overhecrd projector demon~trcrtion~ Using a Motor To Demonstrate Conductivity Sally Solomon' and Annamarla Fiilep-Poszmlk2 Drexel University Philadelphia. PA 19104 The lighting of a bulb, the ringing of a hell, or the deflection of a meter needle provide visual evidence for conductivity in lecture demonstrations. In this experiment, designed to he performed on the overhead projector, the turning of a propeller identifies solutions of strong electrolytes.3 Apparatus The apparatus for performing the conductivity test is assembled from inexpensive components. A miniature gearreduction, high-torque motoq4 measuring z5/8 in. X 13/8 in. X 1'14 in., is neatly supported in a 50-mm X 30-mmmbher filter adapter.5The motor is powered by a 9-V hattery. Mounting the battery in a rectangular battery holder helps to keep the entire apparatus resting firmly on the overhead stage.6 The propeller is cut from a sturdy piece of transparent plastic. Graphite electrodes are taken from AA (nonalkaline) flashlight batteries by carefully cutting thecase and removing the graphite cylinder. With care each electrode can be preserved still attached to its plastic disk and metal cap. Using flexible wire and keeping leads short will make the entire apparatus compact and easy to handle. The motor is connected to the battery holder and to one of the graphite testing electrodes. Note that this particular motor has a metal tab to which one lead is readily connected; the other lead must he inserted into an opening on the opposite side of the motor. T o complete the apparatus, the hattery is hooked u p to the second graphite electrode.
160
Journal of Chemical Education
edited by DORIS KOLB Bradley University Peoria, IL 61625
Procedure To perform the demonstration, pour solutions to be tested in 50. mL beakers placed near formulas of the dirsoived solutes. Formula and discussion can be written on a transparency slide that protects the overhead from rpills. It is effective LO ask student* what they expect fur each solution. Put the electrodes in the solution LO be tested making sure that they do not toucheach other and that your hands do not get in the way. You may find that electrode "holders" fashioned from naoer . . c l i ~ shune over the side of the beaker make this easier. The minimum concentration of KC1 needed to demonstrate conductance using this setup is roughlyO.l M. Uote that it is nor poasihle to make a quantitative study of the effect of concentration on conductance by measuring the rate of propeller revolution. For instance, when electrodes are immersed in a 1M KC1 solution using this setup, the number of propeller revolutions per second is about twice what it would he with a 0.1 M KC1 solution. However..the soecific . 20 O C I of a 1M KC1 . conductance (at solutiwt is 101.9: mmho~crn,about nine trmes that of a 0.1 M KC1 aolution l11.6>mmh~,, rm). The rateat which the motor shaft turns depends primarily on thegear arrangement and the resistance in the entire circuit, ~
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' Author to whom correspondence should be directed.
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Central Research lnstltute for Chemistry of the Hungarian Acads my of Sciences. Presented at the 1lth BCCE, Atlanta. GA, August 1990. Edmunds Scientific, catalog number F35.116. Fisher Scientific.catalog nubmer 10-184. Newark Electronics, catalog number 81F3822. Temperature Correction Data for Typical Solutions; YSI Conductance Meter instruction Manual.
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