In the Classroom edited by
applications and analogies
Ron DeLorenzo Middle Georgia College Cochran, GA 31014
The Membrane Analogy for Surface Tension in Liquids Francisco J. Arnáiz Laboratorio de Química Inorgánica, Universidad de Burgos, 09001 Burgos, Spain The concept of surface tension is usually introduced by referring to liquids and comparing their surface with a thin film that requires some effort to be perforated. That surfaces have special properties is easily assumed by students, since it is obvious that surface particles interact with the surroundings in a different way than those inside. However, students frequently have trouble envisioning the surface of a liquid to be like a thin membrane. (This is probably because common macroscopic membranes present a behavior different from that of liquid membranes—after perforation they are not spontaneously regenerated. However, it is also important that students realize that continuous membranes can be made discontinuous and rapidly restored.) To alleviate this trouble and to reinforce the membrane analogy for surface tension, the following experiments, easily performed with kitchen products, are suggested. Take a container (basin, bowl,…) and cover it with a piece of thin polyethylene film—such as that used to protect foods—so that it remains firmly in place. (If necessary, wrap the container with the film.) Let fall, parallel to the film, a sharp-pointed knife from a height of several centimeters (Fig. 1a). The knife will lie on the film (Fig. 1b) because the pressure exerted on the film (weight of the knife per unit surface area on which it strikes) is very low. Remove the knife and let it fall again, from a similar altitude, but perpendicular to the film (Fig. 2a). The knife penetrates the film immediately (Fig. 2b) because the pressure applied on a small part of the film is now very high. Break some spaghetti into pieces 1–2 cm long. Take a piece and release it on a vessel filled with water, parallel and close to the surface (Fig. 3a). The piece of spaghetti will probably lie on the surface (Fig. 3b). In case it sinks, try it again with another dry piece. Repeat the above operation by releasing a piece of spaghetti from a similar distance, but perpendicular to the surface of the water (Fig. 4a). Now it sinks (Fig. 4b).
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Figure 1. The knife does not break the film.
Figure 2. The knife perforates the film.
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Figure 3. The piece of spaghetti stands on the surface of the water.
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Figure 4. The piece of spaghetti penetrates the surface of the water and sinks.
Of course, polyethylene film may be replaced by aluminum film. If an appropriate knife or appropriate spaghetti is lacking, similar meaningful experiments can be performed with other common products. For example, let a potato fall on the film from different heights until the film is broken. Compare the results with those obtained from releasing a lentil (or a grain of rice) onto a vessel with water so that the object floats or sinks depending on its size, the manner of contact, and the altitude from which it was dropped (a range of 5–50 mm from the surface of water is appropriate).
Journal of Chemical Education • Vol. 74 No. 11 November 1997
