of malachite. We had to exclude the methodof preparation of a suhstance and other aspects of its history explicitly from the list of relevant characteristics, and we did this by redefining our concept yet again. (It is of course true that in many cases the preparation method or the thermal history of a sample of a substance determines for instance its catalytic properties, hut we considered this to be irrelevant at this elementarv level.) We now declared objects to be identical if they did not show any difference. excent or in motion. With this concent . in Dosition . our students could, in a number of cases, come to an agreement on suhstance names. It was generally accepted that tenorite can he prepared in a t least two different ways. The Substance Molecule Bridge Clearly our approach does not solve all problems. Whereas most students manage to remember the definition of identical molecules, many of them have difficulty in applying it. Some of their difficulties have to do with the hypothetical nature of the corpuscular world: many students feel uncertain when they are asked to draw conclusions about molecules on the basis of their observations during practical work. From Piaeet3 we know how difficult it is to areue logirtilly start iniirorn a hyputhesis. Ne\,ertheless, we l,el;ve we hund a track that lead\ 'tudents from thcir vrimiti\,e but authentic corpuscular ideas to a t least a basic understanding of the substance and reaction concepts in chemistry. It is relatively easy to introduce the chemical reaction in the first chemistry lessons as a spectacular and unexpected event. But this reaction concept does not help students very much in their subsequent chemical education. The key to the reaction concept is not the spectacular nature or the unexpectedness of the event but the fact that substances change into other substances. This requires an analysis of the situation before, during and after the event. I t also reauires a criterion for interoretine an observed chanee. - Bv. linking the concept of substance Lo a simple molecular species concept we made it possible for students to derive this criterion from their own Eorpuscular ideas. With this criterion, they can recognize the hlack layer on heated copper sheet as a black substance and its formation as a chemical reaction. A new and useful tool has become available. I t should be realized that the bridge between substance and molecule is still very weak and tenuous. Students have learned that different molecules means different substances, hut they still do not grasp which molecular features correspond to which substance properties. However, it is important that students be made aware of a relation between molecular features and suhstance properties, even if for the time being they believe that, since sulfur is yellow, each sulfur molecule must be vellow. A t this earlv . stage in chemical education we should i e satisfied if students can recognize and describe a chemical reaction. and we should not force upon r h m rnrm explanatory throry than they need t'or that purlwac Much later, imnt.ot'them ma\. lean1 that the sellrw color of sulfur can be explained by transitions of elekrons between energy levels. -
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Piaget. J.; Inhelder. B. ThePsychologyof the Child: Routledge and Kegan Paul: London. 1969.
