In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb
Turbulent Motion in Ethyl Acetate–Water System
Bradley University Peoria, IL 61625
Jamil Ahmad Department of Chemistry, University of Botswana, Private Bag 0022, Gaborone, Botswana;
[email protected] Several demonstrations are based on interesting phenomena observed when certain liquids are added to the surface of water. Two drops of carbon tetrachloride placed near each other on the surface attract each other and finally coalesce to form one droplet (1). A few drops of 1-octanol placed on the surface of warm water randomly move about, but repel one another (1). A drop of 1-heptanol added onto the surface of an ethanol–water mixture moves about rapidly forming patterns similar to fractals (2). If a few drops of cyclohexanol are deposited on the surface of a solution of potassium dichromate in dilute sulfuric acid, they exhibit rhythmic contraction and expansion in an amoeba-like fashion (3). When the surface of water is covered with a layer of talc powder and touched at the center with a minuscule drop of oleic acid, the layer of talc cleaves instantaneously, giving rise to interesting starshaped patterns (4 ). If liquids that are immiscible with water are poured onto the surface of water, one of three things can happen. The liquid will not spread over the surface, or it will spread to give a film of uniform thickness, or it will spread to a layer one molecule thick (a monolayer) and the rest of the liquid will collect as a lens (5). Carbon disulfide does not spread at all, oleic acid spreads to a uniform thickness, and ethyl acetate will form a monolayer with excess remaining as a lens. The behavior is determined by the values of the surface tension of the two liquids and the interfacial tension between them. If the liquid forms a monolayer, as does ethyl acetate, and the monolayer loses any material through evaporation, it would be replenished by liquid spreading from the lens. The demonstration described below shows this process graphically. Materials Overhead projector Petri dish, 10-cm diameter Two 10-mL graduated cylinders 10 mL of distilled water 4 mL of ethyl acetate A crystal of iodine (optional) Procedure Place 10 mL of water in a 10-cm diameter Petri dish and set the dish on the stage of the overhead projector. Add 4 mL of ethyl acetate to the dish at a point away from the center and near the edge. Within a minute or so, an image of a turbulent motion appears on the screen, at first at a few centers that eventually grow and organize themselves in a line. The image of the line of turbulence is quite striking and resembles a front of dancing flames slowly progressing toward the side where ethyl acetate was added. 1182
If so desired, a crystal of iodine may be dissolved in the ethyl acetate to impart color before the experiment is started. The color makes it easier to see the amount of ethyl acetate progressively decreasing as the evaporation proceeds. The line of turbulence is seen to follow the boundary between the liquids as it recedes. The process goes on for several minutes, until all the ethyl acetate has evaporated. If more ethyl acetate is continually added to replenish the amount evaporated, the duration of the process can be extended. To show that it is not evaporation alone that causes this phenomenon, a control Petri dish with 4 mL of ethyl acetate (but no water) may also be placed on the projector close to the first dish. No turbulence is observed in the dish without water, even though all the ethyl acetate evaporates eventually. Explanation The bulk of ethyl acetate remains at the side of the Petri dish where it is deposited, while a small amount spreads in the form of a monolayer on the surface of water. Evaporation causes loss of the material both from the bulk and the monolayer. To make up for the material lost from the monolayer, fresh ethyl acetate spreads from the bulk. The role of surface tension (or more precisely, the surface tension gradient) can be discussed when the demonstration is performed. What energizes this transport is the change in surface tension that accompanies evaporation. When we first add the ethyl acetate, the surface tension of water decreases as it gets covered with the monolayer; later, as the monolayer evaporates, it increases again. This increase in surface tension upsets the equilibrium in the system, which tries to restore itself by the transport of the material. However, as soon as ethyl acetate gets transported to the monolayer region, it evaporates and the process goes on continuously. In addition to surface tension, viscosity also plays a part in what is observed. If a layer only one molecule thick were spreading to replenish what was being lost to evaporation, the movement would not be visible on the screen. However, because of the viscosity effects, the spreading layer drags along with it underlying layers of the liquid as well, and this movement of the bulk of ethyl acetate can be seen on the screen. This is an example of the Marangoni effect (6–8), namely the carrying along of the underlying bulk of the liquid by a spreading surface film. Some other examples can be mentioned to students. One is the formation of tears of wine in a glass. This arises because the evaporation of alcohol from the wine sticking to the container wall results in a local increase in the surface tension of the alcohol–water solution, causing the surface layer to flow to the region taking along with it some bulk material. The liquid eventually comes down in the form of drops or tears. Another example is what is commonly observed near a
Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu
In the Classroom
kitchen sink. If a drop of a detergent solution falls on the middle of a clean but wet plate, the water recedes to the sides. Here again, the decreased surface tension caused by the detergent causes a surface layer to move toward the sides (where the surface tension is still high), carrying away with it the underlying bulk liquid as well. Hazards Ethyl acetate is a volatile and flammable liquid. Since this demonstration brings it in proximity to the hot bulb of an overhead projector, caution must be exercised.
Literature Cited 1. Prall, B. R. J. Chem. Educ. 1991, 68, 592. 2. Corken, W. H.; Holmes, L. H. Jr.; Higgenbotham, N. A. J. Chem. Educ. 1992, 69, 1025. 3. Kolb, K. E.; Kolb, D. J. Chem. Educ. 1989, 66, 955. 4. Ahmad, J. J. Chem. Educ. 1992, 69, 1029. 5. Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley Interscience: New York, 1990; p 109. 6. Adamson, A. W. Op. cit.; p 117. 7. Ahmad, J.; Hansen, R. S. J. Colloid Interface Sci. 1972, 38, 601. 8. Ahmad, J. J. Chem. Educ. 1975, 52, 534.
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