In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Simple Demonstration Model of Osmosis submitted by:
Joseph G. Morse Department of Chemistry, Western Washington University, Bellingham, WA 98225-9150
checked by:
Ed Vitz Department of Chemistry, Kutztown University, Kutztown, PA 19530
The colligative properties of solutions are based on quite a simple physical model, in principle. Such a model for osmosis in particular was discussed in this Journal at least as early as 1934 (1). Still, many students have not yet developed a good conceptual image of randomly moving molecules when this topic is encountered. Osmosis can be demonstrated in visually striking ways (2), but one can’t see what the molecules are doing. Computer graphics can do much to help develop that image, but no device I have seen is superior to one prepared by one of my teaching mentors, Garth L. Lee, then at Utah State University, for helping students develop a physical grasp of kinetic molecular theory as it applies to osmosis and other colligative properties. The Demonstration The device is shown in Figure 1. Oats and beans are used to represent molecules of two different sizes. The only critical dimension is that of the grid in the wire screen, which must be larger than oats and smaller than beans. (That is, it must be permeable to one and not the other.) The device was constructed from a 2-L beaker and stiff wire mesh with “square” openings approximately 4 mm on a side. The grid was cut and bent to fit rather snugly in the beaker and sealed into the bottom with paraffin wax, though something like Shoe Goo (used for extending the life of running shoe soles) would work equally well. In fact, a good snug mechanical fit is quite satisfactory. One begins the demonstration with the two “fluid” levels equal. Then vigorous shaking is initiated while the beaker is held upright. This simulates random molecular motion. The shaking must be somewhat controlled to prevent loss from the top of the beaker, though a cap could easily by prepared to avoid that problem. One can detect what is happening during the shaking after as little as 20–30 seconds. By the end of that time, a readily noted difference will be observed in the “fluid” levels on the two sides of the screen. The system can be “reset” simply by tilting the beaker so that the pure oats are on the lower side and the “solution” on the upper, and gently shaking the device. Gravity will assist in re-separating the different-sized “molecules.” Students seem to enjoy trying the device themselves and being able to grasp the idea that there is a higher probability that oats will make the net transfer through the grid from the side on which they are in higher concentration. They are able to extrapolate this idea to molecules with considerable success. I find that most also extend the notion to other colligative properties, once they have thought about this model. 64
Clearly there are variants which could be used to adapt to the agricultural crops or products of a particular region, or other materials can be used. The densities of the two seeds used in this model are virtually identical, according to measurement in the laboratory, and that might prove important if one is to avoid the appearance that all that is happening is that the less dense seeds simply float through the screen and over the top of the more dense. Discussion This is a model of a physical–chemical system approaching equilibrium for which other mechanical models have been described in this Journal (3). Like all such models it has limits to how far it can be pushed. Mass transfer across the screen should be due to translational kinetic energy and gravitational effects should be negligible, if the model is to be “realistic”. That appears to be the case, from the results of a somewhat more controlled experiment than is commonly done when the device is used for demonstration. In one such experiment the levels of beans and oats were set so that no difference was discernible to the eye of the experimenter. The device was then strapped onto a shaker in the biochemistry laboratory. Each cycle of the shaker had an amplitude of 1 inch in the direction perpendicular to the screen and the shaker was operated at its maximum frequency (200/min). After 24 h the two levels differed by only about 6 mm. For this system, equilibrium seems to be reached at about 25 mm via vigorous hand shaking for about 5 minutes. The shaker exhibited a very low effective “temperature” relative to the vigorous shaking one can accomplish by hand! (Clearly, a controlled experiment resulting in true equilibrium would be a lengthy one with this shaker.)
A
B
Figure 1. A device for modeling osmosis. (A) Before shaking, the two “fluid” levels are equal. (B) After shaking for approximately 30 seconds, a significant “pressure gradient” has developed
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
In the Classroom
Visual inspection of the mixture on the “bean” side showed an apparently random distribution of oats within the beans, although there were more oats near the screen than far from it. This crude inspection gives the result one would expect if this model of a molecular system is a good one. It is probable that hand shaking generates a greater vertical component of motion and creates more and larger void spaces into which the smaller seeds can migrate. Again, this is not dissimilar from what one would expect in two liquids at higher temperature. After vigorous hand shaking, a “flume” of oats is apparent in Figure 1, which on inspection appears to be due to a small gap between the screen and the glass. The flume is not present on the other side of the beaker. In contrast to what one would expect of a real solution system, there is a tendency for the oats to pack rather tightly below the beans after extensive hand shaking; that is, there is a vertical separation as in a separatory funnel. This is apparently due to packing effects, since the densities of both seeds were measured at 1.21 g/mL. This packing makes the approach to equilibrium much slower when attempted from the other direction. That is, if all the seeds are initially present on one side of the screen and shaking is initiated, vertical
separation occurs before the previous equilibrium position is reached and subsequent transfer becomes very slow because it is difficult to provide the same shaking “temperature” and void space at the bottom of the beaker, probably because of the pressure of the beans in the upper layer. In effect a different equilibrium position results for this “three phase” system. Could one accomplish “reverse osmosis” by applying pressure in some way to the shaken system? Acknowledgment I wish to thank the editor of this feature for thoughtprovoking suggestions. Literature Cited 1. Davidson, A. W. J. Chem. Educ. 1934, 11, 499–501. 2. See for example Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin Press: Madison, WI, 1989; Vol. 3, pp 283, 286, 390. 3. Battino, R. J. Chem. Educ. 1975, 52, 55. Olney, D. J. J. Chem. Educ. 1988, 65, 696. Russell, J. M. J. Chem. Educ. 1988, 65, 871–872.
JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
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