According to Wikipedia, supercooling is a process of lowering the temperature of a liquid or gas below its freezing point, without it becoming a solid. However, a liquid below its standard freezing point will crystallize in the presence of a seed crystal around which a crystal structure can form. This scientific principle actually has real-world applications--for example, this has been used in freezers that cool drinks to a supercooled level so that when they are opened they turn to slush. This principle is also found in this supercooled heat pack. The heat pack consists of sodium acetate in a supercooled state in a sealed plastic pouch. There is a disk in the pouch that is clicked to stimulate the liquid to turn into a solid. The freezing temperature of the liquid is higher than room temperature, so when the liquid turns into a solid, it releases enough energy to raise its temperature towards its freezing point, which heats the heat pack very quickly, right in front of you! To supercool the liquid again, it must first be heated to the melting temperature again and allowed to cool. This is an excellent demonstration to use in science classrooms!
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