An Apparent Exception to the John David Worley University of Oklahoma Norman
First Law of Thermodynamics A demonstration involving liquid diffusion
Introductory courses in chemistry often define a solution as a homogeneous molecular mixture of two or more substances. The key words in this definition (which excludes such things as collodial suspensions and two phase systems) are molecular and homogeneous. I n general one determines whether a system conforms to the above definition by simply observing it with the eye. This demonstration takes advantage of the eye's limited ability to distinguish between a truly homogeneous molecular mixture and a nonhomogeneous molecular mixture. It leads to an apparent repudia tion of the first law of thermodynamics and to a striking illustration of the slowness of liquid-liquid diffusion. It also demonstrates that thermodynamics does not include time as a parameter in describing the equilibrium process. The demonstration may best be performed in two separate stages. A simple U-tube is clamped into an upright position on a ring stand. Carbon tetrachloride is poured through one arm of the U-tube until liquid columns two or three centimeters high are formed. Benzene is then carefully added to one arm of the U-tube using about one-half the original volume of carbon tetrachloride. Benzene and carbon tetrachloride are completely miscible in all proportions when shaken together or stirred. However, if the benzene is added to the CCI, with no stirring or shaking, two phases will form because of the large difference in the densities of the two liquids (benzene at 20", 0.879 g/ml; carbon tetrachloride at 20°, 1.595 g/ml). After one or two hours have passed and the two components have undergone some mutual diffusion, the interface between the two phases will no longer be visible. The result will be a fluid which to all ap-
pearances is completely hon~ogeneous,but defies the laws of gravity by having liquid columns of unequal height. It is a simple matter to propose to t,he class that the liquid might be siphoned from the higher to the lower liquid column, with the cycle always regenerating itself. The apparatus would then be functioning as a perpetual motion machine of the first order and would constitute a denial of the first law of thermodynamics. The fallacy in the construction of such a machine is readily seen by the second stage of t,he experiment, demonstrating the slowness of the mixing process unaided by shaking or stirring. Carbon tetrachloride is added in the same manner as in the previous case. Then benzene which has been colored red by the addition of a crystal of iodine is added to one arm of the U-tube. This permits the class to follow the diffusion phenomena after the interface has disappeared and demonstrates that the fluid in the perpetual motion machine could never regenerate the cycle because the liquid is not homogeneous. When the system has attained a state of equilibrium the liquid inside the tube will have a uniform color and the heights of the liquid columns will be equal. The class mill be surprised to find that before this happens some two or three weeks may have elapsed. An interesting variation especially adaptable to display cases and chemical circuses is to set up the apparatus as in the first stage of the demonstration. Then suspend a bar magnet over the higher liquid column. The resulting "magnetic" fluid should attract a large curious audience. The author wishes to express his gratitude to Professors S. D. Christian and H. E. Affsprung for their very helpful suggestions in writing this paper.
Volume 42, Number 6, June 7 965
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