The ability ofoil tospreadacross a water surface has been known for hundreds of years. As long ago as 1773, Benjamin Franklin observed, on a pond near London, that one teaspoon of oil would spread to form a film with an extent of about 22,000 ft2 ( 1 ) .

The esrimarion uf Avogadro's numher and 18f molrcular dimenaims hy rhe"0il Dmp Experiment" was f i r t presented by King and Sdw! (2) who hnaed their rxperimmt on the trchniqueaof earlier workers (3-5). Theexperiment has herome P O P I I I H ~ in lahmatory manuals for general chemistry nrunea 16-8, and variuus authors have ruggmted im~rovements. --

refinements, and variations on the theme (9-13). Beginning students in chemistry sometimes find i t difficult to conceptualize the principles involved and are often

skeptical about the accuracy of the results. We have greatly reduced this problem hy intraducing an analogous experiment as a preliminary to the oil drop experiment itself. The experimental analogy is outlined below.

1) Determine the "valume" of about 8 mL of 2-mm hall bearings using a graduated cylinder. (These ball bearings are available in bulk from most bicycle manufacturers.)

2) Empty the ball hearings onto a large watchglass and pat them down to form a monolayer. 3) Make several measurements to determine the average diameter and hence the area of the monolayer. 4) Use the volume and area to calculate the height of the manolayer. This height corresponds to the diameter of the

individual ball bearings, thus the result may be verified by direct measurement of the ball bearings' diameters.

This experiment enables a student to clearly visualize the oil drop experiment and enables him or her to confirm the accuracy of the results. The usual discussion of experimental errors and assumptions, particularly those concerning molecular shapes and packing can be applied to this analogy.

The experiment may be prepared and performed easily and quickly. I t utilizes cheap, reusable equipment. If time or equipment is a limiting factor then the experimental analogy may be carried out by the students themselves, followed by a demonstration of the actual oil drop experiment by the instructor. Conversely, if the students are to perform the oil drop experiment themselves, then a demonstration of the ball hearing analogy by the instructor provides a useful introduction.

Literature Cited (11 Halliuoll, H. F., "Laboratory investlgstions." Nuffleld Chcmiatry Series, LongmansDengvin Books. London, 1967. (2) King, L. C., and Neilsen, E. K., J. CHEM. EDUC., 35,198 (19581. (31 Gortner, E.,andOrendel. F. J.,J.E=p.Biol.,41,439 (19251. (41 Jones, K. K., Science, I l l , 9 (10501. (51 Berwist, D. H., and Wells, F., Food Technol., 10.48 11956). I61 Ifft. J. B., and Roberts. J. L., "FrantriMalm'8 Essentials of Chemistry in theLaborafory," 3rd Ed.. W. H. Freeman and Co , Sen Franciaeo, 1975 (71 Bailey, P. S., and Bailey, C. A., "Experimental Chemistry for Contemporary Times,' Allyo end Bacon, Bosfon. 1975. IS1 Andraws. D. H.. snd Kokes, R. J., "Laboratory Manual for Fundsmentalchemktry." John Wiley and Som, Ine.. New York, 1962. I91 Koke8.R. J., Dorfman,M. K.,andMathis,T.,J.C~~~.Eouc.,39.18 (19621.

I101 Mo~nihan,C.T.,andGoldwhife,H., J . C H ~ M . Eouc..46.770 (19691. (111 Demchik,M. J..and Demchik,v.C., J.CHEM. E ~ u c . , 4 8 , 7 7 0 (1971). I121 Hanaon. A. L., J. CHEM.. EDUC., 59.379 119821. I131 FeinsVin, H. I., and Sipson, R. F., J. CHEM EDUC., 59.75, (1982).

Ian J. McNaugM Gavin D. Peckharn University of Natal University of Zululand P.O. Box 375 Private 8ag XlOOI Pietermaritzburg 3200, Sou% Anlca Kwa Dlangerwa 3886, South A h i c a

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