9.4.1 Supercooled Melt

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Photo 1: Experimental setup: 1-chlorine-2-nitrobenzene is melted in a test tube furnished with a temperature sensor. According to the literature, the melting (or solidification) point of 1-chlorine-2-nitrobenzene is 35.4°C. In our experiment, however, we have reached this temperature without yet having it solidify.
Photo 2: At 28.8°C, there is still no sign of solidification. We have now produced a supercooled substance. 
Photo 3: At 23.2°C, the first crystals form, 12.2°C below the actual solidification point.
Photo 4: The heat energy released as the substance crystallizes causes the temperature to rise to 31.2°C.

A supercooled melt is a metastable state of a (usually very pure) substance that remains in this state of aggregation, even in the technically forbidden temperature range below the solidification point. Consequently, at equal pressure, a supercooled liquid (or melt) will have a lower temperature than that of its aggregate state. However, the smallest addition of impurities -- or just physically jarring the substance -- leads to a release of the melting enthalpy and the substance spontaneously crystallizes.
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