MOVABLE SYMBOLS AND FORMULAS AS A TEACHING AID' W. T . LIPPINCOTT and ROGER WHEATON Michigan State University, East Lansing, Michigan STunmiw imagination and understanding in general chemistry courses can be stimulated by the use of demonstrations in which symbols and formulas printed on small squares of cardboard and attached to small magnets2may he freely moved to any position on a verticalmounted sheet-iron background, Specific illustrations in which "movable magnetic cards" may be used in demonstrations include visual illustration of the mechanics of solution of electrolytes and nonelectrolytes, visual concept of mechanics of electrolytic displacement, explanation of acid-base relationships, and demonstration of certain organic reaction mechanisms such as the two-step addition of hromine or hydrogen bromide to olefins. These examples do not exhaust the possibilities or uses of suitably titled magnetized cards. An example of this technique in use nnder classroom conditions describes the operation of a Daniell cell or a similar galvanic cell. In this case the magnet board is divided into a copper compartment and a zinc compartment3 as shown in Figure 1. I n the copper compartment, cards representing the electrode and marked with the symbol Cu are arranged close together and in an orderlv fashion. Other cards containine Cu++ and 'Presented before the Division of Chemical Education at the 128th Meeting of the American Chemical Society, Minneapolis, Septemher, 1955. Magnets are availahle from any of mverd 8cientific supply houses. a Chalk lines are u8ed to mark compartment haundaries if the panel is also a blackboard. Otherwise auitably marked magnetized strips of cardboard are employed.
still others with SO,-- are scattered throughout the compartment to simulate the solution surrounding the electrode. A similar picture is set up for the zinc compartment using cards containing Zn, Zn++,and SOa--. Each Cu++ card has the symbol Cu on the reverse side and each Zn card has Zn++ on its reverse side. The instructor starts the discussion by proposing that the electrical circuit be closed so that electrons flow through the external circuit. At the copper electrode, cards containing Cu++ are deposited and inverted, now showing Cu. The copper electrode model becomes larger as more deposition occurs. For every Cu++ card deposited, a Zn card is inverted showing Zn++ and moved away from the Zn electrode model. Hence this electrode "corrodes" away as illustrated by Figure 2. The equations for the electrode reactions and the overall reaction are then written in the usual manner. Since the student cannot "copy" the movable model illustrations into his notes the equations serve as reminders of the picture he has seen. This picture is made complete by the corresponding lecture demonstration. In a similar manner the atom scale picture of many other phenomena car1 be made more vivid to the beginning student in chemistry. ACKNOWLEDGMENT
The authors wish to thank Drs. F. B. Dutton, R. N. Hammer, J. C. Sternberg, and S. E. Lippincott for their helpful advice during the development of this technique.
