Freezing Curves for Salol As part of our one-semester"Physica1 Science: Chemistry" course for nonscience majom, we introduce the thermodynamiw of phase transitions. Enthelpy and entropy changes are stressed here and throughout the course. An illustrative experiment formerly was the cooling and freezing of liquid Wood's metal. However, we have found that the com~ound~ h e n v salicvlate l ("salol") not only shows more interestine behavior but is ~ e r h a mare ~ s "relevant" since it [s related to-aspirin in its antipiretic andanalgesic properties an: is an ingredient in s familiar commercial medicine for intestinal upset. The convenient freezing temperature of salol (4043°C) meens that the students may eaeily and safely melt the material using warm water. Of a number of substances we tested, all of which frose a t favorable temperatures, salol way the only one which exhibited such a remarkable degree of supercooling (almost 20°C in some cases). Furthermore, its cryrtal growth is beautiful, and the students enjoy watching a droplet of the liquified compound crystallize under the microscope. We have also designed the experiment to illustrste aspects of the kinetic molecular theory. Differences in the ahilitv of water and air to transfer heat are shown by the rate of cooling of liquid salol surrounded by each of these two media.
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Experimental Procedure and Results One small test tube containing melted salol (to a depth of about 1 in.) initially a t a temperature of approximately 50% is damped in an air environment, and asecond small tube containing liquid sald initially a t the same temperature is simultaneously inserted part way in a large beaker of water a t ambient temperature. Thermometers remain in the tubes during the experiment. The students record temperature versus time until room temperature is reached, stirring gently and continually. One set of typicd student results is shown in the figure.
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I A l SURROUNDED BY AIR I B I SURROUNOEO BY WATER
Time
(minl
PATTYHALLLASWICK
Volume 49, Number 8, August 1972
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