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The Basics:
Water has three states: liquid, solid, and gas. It is usually liquid from 0°C to 100°C until it becomes steam, and solid from 0°C and lower. However, when water is cooled in an enclosed container, the crystallization does not occur at its freezing point of 0°C at atmospheric pressure. Instead it is supercooled below the freezing point before nucleation occurs.

Definition:
Supercooling can be defined as the process of cooling a liquid below its freezing point without it becoming a solid mass. Supercooled water is in a state of metastable liquid, despite its temperature being below the freezing point. The metastable state ends when water reaches -38°C, at which point nucleation takes place and a plate-like ice crystal forms within the supercooled region of water. While the crystallization process is occurring, latent heat is being released from the ice crystal and absorbed by the water. Therefore, at the end of the nucleation process, the temperature of the water rises, and returns to its freezing point of 0°C.

Nucleation:
Supercooling will occur if the water contains almost no foreign substances such as dirt or dust. The formation of ice crystals depends on molecules in the liquid finding a surface to crystallize on. These surfaces are called nucleation sites. A liquid below its freezing point will crystallize if there is a presence of a seed crystal around which a crystal structure can form.

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