Demonstrations for Nonscience Majors Using Common Objects To Illustrate Abstract Concepts William Laurla Kutztown University, Kutztown, PA 19530 While attempting to present concrete examples of common ohenomena to a class of nonscience majors. I developed a fed demonstrations that others may find useful for the same purpose. Marbles and Avogadro's Number Laboratory exercises designed t o measure Avogadro's number by determining the thickness of a monomolecular layer of an amphiphilic substance like oleic acid are common'. The idea that the thickness of a "slick" may he measured indirectly seems easier to understand if the method is first applied to a layer of marbles. Equipment needed: one round, 9-in. Pyrex pie plate, about 150 marbles withadiameter of about 1.5 cm, a 500-mL eraduated cylinder, a halance capable of weighing somewhat more than i kg. Carefully dispense the marhles into the pie plate until you have produced a round, tightly packed, single layer of marbles; measure its radius. Weigh the graduated cylinder, and carefullv roll the marhles into it. Note the volume of the &arbleiand the combined weights of the graduated cylinder and contents. After picking up marhles dropped on the floor,2proceed with the calculations. Typical data and calculations are reproduced helow. Data
Marble layer radius = 10 cm Wt. grad. cyl. = 375 g Vol. of marbles = 475 em3 Wt. grad. cyl. +marbles = 1045g Calculations Height: = 314 cm2 Area covered by marbles = r X (lo Height of marble = 475 cm3/314em2 = 1.51 cm
(Can agree to within la with a micrometer measurement.) Auogadro's number: Density of marbles = (1045 - 375)g/475 em3 = 1.41 g/cm3 = 3.4 cm3 Volume of a marble3 = (1.51 Entering the world of fantasy, tell the students that the atomic weight (found from combining weights) of the monovalent marbles is 100 g. Ask how many marbles is in 100 g (Avogadro's number for this fantasy)?
Kina. L. C.: Neilsen. E. K. J. Chem. Educ. 1958. 35.198. Be sure to dropa few marbles so that you can make a comment to the effect that you have lost your marbles. For this purpose, a model marble with cubic shape is superior to one with spherical shape because the measured density is based on a volumeoccupied by many marbles, not just thevolumeof one marble. Check the weight of the number of marbles calculated to weigh 100 g. The agreement is very poor if the volume of the marble is calculated from a spherical shape. 1
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Number of marbles = 100 g (1m3/l.41 g) x (1marble/3.4 em3)= 21 marbles Velcro and "Llke Dissolves Llke" One of the rules that comes out of a discussion of intermolecular forces is "like dissolves like". A convincing demonatration that intermolecular forces are responsible for this observation can be made with some strips of Velcro in a mayonnaise jar. Equipment needed: about 3 yards of %-in. white Velcro (1.5 yards of hooked and 1.5 yards of looped), ahout 3 yards of 3/4-in.black Velcro (1.5 yards of hooked and 1.5 yards of looped), a l-quart mayonnaise jar. Cut 100 squares of white Velcro (50 hooked and 50 looped) and 100 suuares of black Velcro (50 hooked and 50 looped). Place black squares together so that the hooks and loops are engaged with the smooth surfaces exposed. These squares may be stapled or sewn together. Place white squares together so that the smooth surfaces are touching with the hooks exposed on one side and the loops on the other. These squares should he sewn together hut four staples may he adequate. Stack the squares so that they alternate black, white. Dumo the stacks into the mavonnaise iar. Shake vigorously. The white squares are separGed from ihe black squares and form a single clum~-the "dipolar" white squares separate from the nonpolar black squares. A Pencll, A Wrench, and Momentum The idea that pressure results from a change of molecular momentum may he illustrated without reference to equations or mathematics of any kind. Equipment needed: an 8- X ll-in. writing tablet with cardboard backing, apencil, awrench (weighing about a half pound). With the accompanying figure in mind, hold the cardboard hacking a t an angle of 20 or 30" with respect to the vertical paper. With wrist motion only (not arm motion), vibrate the pencil rapidly with a frequency of 3 or 4 cyclesls between the bottom thirds of the cardhoardand the paper so that the paper is no longer vertical. The pencil should trace an area with the shape of a pie wedge. Now replace the pencil
with the wrench. Vibrate the wrench while trying to estahlish about the same deviation of the paper from the vertical. The wrench moves much more slowly than the pencil to establish roughly the same deviation from the vertical. Although the paper may bounce about under the infrequent blows of the wrench, the student is convinced that both velocity and mass are involved in the calculation of pressure. I t should, of course, be pointed out that pressure is propor-
tional to force and that the force is proportional to the dinplacement of the paper from the vertical position. This demonstration may help the student to assimilate the idea that a hvdroeen . " molecule and a radon atom with the same kinetic energy exert the same pressure when confined to the same volume. The acceptance of this model for gas pressure may help the student to accept the very general nature of the ideal gas law.
Volume 67
Number 1 January 1990
61
