lsopentanol in an Aqueous Environment When isopentanol ((CH&CHCH2CHzOH) is added to a shallow layer of water, a number of interesting phenomena can be observed. The addition of the first few drops of the alcohol to the water will cause turbulence on the surface and may even leave a dry spot in the dish holding the water. This is a fairly familiar occurrence which is, of course, caused by the difference between the surface tensions of the two liquids. As more isopentanol is added, and its concentration in the water builds up, the rate a t which it dissolves decreases. This causes the formation of irregularly shaped globules of the alcohol, which violently thrash about on the surface of the water until they dissolve. This motion, too, is due to differences in surface tension around the perimeter of each dissolving globule. As soon as the water is saturated with isopentanol, excess of the alcohol takes the form of perfectly circular, rather than irregular globules. As the dissolving stops, the thrashing also ceases. If a drop of glycerine is placed upon a circular globule of the alcohol, the globule breaks up into many smaller particles which go shooting outward in all directions. This action can be explained by realizing that the polarity of the two substances differs (as is evidenced by their immiscibility). Since the glycerine is much more viscous than the a k a hol, the easiest course of separation is for the isopentanol to be dispelled from the mixture (rather thanvice versa). It should be noted that the shallowness of the water is particularly important for the success of this experiment. If the water is too deep, the glycerine may just settle beneath the floating globule of isopentanol, and have little or no effect on it. Only if the water is sufficiently shallow will the two substances come in close enough contact for such a spectacular separation to occur. Several drops of carbon tetrachloride can be placed in a n area highly populated by the alcohol globules, which, in passing, are readily drawn into the carbon tetrachloride, owing t o the miscibility of these two substances. This gives the impression that the carbon tetrachloride globules "grow" as they "eat" the alcohol globules. Finally, if a drop or two of glycerine is added near the inner edge of a fairly large isopentanol-carbon tetrachloride globule, it appears to he rejected from the mixture. The apparent rejection is the result of the nonpolar molecules of carbon tetrachloride and isopentanol moving away from the more polar, more viscous glycerine. Again, the depth of the water is crucial. The series of phenomena just described can and have been used as classroom demonstrations. From the preeeding paragraphs, it is seen that surface tension, molecular polarity, and saturation of solutions can all be discussed in reference to them. It has been found quite convenient to perform these experiments on an overhead projector so an entire class may readily look on. Proper water depth can be obtained by using 10 ml of water in a Petri dish which is 9 em in diameter. Coloration of the alcohol with iodine is necessary to make it readily visible. This has the added advantage in that, as the alcohol dissalves in the carbon tetrachloride, the iodine can be seen to change from its brown color (in the alcohol) to its purple color in carbon tetrachloride. Elmont Memorial High School Elmont, New York 11003
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/ Journal of Chemical Education
Mary Duffy Kenneth Leopold
