freezing, change of a substance from the liquid to the solid state. The temperature at which freezing occurs for a pure crystalline solid is called the freezing point and is a characteristic of the particular substance. The reverse process, the change of a solid to a liquid, is called melting. See melting point; refrigeration.

Temperature at which a liquid becomes a solid. When the pressure surrounding the liquid is increased, the freezing point is raised. The addition of some solids can lower the freezing point of a liquid, a principle used when salt is applied to melt ice on frozen surfaces. For pure substances, the freezing point is the same as the melting point. In mixtures and certain organic compounds, the early solid formation changes the composition of the remaining liquid, usually steadily lowering its freezing point, a principle that is applied in mixture separation. The freezing point of pure water at standard atmospheric pressure is 32°F (0°C). To change a liquid at its freezing point to a solid at the same temperature, the heat of fusion (see latent heat) must be removed.

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Method of food preservation in which low temperatures (0 °F [−18 °C] or lower) inhibit the growth of microorganisms. Used for centuries in cold regions, it was not until the advent of mechanical refrigeration in the mid 19th century that the process became widely applicable commercially. In the 20th century, quick (or flash) freezing was developed by Clarence Birdseye. Except for beef and venison, which benefit from an aging process, meat is frozen as promptly as possible after slaughter. Fruits and vegetables are often frozen in a syrup or vacuum-sealed to exclude air and prevent oxidation and desiccation.

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