Can Elementary Chemistry Teaching Be Logical? ANTHONY STANDEN S t . John's College, Annapolis, Maryland
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T IS generally agreed that part, a t least, of the object of scient~fic . . courses is to teach the scientific method, and that this method consists in learning from experiment. The little historical essay at the beginning of a science textbook often says that, in the Middle Aces, learned men believed things on the authority of &istot1e, hut that the student as Xientists do, and learn Truth from Experiment only, giving no further credence to argument from Authority. rt is extremely difficult to teach elementary chemistry in the light of these precepts. The difficulty arises from the very advanced state of modern science, in which our theories denend uDon a vast mass of interrelated experimental evidence. In teaching some simthings in physics-laws of motion, for example, or falling bodies-a few experiments can be carried out, and other evidence cited, and a t the same time the theory can he Dresented and it can be shown that the theor; account's for the facts, as a good theory should. Modern theories of the atom cannot be presented to the elementary student in this manner, for the experimental evidence is far beyond him. Unfortunately i t is con. sidered necessary to present some of these theories to the heginning student for the sake of their utility. or example, valence is always explained in terms of electrons. But unless great care is taken to explain to the student that there are valid experimental reasons for believing in electron shells, octets, K-levels and so on, and that he will learn them all later if be sticks to physical science, then he is believing in them purely on Authority, and has been given no conception of the scientific method. This difficulty is reflected in elementary chemistry textbooks. Many of them are a hodge-podge of faulty reasoning and unscientific presentation. This assertion can be best illustrated by a close examination of just one textbook, a popular college text similar to those illuson general chemistry. trated below, perhaps even the same faults, can easily he found in many other textbooks. At the end of Chapter 1 we have: "Scientific Reasoning. Many illustrations of sound reasoning will follow later but here is an interesting example of faulty reasoning. ,,Van Helmont a willow in a weighed quantity of dry earth, supplied it with water only, and at the end of five yean found that it had gained 164 pounds in weight, while the earth had lost only 2 ounces. Here was ingenious proof that practically all the uew substance of the willow was madc of water-onvincing proof-onti! 100 years later Ingenhousz and Priestley showed that plants carbon from carbon dioxide it, the air.*r
There are, of course, many illustrations of sound rea-
soning in the book. But there are also illustrations of faulty reasoning, and it would be a clever and attentive student who would distinguish them. For example, in Chapter 3 a diagram of a cathode ray tube is given and we read: wires at A and support the anode and cathode, With any gas at l/lmo of an atmosphere or less, and with any conducting cathode, peculiar radiations called 'cathode rays' emanate from the cathode with the velocity of light and cause a greenish fluorescence where they hit the gbss wall of the tube. . . . When a cathode tube is placed between two metal plates, one a heavy positive charge and the other a heavy negative charge, the rays are deflected towards the positive plate. Crookes surmised and 1. -1. Thomson later roved 118971 , , that the cathode '.?Ys are a stream of negatively charged particles. And since these sameparticlesare throw11 offfrom thecathode (or thegas?), no matter what the cathode or gas, they must be constituent parts of atoms of all elements,u
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The reasoning in the last sentence is just as false as Van Helmont's. Why do the electrons have to be constituent parts of the doms? Couldn't a piece of platinum, or of any other substance, be like a sponge, impregnated with electrons, which would occupy some of the interstices between the atoms? Alternatively, another objection is this: the tube diagrammed will give no rays emanating from the cathode with the velocity of light, and there will be no greenish fluorescence~if A and are not connected to a battery, or an electrical generator of mme description. Why do we have to say, then, that the negatively charged particles come from the cathode (or the gas)? .Couldn't they be supplied by the electrical generator? Later in the same chapter we have: "The radio has made everybody familiar with electrons." No, it hasn't. I t has made everybody familiar with radios. I t has made everybody familiar with talk about elect r o n ~ ,just as the atomic bomb has made everybody familiar with talk about atoms. A student who bas worked with radios, and understands something about them, knows something about electricity, but for all the talk he has heard about electrons he knows nothing about them (unless perchance he is familiar with something like Millikan's oil-drop experiment), because electrons are not needed for an elementary explanation of radio, ~ 1 that 1 the student knows about radios can be explained just as well without assuming the discontinuOUS nature of electricity: thus, when the filament is heated electricity is "boiled" out of it and is impelled toward the positively charged plate, but its velocity will be modified by the variable charge on grid, Electrons are more necessary to explain this than molecules are to explain why water, thrown out of a bucket, falls to the ground.
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Other departures from scientific method come in later. In Chapter 6 the author introduces the kinetic theory of gases. He describes Boyle's law, Charles' law, Dalton's law of partial pressures and Graham's law of diffusion, and then gives the kinetic theory. "Since all gases react alike to changes in pressure and temperature, they must all have the same structure." Must they? Does this really follow? The graduate student who would argue thus in a research problem might soon find himself in a terrible mess. A few pages later we have a summary of the reasons for believing in the kinetic theory. Two columns are drawn up, the left-hand one for the phenomena-nine of them-and opposite each its explanation on the kinetic theory. This is an excellent example of presentation of a scientific theory; unfortunately, i t is headed, "Gases must be composed of flying molekules because such a kinetic theory explains the observed facts." This is extremely unscientific,for i t is a non seqwitur. It would be scientific to write, "We have good reason to suppose that gases are composed of flying molecules, because such a theory explains the facts," or in this case one could be more positive and write, "Gases must he composed of flying molecules because only such a theory can explain the facts." If we have just a theory that explains the facts, we may adopt this theory as a working hypothesis, but i t is not proved. We accept the kinetic theory because i t explains such a vast array of facts, and explains them so well, with excellent quantitative agreements, that i t is in the highest degree unlikely that anyone will ever explain them by any diierent theory. If the reason for our confidence in modern theories is not thus explained, a t least somewhere in the book, then the hook does not illustrate scientific method. But the most difficult problem confronting the teacher of elementary chemistry is the introduction of the atomic theory, and the explanation of the determination of atomic weights. This problem is exceptionally d&cult, more diicult probably than any problem confronting a teacher of any other science, on account of the cyclic way in which the atomic weights were, in historical fact, discovered. It took 50 years to agree that water is H1O and not HO. The first real confirmation that the atomic weights had been properly selected came from the Periodic Table, but this could not have been set up until a t least the majority of the atomic weights had been selected correctly. (I speak here not of thedetermination of anequivalent weight, but of the selection of an atomic weight, from among the multiples of the equivalent weights.) Modern discoveries, such as atomic numbers, positive rays, etc., have abundantly confirmed the selection of atomic weights used by Mendeleeff, in all cases, hut these discoveries could not possibly have been made if a mass of information from electrolysis, spectnun analysis, radioactivity, etc.-all dependent upon the knowledge of the atomic weightshad not been available previously. Chemistry may be said to have pulled itself up by its boot-straps, kicked a table underneath itself (the Periodic Table, of course), and then stood on it. We have
confidence that the table forms a sound support; but to explain i t to a student, and a t the same time preserve a t least the appearance of scientificmethod, is a formidable task. Here is how most elementary chemistry books say that atomic weights are determined. Avogadro's hypothesis is stated; sometimes reasons are given for believing it, sometimes not. Believing in the hypothesis implicitly, without a shadow of scientific doubt, it is shown that there must be two atoms of hydrogen in the molecule, and similarly for oxygen. This gives 0 = 16, and not 8. Cannizzaro's ideas are then explained, and are used to give C = 12, and a few other elements that have readily available gaseous compounds. The metals do not come in here, because carbonyls, organo-metallic compounds, and the like are deemed too exciting for an elementary text. From the few elements that can be settled in this way, several of which are gases, the student is supposed to believe that Dulong and Petit's law can he set up, and this in spite of the fact that carbon, boron, and silicon are exceptions (at ordinary temperatures). Dulong and Petit's constant is traditionally given as 6.4, but the examples given may run as low as 6.0 or even lower. From then on, of course, it is plain sailing, the atomic weight of any element can be selected, either from vapor density determinations on a number of volatile compounds, or from a specific heat determination. The Periodic Table c o n k s it, all the modern stuff confirms i t a hundred times again, and that is that. What if the student should ask, "How could it be first determined that the alkali metals are monovalent and the alkaline eaxths divalent?" They might be the other way round, or both divalent, or both monovalent. Each of these possibilities would knock Dulong and Petit's law into a cocked hat, especially since all the other metals are subject to a similar uncertainty. The truth is that the original selection of the atomic weights was a long and diicult process of trial and error, which is now fully understood only by a few specialists in the history of chemistry. It is not necessary, of course, to give students anything like a full account of this process, but an account which does not mention isomor$hism is not even logical. The textbook here being considered deals with these matters as follows. Chapter 1 gives basic principles, Chapter 2 gives combining proportions, and then Chapter 3 takes up the atomic theory. It first gives the basic assumptions made by Dalton-including those now known to be w r o n g a n d then shows how they explain conservation of mass, definite proportipus, and multiple proportions (reciprocal proportions were glossed over in Chapter 2). It then jumps straight into electrons, protons, octets, covalence and all, and is followed by a chapter on symbols, formulas, and equations. Getting hack to the actual phenomena of chemistry, Chapter 5 deals with oxygen. Afterwards we have kinetic theory, hydrogen, and "method of solving problems." All the time there is a promise, from the end of Chapter 2, that "in an early chapter detailed
methods will be given for the determination of relative weights of atoms," but we have not found them yet, although we are balancing equations and even solving problems. Atomic weight determinations are finally taken up in Chapter 11. But before this we have Chapter 8, on valence. It begins, "Valence is most easily understood by reasoning from experiment." The author seems to have remembered that science is, after all, based upon reasoning from experiment, and to have decided to let the students do some. The experiment considered is this: You look up in the book the atomic weights of, say, sodium, magnesium, and aluminum, and weigh out, respectively, 23, 24, and 27 mg. of the metals. You introduce them under inverted tubes of dilute hydrochloric acid, and note that the volumes of hydrogen given are in the ratio, 1, 2, 3. You have therefore found, by experiment, the valences of sodium, magnesium, and aluminum. Now what, in Heaven's name, is supposed to be the position of the person making this experiment? If he knows the atomic weight by experiment, then he must know the valence, for the two pieces of information come together. If he does not know the atomic weight, then he cannot do the experiment. If he is allowed to look up the atomic weight, but not the valence, then he must obtain a very strange idea of "reasoning from experiment." It is true, the author only says that valence is "most easily understood" by reasoning from this experiment. But since, in this book, we have already had formulas such as HzO, MgO, AlzOs, HCI, CaC12, NHa, and CHI there should be little difficulty in explaining the concept of valence in words. It is perhaps true that the concept of valence is easily explained to the student by this demonstration, but there must be many students who mistake i t for an experimental way of finding the valence of an element, especially since the author himself, two pages later, says, "We have just learned by experiment that the valence of aluminum is three." This experiment depends upon a knowledge of the atomic weight. And how is the atomic weight found? By looking i t up in the book. When we reach Chapter 11, the valences of various elements have been determined (or a t least understood) by experiment, and the atomic weight has been looked up in the book numerous times. The student has also had crystal lattices, diagrams of electron orbits, electronic formulas, hydrogen bridges, atomic radii, and very much else that depends entirely upon knowledge of atomic weights. If he still has any interest in the original process by which atomic weights were once found out, he gets i t in this chapter, which is devoted to a lot of ancient history, with dates, the latest of which is 1858 (Cannizzaro). At the end of i t we "deduce the formula of water." Earlier in the book we were reasoning in this manner: In Chapter 4, "H20 as a formula for water tells us that two atoms of hydrogen and one atom of oxygen make up a single molecule of water"; in Chapter 8, "Oxygen, as shown by the formula HzO, must have a valence of two, since one atom holds two
atoms of monovalent hydrogen." Already the famous formula of water, the well-known HzO, has been telling us things, and showing us things. I do not see why anyone should retain any interest in "deducing" that hoary old chestnut, HzO. The general impression of the first chapters is of a breathless rushing backwards and forwards between ancient history and the very latest electronic "dope" on the atom. This is caused by scrambling together two very different objectives which the course is supposed t o achieve: (1) to inform the student, pontifically ex cathedra, what are the principal findings of modern science; (2) to give the student some idea of the scientific method of reasoning from experiment, by showing how we can learn, first of the existence, and later of the structure, of minute invisible particles called atoms. The confusion is increased because i t is deemed necessary to pep the book up constantly with practical applications. For example, hydrogen no sooner comes in than we must have the atomic hydrogen flame. The result is that the book never stays for two pages together a t any one level of science. The student never knows what is coming to him, for he is kept hopping about between history of chemistry, marvels of modern science, balancing equations, solving problems, and latest applications. Yet another confusion is latent in this treatment. Why history of chemistry a t all? What precisely is intended to be achieved by all this historical stuff? Here again there are two objectives, scrambled. To a certain extent, the history of chemistry is an end in itself, for a chemistry course, whether for science majors or liberal arts students, would be lacking in something if i t did not help the student to place the development of chemistry in some sort of historical perspective. The other objective is much more important; it is nothing less than illustrating the scientific method itself. But this requires, not so much the historical order, but the logical order, or a t least a logical order. The two are not the same. They happen to coincide very closely (for very clear reasons, of course) but for a logical order there is no objection to discovering atomic weights by a method which was not, in point of historical fact, the method used by Berzelius. But i t must be a logically possible method, it must not be a method which could only be possible after the atomic weights were already known. The mass spectrograph probably cannot be used, in the logical order, for justifying the selection of atomic weights, for in the course of the explanation it would be necessary to say "positively charged atoms are formed and are impelled toward the cathode." How could i t be shown logically that i t really is positively charged atoms that are formed, and not something else, without drawing upon the whole mass of chemical and physical knowledge? Perhaps someone can do this, honestly, and without begging any questions. I would be most interested to see it done. Before giving some constructive suggestions, one more criticism can be made of almost all elementary chemistry books-and physics books too, for that mat-
ter. They state, or imply, that a measurement of the size of atoms is evidence of the existence of atoms. This is not true. The fact that measurements of the size of the atom made by methods which diier widely in principle give very closely the same result is, of course, excellent confirmation of the atomic theory. But an alleged measurement of something is no proof whatever that that something exists. Any scientist engaged in active research should know that i t is possible to believe that you have measured what you wanted to measure, and yet to find out later that your apparatus has actually measured something else. And the falsity of the argument, "I havemeasured, therefore I know," can he illustrated by the following two examples. (1) There are some who say that they have measured intelligence. I believe in intelligence, hut my belief in the existtnce of intelligence is in no way incrkascd bv the alleeed measnrement of the s3me. (2) Suppose that one could converse with Ptolemy, or Copernicus, and dispute with them concerning the reality of their astronomical epicycles. Both these scientists would he in a position to say, "But I have measured epicycles." And yet belief in epicycles is deader than the dodo. What shall we do, then, to keep as much logic as possible in elementary chemistry teaching? The solution must depend very greatly upon the objectives of the course. If it is a "quickie," that is one thing, but if it is intended seriously, that is quite another. A course may be designed to turn out competent operators in chemistry, a t any level from the routine technician to the Ph.D., in the minimum of time. Such a course may pay no attention to the historical order, and little, or perhaps none, to the logical order. Such students could be given completely modern conceptions of the atom from the very beginning. The first sentence in the book could almost be, "A hydrogen atom consists of one proton.and one electron," and Chapter 2 could give watered-down wave mechanics. There would be no need to pay lip-service to the old law of multiple proportions--no need even to mention it. There would he no need to bother with what Cannizzaro said in 1858. Why bring in any of this grubby ancient history? Students would be taught to solve prohlems-plenty of t h e m a n d then every problem solved would be a confirmation of the theories. A course designed for serious teaching of chemisfry, either for science majors or for liberal arts students, faces much graver problems. I can make two alternative suggestions. (1) Arrange the course as much as
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possible in a logical order, with perhaps a very few anticipations, clearly labeled as such. Or else (2) start with the full modern apparatus of electrons, octets, covalence and so forth, and use nothing else, until the student has some familiarity with atomic weights, equivalent weights, formulas, balancing equations, solving problems, acids, bases, and salts, oxidation and reduction, and all such regular stuff. Then, and only then, have a chapter which might begin: "We can nno ask the question, 'How could all this mass of information, which deals with invisible particles such as atoms and electrons, ever have been discovered in the first place?' For the time being, forget all the theory that you have learned, and let us begin again a t the beginning, that is to say, with the phenomena." Such a course would spare the beginning student the present razzle-dazzle technique of leaping from the cyclotron to Lavoisier and l'rirstlcv. then to the classical billiardball atom and then to ;he early 19th century. The first suggestion-logical order as far as possible-requires a few further comments on account of its inherent difficulties. It would roughly parallel the historical order, but would not be bound to it. It would not be necessary to review the early 19th-century steps in selecting atomic weights. The problem should he stated, a few guiding principles could be given, and then we could go a t once to the Periodic Table, which confirms all the selections. Valence would have t o be explained and understood first according to preelectronic ideas. This would be a great advantage, for it would force students to he clear about what valence is before learning the modern electronic theories which, to a large degree, account for it. Ionization would probably have tq be explained first in the terms of Arrhenius and van't Hoff, unless someone can think of a truly logical way of introducing the electron a t this point. When well on in the course, modern theories could he explained and used, with the very clear statement that the student will not understand the full experimental reasons for believing in them unless he will go out and take a stiff course in physics. I n one way such a course in elementary chemistsy would be difficult, for it would make students think rather than memorize. But i t would in another way he easier than present courses, for it would he doing only one thing a t one time. It would not he pumping the student full of complicated modem theories and simultaneously making half-hearted attempts to teach him the history of these theories. At any rate it would be logical. And there is no reason why students should not like i t on account of the resulting clarity.
The 8 - 2 9 Superfortress uses e m u g h gasoline in one hour to take core of the average civil& for five and one-half years.
