Symposium on Lecture Demonstrations DEMONSTRATION TECHNIQUES1 HUBERT N. ALYEA Frick Chemical Laboratory, Princeton, New Jersey
THE
BEST lecture demonstration in the world is a and the reactions were carried out i n these veru vials. failure if poorly staged. By intensely lighting from To make the colors of solutions and precipitates visible below, or spotlighting from above, color changes, the to a class of 100 students is not at all difficult if the formation of precipitates, or the evolution of gases vial is spotlighted. A convenient arrangement is to can be made visible to large audiences. A vortable lecture demonstr&n box to serve this purpose is shown in Figure 1. Toggle switches, on the back of the box, control the spot and photoflood lights. The extension back is raised whenever reactions are to be carried out in tall cylinders; the remainder of the time it is down to permit the lecturer to mix solutions from directly behind the box. Demonstrations on a Semimimo Scale. Just as semimicro techniques save time and materials for the student, so also lecture demonstrations on a semimicro scall: are a boon to chemistry teachers who lack a lecture assistant or expensive chemicals. The author conducted an evening course in General Chemistry fifty miles from home and performed dozens of experiments with equipment which could be carried to the lecture hall in a brief case and set up in five minutes. To accomplish this, the reagents--solids and solutionswere stored in plastic-capped, 1 X 3-inch, glass vials; 1 Presented before the Division of Chemical Education, Symposium on Novel Lecture Demonstrations, at the 112th meeting of the American Chemical Society, September, 1947, NewYorkCity.
JOURNAL OF CHEMICAL EDUCATION
fasten a glass shelf six inches below the fluorescent lamp of a "titration illuminator" (Fischer Scientific Company No. 11-991-5). In many of the experiments the reagents are not consumed, so the vials may be capped and laid away with the reagent still in them for repeated use. I n fact, preparing a thousand vials, which were placed in fifty different drawers corresponding to fifty lectures, was the work of but a few days, and these vials could be used over and over, from year to year, refilled from stock solutions, 100 ml. of which would last for several years. No need of an assistant tl Typical demonstrations follow: (a) Has water is added to a row of vials containing various cations. ( b ) Chlorine is generated from NaCl MnOe HZSO,, the vial being placed on a hot plate. ( c ) A polished steel plate is dipped into a vial containing cupric sulfate solution. (d) The law of conservation of mass can be demonstrated on a semimicro scale. Often it is demonstrated by inverting a 2-liter flask containing 1000 ml. of lead nitrate solution in which is placed a test tube filled with potassium iodide solution; and showing, by the use of a large delicate balance, that no change in weight accompanies the
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reaction. It is just as instructive t o carry out the experiment in a 1 X 3-inch vial containing 20 ml. of lead nitrate solution in which there is a 6-mm. tube holding 3 ml. of potassium iodide solution. The vial and contents are suspended by a rubber band against the blackboard and the position marked. After showing that a small additional weight, such ss a paper clip, stretches the band, the vial is inverted, mixing the two solutions. No increase in weight is observed. ( e ) Conductivity can be illustrated by means of a special apparatus. Two lamp sockets are connected by a two-inch nipple. A hole is bored in the side of the nipple and the sockets are wired in series. A lamp bulb is screwed into one socket. Into the other is screwed a burned-out 150-watt bulb whose glass has been discarded, the stout lead-in wires of the lamp serving as electrodes. The device is clamped upright, plugged into 110 volts a. c., and various vials containing electrolytes, or nonelectrolytes, raised up to immerse the electrodes. After this experiment the solutions are left in the vials, which are laid away for use again. Many other demonstrations on a semimicro scale will suggest themselves to the imtiginative teacher.
