Laboratory Work and the Scientific Method W . B. THOMAS. Bates College, Lewiston, Maine*
carbonate. The impulsive student notes that both original and final substances effervesce with acid and concludes that the change was physical. These and other errors in technique and judgment are discussed in detail with the class after, but never before, the experiment is performed. In a similar fashion many of the classical experiments of general chemistry can be revised to include an unknown. (b) Avoid detailed directions. The authors of some of the newer manuals have incorporated this suggestion, thus minimizing the "cook b o o k chemistry that all teachers deplore. Great care must be taken to provide adequate guidance while a t the same time encouraging the student to exercise his own judgment. (c) Segregate questions dealing with background material from those based upon experimental results. The distinction between seeking the answers to questions by reading and by experimenting must be made clear to the student. The research worker covers the literature dealing with a problem and then sets up experiments, the results of which may or may not agree with his predictions; the student must be taught to do likewise. (d) The resnlts of an experiment should not be given in advance. The following quotation from a wellknown laboratory manual serves to illustrate the point. "Drop a small fragment of zinc into dilute nitric acid. The latter is reduced to a poisonous red-brown gas, nitrogen dioxide, NOz." Such an experiment may help the student to learn the properties of nitric acid but will hardly introduce him to the scientific method. Garard3 gives other examples of the same faulty approach. (e) Experiments should be so designed that the student uses the results of certain experiments to plan further work. Schlesinger4elaborates on this point. The writer has attempted to incorporate the suggestions just discussed, in a series of five experiments on qualitative analysis to be used near the end of the freshman course. The experiments were not designed to teach qualitative analysis as such (the regular course in that subject is given in the sophomore year a t Bates College), but rather to serve as a tool for the teaching of ionic reactions as well as the scientific method. The * Present address: Maine Mills Laboratory, Lewiston. Maine. ' CARMODY, "Elementary laboratory instruction," I. CHEM. experiments have been used for several years with Eouc.. 12. 2334 (1935). considerable success. Since the five experiments are a F&I&GER, "&me ' departures from classical laboratory similar in approach, only one is presented here. methods in general chemistry," Ibid., 13,487-90 (1936).
T .
HE LABORATORY work in courses in general chem~stry has been subjected to an increasing amount of critical evaluation in recent years. This is due, in part, to the controversy over lecture demonstration methods vs. independent laboratory work; but there has also been general dissatisfaction with the results of laboratory work in elementary courses. This has led to numerous attempts to list the objectives of laboratory work. Every list that the writer has seen has included as one objective the teaching of the scientific method. In spite of this unanimity of opinion, it is probably fair to state that the orthodox laboratory course in general chemistry largely fails to attain this objective. Other writers', 2, 3, have expressed similar viewpoints. It is the purpose of this paper to suggest several means whereby the laboratory work in general chemistry may be made to approximate more nearly the scientific method. (a) Wherever possible, plan the experiment to include an unknown. In this way the student is compelled to make observations and to draw conclusions therefrom. By way of example, consider the usual experiment on chemical and physical changes in which the student carries out certain operations on known materials. In most cases he decides whether the change was chemical or physical on the basis of previous knowledge or by consulting the textbook, thus missing the real point of the experiment. On the other hand, if the student is told to heat an unknown substance and to compare the properties of the original and final substances, he must employ the observationconclusion sequence which is so vital to scientific work. The writer has found sodium bicarbonate to be an excellent unknown for the experiment under $cussion. The substance is heated and the original and final substance tested for solubility in water, effect on phenolphthalein, and reaction with dilute acid. The number of p i t f a l l s a n d hence opportunities for some real teachingis surprisingly large in this simple experiment. The careless student uses too much of each salt and concludes that both are insoluble in water; the keen student discovers a decided difference in solubility of the original bicarbonate and the final
a GA-, "Scientific method in general chemistry laboratom work. Ibid.. 11.42-4 (1934). ~Scm&rrro&, he contribution of laboratory work to general chemistry." Ibid.. 12,524-8 (1935).
SOME IONS OF THE COPPER GROUP
I. Put 10 ml. of solutions of HE++, Pb++, Bi+++,
and Cu++ in four different test tubes and treat each in succession with H2S. Generate the gas by the action of dilute HCl (technical-sideshelf) on FeS. As soon as this is completed, stop the evolution of H2Sby washing the acid out of the generator with water. Allow the precipitates to settle. (Heat may hasten this.)
v.
cu++ A . Repeat I V A and B, using Cu++. The final result serves adequately as a test for Cu++. VI. Devise a method for the separation and identification of the four ions studied in I to V. Analyze 25 ml. of a solution known to contain the four ions. This is to be followed by the analysis of an "unknown." 11. Hg++ (Note: Decantation is satisfactory for the prekninary A. Decant the liquid from the HgS; wash by tests, but filtration must he used in the analysis in decantation; heat with HNOa. order to get complete separations. Precipitates should B. Add HC1 to mixture A and heat. also be washed with water and the washings added to C. Cool B. Dilute with water. Filter if not the filtrate.) clear; add SnCl, drop by drop. Students who have had contact with the type of experiment just described, and also with the detailed 111. Pb++ directions previously employed, &e enthusiastically A . Repeat I1 A, substituting Pb++. in favor of the present plan. Of course, errors in judgB. Filter A if there is a precipitate and treat ment, procedure, and technique are numerous; but filtrate with HzSOI. Evaporate almost to dryness (evaporating dish). Cool. is that not actually an advantage? Does not the Add water; transfer to test tube; student learn through such a process? A thorough discussion subsequent to the lahorato~ywork provides decant. C. Add CzHsOz- to precipitate B. (Use a ample opportunity to point out the best procedure and technique as well as to explain the reason for student salt, not HGH802. Why?) errors. D. Add CrOr- HC2H302to solution C. Some authors of manuals of qualitative analysis IV. Bi+++ take pride in presenting such detailed directions that A. Repeat I11 A and B, using Bi+++. "men, though fools, shall not err therein." The B. Add excess NHaOH to resulting solution. writer is not prepared to advocate the use of the C. Filter B and pour a solution of S n O r over method here presented in the full course in qualitative the precipitate on the filter paper. ( T o analysis, hut he is convinced that indoctrination in prepare Sn02- add excess NaOH to the scientific method is an important part of the Sn++.) general chemistry course.
+
