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IIIIIH-SCHOOL C H E M I S T R Y
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A Plea for the Plaintiff CHARLES H. STONE Vermont Junior College, Montpelier, Vermont
INCE every statement of chemical fact that appears m our preparatory school texts had its origin in an experiment by somebody, sometime, somewhere, it seems strange that the compilers of these texts offer so many statements of fact without supporting evidence. Is this the scientific method? It is true that many of the theoretical statements are not easily verified experimentally. You cannot prove the truth of Dalton's hypothesis, of Avogadro's principle, of the Bronsted theory; you cannot demonstrate the structure of an atom, nor visibly exhibit the movement of electrons along a copper wire. But there are so many statements of fact that are easily demonstrable that every reason exists for placing this evidence before the student. We offer the following: 1. The statement is made in our texts that manganese dioxide is a catalyst when used with potassium chlorate in the preparation of oxygen; often an experiment in the laboratory manual covers this topic. But as Johnny performs the experiment is there proof that catalytic action really occurs? In order to prove it, he must fist show that the chlorate alone will yield oxygen if heated sufficiently; that, when the exterior heat is removed, the chlorate presently cools to a point where the evolution of oxygen ceases; that manganese dioxide added then stimulates the evolution of oxygen to a point exceeding that when the chlorate alone is used, and obviously the temperature is lower than that of the first decomposition. Few of our textbooks or manuals bring out this sequence. The student is expected to believe the statement because the book says so. This is hardly sound pedagogy. 2. Some textbooks say that hydrogen peroxide is prepared by the reaction between barium peroxide and a dilute mineral acid. Where is the experimental evidence to demonstrate this point? Any teacher can sift barium peroxide into dilute hydrochloric acid until there is no further reaction, filter off the residue, test the acid filtrate with potassium dichromate solution, and observe the blue color which results. The test may be confirmed by applying i t to a solution known to contain hydrogen peroxide. 3. The textbook says that lime, CaO, is obtained by heating limestone, an impure calcium carbonate, CaC08; but neither the text nor the manual offers experimental proof.. Any teacher can, of course, prove this point by first dropping a bit of marble chip into an alcoholic solution of phenolphthalein; no reaction results. Another bit of marble is strongly heated by holding i t by its edge with the tongs in the tip of the Bunsen flame for a t least one minute, and then drop-
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ping it into the phenolphthalein solution; a red color results. .We have then this succession: CaCOa
- CaO
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Ca(OH)% hydroxyl ion
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color test
4. The text says that the gas masks provided soldiers contain activated charcoal and other substances that have the power to adsorb gases. But where is the experimental evidence of the adsorptive power of charcoal? Any teacher can show that a small cylinder of wood-charcoal, cut to slide easily up and down inside a test tube, will adsorb large amounts of ammonia gas. Boil some moderately strong ammonium hydroxide in a test tube, pour out the liquid, keep the tube mouth down and slide up into it the charcoal cylinder, and stand the tube in a dish of mercury. The steady rise of the mercury shows that the gas is being adsorbed, and the rise is greater than can be accounted for by the cooling and consequent shrinkage in volume of the gas. For proof, remove the charcoal and let some student test the odor of it; i t is very strongly ammaniacal. Teachers need a good list of simple but convincing experiments for demonstrations. Such a list should not be merely a repetition of experiments set forth in the laboratory manual. Many such experiments are available. Because teachers are not always able to devise suitable desk demonstrations and because full directions are not contained in most of our textbooks, many teachers do little with these demonstrations. We have another charge against the compilers of our chemistry texts. They dwell upon the isolated instance with some detail, but frequently do not enunciate the general principle. Priestley's experiment with mercuric oxide is cited as an example. This is a case of the decomposition of a simple oxide by means of heat. But where is the statement of the general principle that the simple oxides of most metals do NOT yield oxygen when heated? This is easily verified by the teacher a t the desk or by a group of students working together in the laboratory. Lime is obtained by the decomposition of calcium carbonate by heat. But where is the statement of the general principle that most metal carbonates are similarly decomposed when heated? Any teacher can show that the carbonates of lead, copper, cadmium, cobalt, and nickel are decomposed into the oxides with liberation of carbon dioxide when heated; or he may turn a p u p of students to the solution of the problem in a laboratory exercise. Carbon dioxide is prepared by the reaction between marble chips and dilute hydrochloric acid. But where
is the statement that all carbonates act the same way when treated with acid? The effect of the formation of an insoluble compound in a reaction between a solid and an acid is seldom presented adequately. It can be shown that marble chips and dilute sulfuric acid react vigorously a t 6rst. hut the reaction soon stops, although acid remains. Why? Manganese dioxide is recommended for use with hydrochloric acid in the preparation of chlorine. But there are half a dozen other oxidizing agents which may
be used, some of them much better than the dioxide. Is this principle set forth in the text for the student? Cost as a determining factor hardly receives the attention it deserves, but it must be apparent to anyone that if there are six ways to prepare a certain product, the method which produces a satisfactory yield a t the lowest cost in a given locality is the one that will win out on a commercial scale. Do we ever solve problems that involve a dollar sign and approach manufacturing costs?
