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Supercooling to the Max

By Nüsret Hisim

Water is an obvious choice when studying phase change because students regularly encounter phenomena that involve water. While data-collection technology helps students visualize what is happening to the temperature as water cools, the process occurs through a blurry, ice-filled beaker. For my students, the temperature increase of the ice/water mixture in the test tube as it was stirred relative to the external ice water/salt bath was the most surprising aspect of supercooling. This is an interesting aspect of the experiment but doesn’t get to the heart of understanding phase change.

Recently, a colleague and I were pondering alternative ways of doing a phase change experiment. We rejected common alternative solutions such as para-dichlorobenzene and acetamide because of safety concerns, and I vaguely recalled the long-past ChemStudy movies that used phenyl salicylate in a variety of demonstrations. Phenyl salicylate is an attractive alternative because it is relatively inexpensive, there are fewer safety concerns, and the freezing temperature is 41.5°C (106.7°F).

Graph showing cooling curve for phenyl salicylate.
Cooling curve for phenyl salicylate

To investigate the viability of studying phase change with phenyl salicylate, I warmed a test tube of solid phenyl salicylate in a hot water bath and inserted a Go Direct Temperature Probe when the phenyl salicylate liquefied in the test tube. I started data collection after the test tube and probe reached the temperature of the warm water bath. After removing the test tube from the warm bath, I placed it in a cold water bath to increase the rate of cooling. Once the temperature dropped to near room temperature, I removed the water bath and dropped a seed crystal in the test tube. The crystallization of the liquid started immediately.

I was surprised at how low the temperature dropped while the material remained liquid. With a melting point of 41.5°C, I was expecting a small drop below this value while I moved the probe up and down a little to minimize supercooling. However, in one trial the drop was all the way to room temperature and the material could only be made solid with a seed crystal.

The beauty of using phenyl salicylate is that while the crystals formed, I could watch the temperature increase on the graph screen and also feel the temperature change when I touched the surface of the test tube in front of me. This type of experiment is a quick and easy way to help students make connections that they don’t normally see when they use water as a solution to study phase change.

It should be noted that is not the same process as with sodium acetate and water. The temperature change observed when sodium acetate solutions crystalize after reaching room temperature is a result of the supersaturation of the solid in the waters of hydration of the salt. The effect seen with phenyl salicylate is completely due to change in phase of the pure substance.

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