Experiment 6: Supercooled Chemical Melt

Fill a clean test tube half to three quarters full with sodium thiosulfate. Hold over flame or melt in a double-boiler (described in Growing Large Crystals with a Seed Crystal experiment). This salt melts at about 48.5°C (119°F). When all the solid has turned into a colorless, watery liquid, remove from heat and cap the test tube. Carefully place the capped test tube on a test tube stand and allow to cool in an undisturbed location. If all goes well the melted salt will cool to room temperature and remain in a liquid state. This is a supercooled liquid, that is, a chemical that remains a liquid far below its freezing point. Measure and record temperatures during melting and cooling. Be certain the thermometer is very clean and do not stir the melt while measuring.

Now, if you uncap the test tube and add even a tiny particle of dry sodium thiosulfate, watch what happens. Crystallization immediately begins around this "seed" crystal. What is the temperature now? This rapid crystallization process produces a considerable amount of heat. Record temperatures during crystallization.

When conditions are right, some liquids or solutions can be cooled below their freezing point (for water = 0°C). Supercooling is that strange state when a liquid remains liquid at temperatures below its freezing point. (Sometimes water may remain unfrozen at temperatures as low as -20°C!)

Solids, like ice, form a definite crystalline structure upon freezing. For example, ice crystals have a regular hexagonal (six sided) arrangement of water molecules. However if a cooling liquid or solution is super clean it can't form that first crystal needed for nucleation. When nucleation sites such as gas bubbles, dust particles, seed crystals, or surface irregularities in the container holding the liquid are absent, crystallization may be prevented until the temperature is finally so low that crystals form spontaneously.

Once crystals start to form in a supercooled liquid they will grow very rapidly. By adding a single grain of sodium thiosulfate to the supercooled melt, crystallization proceeded very rapidly. In addition, there was a considerable amount of "heat of crystallization" generated.