LABORATORY WORK for fhe CHEMISTRY PART of a GENERAL COURSE in the PHYSICAL SCIENCES E. J. ROSENBAUM The University of Chicap, Chicago, Illinois
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simplicity of the technic required permits the attention of the student to be focused on the point which the experiment illuminates. A similar treatment of the chemistry experiments might seem an attractive possibility, hut experience has shown that this approach is quite impractical. It is relatively difficult to coustmct semi-automatic chemical apparatus and even more difficultto keep such apparatus in good working coudition. Instead of attempting to imitate the physics treatment of the lahoratory work, it seemed more desirable, in connection with the Physical Science General Course a t the University of Chicago, to re-examine conventional chemistry experiments and to inquire to what extent they could he adapted to general course requirements-to what extent they could he "streamlined." The guiding thoughts in this analysis were the following. (1) The work should be planned specifically for those who have not had high-school chemistry. Consequently, (a) The experiments must he so extremely simple that technical difficulties do not obscure the point to he illustrated. (b) The experiments must he so foolproof that even with unskilled hands thereisno real danger. (2) Each experiment should have a definite objective which is intimately related to the material covered in the lectures. (3) No attempt should be made to teach technic heyond a general appreciation of the fact that carefulness is essential in the lahoratory. On the basis of these premises a set of chemistry experiments has been developed here which seems to meet the needs of a general course particularly well. Participation in the laboratory work is entirely voluntary. The average student can complete the assigned work by spending a minimum of one-half hour per week in the lahoratory (although many spend more time than this) while chemistry is under consideration. This means that a lahoratory holding fifteen to twenty students is sufficient to take care of a fairly large class. ' Presented before the Division of Chemical Education at the The apparatus required consists almost exclusively of ninety-eighth meeting of the A. C. S., Boston, Mass., September, test-tubes, and the amount of supplies used up is rela14, 1939. tively small. a LEMON. H. B.. Am. Pkys. Teacher, 2, 10 (1934); HUTCHIS- Before some of these experiments are described, it L. R., SON,E. AND V. HICKS,ibid., 3, 65 (1935); INGERSOLL, may be well to sketch the background in which they ibid.. 4. 112 (1936).
ITHIN recent years the development of new general ("survey") courses in the Physical Sciences has brought with i t many new problems, among which is the question of suitable lahoratory work. Although many institutions have evaded this problem, there is general agreement that some sort of lahoratory work is highly desirable, for otherwise students memorize definitions and generalizations without adequate recognition of their observational background. But i t is very clear that the conventional type of technical training offered in courses in physics and chemistry is out of the question for reasons directly related to the fact that all students are usually required to take the general course. In the first place, the classes are relatively very large. It is impractical to have available a large amount of laboratory space and equipment which is used for only part of the academic year. Even more important is the fact that a large fraction of the students has had neither high-school physics nor chemistry and, consequently, no lahoratory experience. These students, whose need for the lahoratory work is obviously the greatest, are the ones who, in general, have neither talent for nor interest in this aspect of the physical sciences. For these students the development of technical skill within reasonable time limits is impossible and should not be attempted. Liberal use of lecture demonstrations is, of course, a great help in ameliorating this situation* Likewise the use of motion pictures broadens the students' ohservational horizon. But with large classes many details of a lecture demonstration are not easily visible, and the number of topics which can be covered by motion pictures is limited. Neither one of these valuable aids completely replaces that direct, intimate contact with phenomena which can he obtained only in the lahoratory. The difficulty of supplying suitable lahoratory work in physics has been overcome to a large extent by the development of "Physics Muse~ms."~In these the student can perform experiments with very primitive manipulations, often by merely pushing a button. The
are performed. As the student enters the laboratory reducing agent, is prepared by dropping a few pieces of he takes a mimeographed sheet of instructions. After zinc into a tube of hydrochloric acid. The first week's work is closed by two experiments on he has read this through to get some idea of what is ahead of him, he receives from the laboratory assistant the behavior of strong oxidizing agents. In one of these a test-tube rack filled with tubes containing most of the a dropperful of concentrated sugar solution is added to chemicals he needs, and takes i t to a section of labora- a tube containing concentrated sulfuric acid. In the tory desk equipped with a Bunsen burner. If he must other one drop of glycerin is added to a small pile of wait because all the desk space is occupied, or when he pulverized potassium permanganate on a glass plate. has finished his experiments, his attention is directed to (This experiment can also be used as an illustration of the exhibits and automatic experiments on display. spontaneous combustion.) The experiments of the second week are intended to He might find laid out on a table a periodic chart consupplement some descriptive lectures on the comtaining samples of a large number of elements or a collection of particularly interesting compounds. In an- pounds of the more common elements. First, the stuother corner of the laboratory a distillation or the elec- dent is directed to a series of beakers containing labeled solutions of common bases and acids and a few "untroplating of a silver spoon might be in progress. After he has become familiar with the color -When the student begins his experiments he finds knowns." that the mimeographed sheet referred to above is an changes of litmus paper, he is ready to proceed. essential part of the laboratory instruction. It canies Ask the assistant for test-tube rack and some "dry ice." This not only complete directions for each experiment but is solid COT. Since it is very cold do not handle it with bare also many explanations of observed phenomena and hands. Put a piece in test-tube 1. The tube is now full of COr leading questions aimed a t more careful observations gas from the evaporation of the solid (more correctly, sublimaand closer correlation between laboratory work and lec- tion). Light a splint and put it into the tube. Does C09 supcombustion? Add a half dropperful of lime water (solution ture material. The directions are made extremely port of &(OH),) to tube 2. CO* is heavier than air and may be specific by numbering the holes in the test-tube rack poured from one tube to another just like a liquid. Pour same and then refening to the tubes in these holes by num- CO, from tube 1 into tube 2. Cork the tube and shake. Have you seen this reaction befare? To a small piece of CO* in tube 3 ber. a dropperful of distilled water and cork the tube. After the The first week's work covers the general topics of add solid has disappeared, test the solution with blue litmus paper. elements, mixtures, and compounds, nature of a chemi- Is the solution acidic or basic? cal reaction, and the concepts of oxidation and reducFollowing this, the bleaching action of chlorine water tion. Each laboratory section opens with a brief discussion by the assistant of methods of separation of on dyed cloth and on litmus paper is observed. Then mixtures into pure substances. This is illustrated by a comes the only experiment in the course which requires demonstration of the separation of NaCl and MnOz by the use of a hood-the reaction of nitric acid with copatration, and the separation of CUSOIand HzO by dis- per. All that the student needs td do is drop a few tillation. The first experiment is on the reaction of copper pellets into' the nitric acid contained in his tube. In spite of the possibility of an accident with the nitric iron and sulfur. The directions are as follows. acid there has been no difficulty with this experiment. Test-tube 1 contains a mixture of sulfur and iron. The iron During this week a number of alloys, ores, and nitrocould easily be separated from the sulfur by use of a magnet. gen compounds are on exhibition. , Put a little of this mixture into test-tube 2 for later use. Heat The principal topics of the third week are reaction vetube 1 over a burner, gently a t first, then briskly until a red glow appears a t the end of the tube. Remove from t h t flame and oh- locity and chemical equilibrium. The experiments can serve the glow. Why does it continue? Ask the assistant to best be described by quoting the directions. break the test-tube for you and observe the contents. Is any part attracted by the magnet on the reagent sbelf? Add a few drops of hydrochloric acid (HCI) (reagent shelf) and notice the odor. Now add a few drops of HCI to the mixture in test-tube 2. Do you notice the same odor? What must have happened when tube 1was heated?
A test for the most common product of combustion is then made by sucking the gases from a candle flame through Ca(OH)* solution with an aspirator. The student blows into another tube of the solution and thus identifies COz as a product of biological oxidation. To show that the more active metals bum in air a piece of magnesium ribbon is ignited. To show that substances other than oxygen can support combustion one of the tubes contains chlorine. Into this tube a piece of filter paper soaked in turpentine is dropped, and the tube is quickly re-corked. The observed charring is very convincing. Hydrogen, which is referred to as a typical
Test-tube 1 and 2 contain two different concentrations of HCI. Add a few pieces of granulated zinc t o eacb. Which tube contains the more concentrated acid solution? Test-tube 3 contains a piece of iron'and tube 4 contains iron oowder. Add to eacb a small amount of dilute HCI from thereagent shsf. Observe the effect of the state of subdivision on the reaction velocity. Tube 5 contains a mixture of two compounds (NalCO1 and citric acid) which bas been standing for days without reacting. Add a few drops of water. Why is there so large a difference in reaction velocity between the mixed solids and the solution? Tube 6 contains Dure ~otassiumchlorate (KCIOd. Heat this compound and testfor the presence of OXwith a glowing splint. Tube 7 contains a mixture of KCIO. and MnO.. Heat the contcnlr of this tuhc and again test for the prcsence of 0.. Which Illhe gives 0.more rapidly? The NnO, could bc recovered unchanged after the reaction is over. See Schlesinger, pp.156-7. for a discussion of its action. I n tube 8 is a saturated solution of BiCIs. Add a few drops of water and observe the reaction. Add a drop or two of HCI and observe. Repeat the addition of water and acid. How is the
equilibrium of the reaction BiCIs fected by HtO? By HCI?
+ H 9 0 = BiOCl + 2HC1 af-
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Since these exoeriments can be oerformed raoidh. -. there is tirne fo; a preview of the iext work, which is mainly on the theory of ionization. ~
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PbCI1. Add three dropperfuls of NH40H,cork with thumb, and shake well. Secure a funnel and filter paper from the assistant. Filter the turbid solution into tube 8. The white PhCL remains on the filter . PaDer. What has happened t o the AgCI? How . can YOU precipitate it again? hisi is a test for the presence of Ag ions. The technic you have used here is typical of qualitative chemical analysis.
Near the reagent shelf you will *d a series of snlutions and compounds whose electrical conductivity is to be tested. Remember that the more brightly the flash-light lamp ~hines,the larger is the current passing through it. TO test the circuit, touch the wires from the battery for a moment and observe the light. Then insert the wires in the sample to be tested. being sure that the wires do not touch each other. Can you account for the fact that some solutions are good conductors and others are not? This will be discussed later in the course.
~h~~~students who are sufficientlyinterested are given Out an acid-base titration the under the guidance of the laboratory assistant. The fifth and final week includes an experiment on the displacement of copper by zinc and of mercury by copper, thus furnishing the simplest approach to the electromotive series. The remaining experiments are all on organic chemistry. The student observes the The experiments of the fourth week are on the reac- decolorization of bromine solutions by amylene, and tions of ions. By this time the students have had a the lack of decolorization with hexane. He cames out moderate amount of experience with chemical equa- the reaction of Fehling's solution with glucose. He contions, and they are urged to write the equation for each trasts the behavior of silver nitrate toward a sodium reaction they observe. This week's exhibits include a chloride solution and toward carbon tetrachloride. large-size galvanic cell connected to a demonstration- He adds a few drops of water to a piece of calcium cartype galvanometer, and an electrolytic cell in which bide and ignites the gas which is evolved. The chief exhibit this week is the lawest assortment silver is deoosited. of interesting organic compounds which can be colHydrochloric acid is contained in tube 1and acetic acid just as lected, Such compounds as benzene, aniline, trinitroconcentrated is contained in tuhe 2. Add a few pieces of zinc t o which toluene, aspirin, and various dyes are on display, each each tuhe. Which reaction has the greater velocity? tube is the concentration of hydrogen ions the greater? which with a card bearing its structural formula and a few deis the stranger acid? scriptive sentences. Tuhe 3 contains concentrated HCI and tuhe 4 contains coniyOspecial meritcan be claimed either for the parcentrated NaOH. Mix these two slowly and carefully .without ticular experiments chosen here, or for the order in allowing any t o spill. Pour a small amount of the resulting solution in a small watch glass and place on the hot plate near the which they are to be performed. These details were door. When the solution has evaporated remove the glass with determined entirely by the desire to furnish effective a pair of forceps After the solid has cooled taste a few crystals. supportfor the subject matter of the lectures given in What is the compound? the general course a t Chicago. It would be a relatively Tube 5 contains washing soda. Nad203. Add same vinegar to lib orator^ work to any task t? it. I n accounting for the reaction, remember that vinegar is a other arrangement of lectures. It is worth emphasizdilute solution of acetic acid. Tuhe 6 contains a solution of AgNOs. Add a few drops of ing, however, that the general course laboratory work NaCl solution. The white curdy precipitate is AgC1. Add a described here has been successful in bringing some dropperful of concentrated NHIOH and shake for a while. What modicum of experience with experimental chemistry to happens? Add a dropperful of HCI. What is the precipitate? T U 7~contains ~ a solution of salts ,of silver and lead. ~ d ad students who otherwise would haye had no contact with few drops of HCI. The precipitate is a mixture of AgCl and laboratory work.
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