overhead projector demonstrations Solubility of Alcohols Walter H. Corkern and Linda L. Munchausen Southeastern Louisiana University Hammond, LA 70402
One of the topics generally discussed in introductory organic lectures is the solubility in water of organic com~ ksunds, surh as alcohols. Rrcn;~semost o r g a ~ c i i q u i dare colorless, it is somewhat difficult to demonstrate solubility for any other than nonpolar compounds (which,when added to water, give two obvious layers). It is more difficult to demonstrate relative solubilities of low molecular weight polar organic compounds. One of the generalities of science is that properties such as solubility change gradually from one extreme (soluble) to the other (insoluble).This gradual progression seems to be a difficult concept for some students. The Nature of Solubility Textbooks usually state that alcohols and other polar organic compounds are soluble a t low molecular weight but gradually become insoluble a s the length of the carbon chain increases. A s a "rule of thumb" students are told that polar, hydrogen-bonding compounds of five carbons or less are soluble (I).Students oRen assume that there must be a sharp change in solubility between C-5 and C-6 compounds. However, if one looks a t the solubility data for alcohols, amines, or carboxylic acids of 1-10 carbons, this is not the case. Demonstrations can be conducted using C-4 to C-10 straight chain alcohols to emphasize the gradual nature of the change in solubility. (Amines or carboxylic acids also can be used, but the demonstration should be conducted in a fume hood or well-ventilated area if such malodorous compounds are used.) Demonstrations and Obse~ationS In order to increase visibility, a small amount of Oil Red EGN dye (Mathesou Coleman & Bell) is dissolved in each of the alcohols to be tested. Each alcohol is then added one drop at a time to a Petri dish about half filled with water. The demonstration is observed easily by large groups of students by putting the Petri dish on an overhead projector and displaying the image on a screen. As expected, lower molecular weight alcohols (C-4 and C-5) dissolve rapidly, and the dye disperses over the surface (Oil Red is not water soluble). The C-10 compounds produce a surface layer immediately; then no further changes are observed. The most interesting demonstration is with the C-6 and C-7 alcohols, the effect being most pronounced for heptanol. Initially, there is rapid movement on the surface and the drop breaks into smaller drops, which appear to be spinning and moving rapidly in all directions. Observation under a microscope shows the water climbing up on the edge of the drop and then falling off. These observations are consistent with a slow, equilibrium-like dissolution. which would be intermediate be-
Presented in part at the 65th Annual Meeting of the Louisiana Academy of Sciences, February&1991.
928
Journal of Chemical Education
edited by
DORISKOLB Bradley University Peoria, lL61625
tween soluble and insoluble. The nature of the motion is also consistent with the changes in surface tension that would be expected because of the changing concentration of the solution near the drop. Interestingly enough, the movement of the drops is not affected by a magnet or a charged plastic rod, which indicates that it is not a static electricity effect. Literature Cited R. T;Boyd,R.N. 01gonie Ch
