NOVEMBER, 1951
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THE OVERHEAD PROJECTOR AND CHEMICAL DEMONSTRATIONS W. H. SLABAUGH Kansas State College, Manhattan, Kansas
MANY
c1assroom demonstrations of fundamental chemical experiments can be conducted on a scale which permits those in the back rows of the lecture hall to witness the operation. I t is not probable that lecturers have resorted to wash tubs and bath tubs for these larger demonstrations, hut there are times that if glass containers of comparable size were available they might have been used. On the other hand, there are chemical phenomena which take place only on a small scale and must consequently be eliminated from demonstration experiments except for small groups. Thus, it would be almost hopeless to expect a typical member of a lecture section which exceeds a few dozen in size to observe critically a demonstration of surface tension wherein a steel needle or a razor blade is suspended by the surface film of water. In an electrolysis experiment, to report to the students in the back row that gas bubbles are forming at the anode constitutes a ridiculous situation.
In order to solve this problem the writer has adapted an overhead projector' to several chemical demonstrations, some of which are described below. The advantages of this type of presentation are at once obvious. In classes numbering ove1 200 students, all members can easily observe the demonstration as it is projected on the screen even though the room is partially illuminated. This permits the student the opportunity of taking notes a t the same time that the demonstration is being projected on the screen. There is a minimum amount of disruption to the course of the lecture because the projector is on the lecture table and the lecturer who operates the projector remains in visible contact with the class a t all times. Noller2 has suggested the use of the overhead projector for the demonstration of optical activity. There
' Overhead projectors are available from the Charlen Beseler Co., American Optical Co., and Bausch & Lomb Optical Co. ' NOLLER,C. R., J. CHEY.EDUC.,26,269 (1949).
JOURNAL OF CHEMICAL EDUCATION
are undoubtedly others who have used this medium for specific demonstrations, hut there appears to be a great number of commonly used demonstrations which can be performed in this manner. DEMONSTRATION EXPERIMENTS
(1) The Activity Series for Metals. Symbols for common metals such as magnesium, zinc, iron, tin, and copper are written with wax pencil on the bottom of a crystallizing or culture dish. Dilute hydrochloric acid is then placed in the dish t o a depth of 5 mm. Into the dish on the projection table are dropped small pieces of the appropriate metals, whereupon hydrogen bubbles form at a rate relative to the activity of the metal. The figure shows the projected image.
The Relative ActMt,. of Certain Met& in Displacing Hydrogen *om Dilute Hydrochloric Arid
(2) Electrolysis of Water. A miniature Hoffman apparatus with electrode chambers about 10 cm. long can be easily constructed using glass tubing of oval cross section. Circular cross-section tubes filled with liquids will appear as solid shadows on the screen. The arms and reservoir are mounted at 45' to normal so that the image on the screen will show these parts. I n performing this experiment the gas bubbles which form a t the electrodes will appear on the screen as large as 20 cm. in diameter. In the traditional manner, organic dye indicators may be used which will indicate the acidic and basic nature of the electrolyte a t the cathode and anode, respectively. (3) ,-,-Random Motion of Gas Molecules (Kinetic-Molecular Theory). A dozen or more pieces of metallic sodium approximately 2 mm. in diameter are dropped into a crystallizing dish which contains a few ml. of water. The degree of activity of this reaction causes the metal to melt into spheres almost immediately, and to travel a t speeds of 10 to 20 cm. per sec. across the surface of the water. At the edges of the dish, the water meniscus provides a suitable barrier which turns
the globules of sodium back an angle of reflection apparently equal to the angle of incidence. The turbulent wake caused by this motion leaves a record of the path which the metal sphere follows, but this wake soon subsides so that the greater portion of the surface of the water remains undisturbed. The presence of phenolphthalein in the water produces a very colorful wake which adds considerably to the spectacular nature of this experiment. (4) The Action of Metal Couples. Pieces of zinc and copper screen are placed in dilute hydrochloric acid in a crystallizing dish. Hydrogen gas is evolved at the zinc screen until the two metals are placed in contact with each other. Then the gas bubbles form on the copper. Metal in the form of screen has been found advantageous because of the greater degree of visibility provided by this form in rontrast to solid pieces of the metals. (5) The Relative Strength of Acids. Five small crystallizing dishes containing dilute solutions of HC1, H2SOa,CH3COOH,citric acid, and boric acid are placed on the projector. When small pieces of zinc are added to each dish the relative activity of each acid is easily deduced on the basis of the quantity of hydrogen bubbles which appear on each metal. (6) The Qualitative Aspects of Optical Activity. A crystallizing dish containing a solution of an optically active compound is sandwiched between two sheets of Polaroid. Orientation of the upper sheet d l readily show the effect of rotation of the plane of polarized light by the optically active compound. (7) Electrochemical Experiment. Not only small scale cells may be projected but also the electrical measuring instruments, if constructed on transparent plastic bases; all information concerning the experiment becomes directly available to each member of the class. Wiring diagrams of the electrical circuits can be made an integral part of the actual apparatus. Many other lecture experiments can be performed in this manner. The color response of indicators to acids and bases, the formation of insoluble products, the speed of chemical reactions as a function of state of subdivision, and the exhibition of molecular models are examples. I n setting up experiments with the overhead projector it is well to bear in mind that the apparatus for the most part must he transparent and that the view on the screen will be as if one were to look directly downward at the experiment. I t has recently been suggested by Kharascha that lantern slides can be used to an advantage in organic chemistry courses. The overhead projectors now available not only project the standard slide from the lecture table, but permit the use of slides which are made on cellophane, Kodapak, or other transparent media. Diagrams, charts, graphs, etc., up t o 7 X 7 inches can he easily reproduced by simply tracing the original onto the transparent sheet by means of a wax pencil. The st,orage of slides of this type resolves into the use of an ordinary letter file. a
KHARASCH, NORMAN, J. CREM.EDUC., 28,280 (1951).
