Freezing

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In physics and chemistry, freezing is the process of cooling a liquid to the temperature (called freezing point) where it turns into a solid. Melting, the process of turning a solid to a liquid, is the opposite of freezing. For most substances, melting and freezing temperatures are equal. For example, the melting point and freezing point of the element mercury are the same. Rapid cooling by exposure to cryogenic temperatures can cause a substance to freeze below its melting point, a process known as flash freezing.

For some pure substances, such as pure water, the freezing temperature is lower than the melting temperature. The freezing point for water is only the same temperature as the melting point when nucleators are present to prevent supercooling. The melting point of water is 0°C (32°F, 273 K). In the absence of nucleators water will supercool to −42°C (−43.6°F, 231 K) before freezing. But in the presence of nucleating substances the freezing point of water is the same as the melting point. Nucleating agents, such as dust, are commonly present in the environment, which is why rain water and tap water will normally freeze at the melting point of water.

Freezing is a common method of food preservation which slows both food decay and the growth of micro-organisms and, by turning water to ice, makes it unavailable for bacterial growth and chemical reactions (see frozen food).

In biology, freezing is the reaction of an animal to a fear-eliciting situation, enabling it to remain undetected by a predator and prepare a fight-or-flight-reaction.