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 POPIIIH~ 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 t o 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 t o determine the average diameter and hence the area of the monolayer. 4) Use the volume and area t o 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 t o 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 (2) (31 (41 (51 I61 (71 IS1 I91 I101 (111 I121 I131
Halliuoll, H. F., "Laboratory investlgstions." Nuffleld Chcmiatry Series, LongmansDengvin Books. London, 1967. King, L. C., and Neilsen, E. K., J. CHEM.EDUC.,35,198 (19581. Gortner, E.,andOrendel. F. J.,J.E=p.Biol.,41,439 (19251. Jones, K. K., Science, I l l , 9 (10501. Berwist, D. H., and Wells, F., Food Technol., 10.48 11956). Ifft. J. B., and Roberts. J. L., "FrantriMalm'8 Essentials of Chemistry in theLaborafory," 3rd Ed.. W. H. Freeman and C o , Sen Franciaeo, 1975 Bailey, P. S., and Bailey, C. A., "Experimental Chemistry for Contemporary Times,' Allyo end Bacon, Bosfon. 1975. Andraws. D. H.. snd Kokes, R.J., "Laboratory Manual for Fundsmentalchemktry." John Wiley and Som, Ine..New York, 1962. (19621. Koke8.R. J., Dorfman,M. K.,andMathis,T.,J.C~~~.Eouc.,39.18 Mo~nihan,C.T.,andGoldwhife,H.,J . C H ~ MEouc..46.770 . (19691. Demchik,M. J..and Demchik,v.C., J.CHEM.E ~ u c . , 4 8 , 7 7 0(1971). Hanaon. A. L., J. CHEM..EDUC., 59.379 119821. FeinsVin, H. I., and Sipson, R. F., J. CHEMEDUC.,59.75, (1982).
Ian J. McNaugM
Gavin D. Peckharn
University of Natal
University of Zululand Private 8ag XlOOI Kwa Dlangerwa 3886, South A h i c a
P.O. Box 375
Pietermaritzburg 3200, Sou% Anlca
Volume 62
Number 9
September 1985
795
