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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In physics and chemistry, freezing is the process whereby a liquid turns to a solid when cold enough. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is almost the exact opposite of freezing. All known liquids undergo freezing when the temperature is lowered with the sole exception of helium, which remains fluid at absolute zero and can only be solidified under pressure. For most substances, the melting and freezing points are the same temperature, however, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F); this process is known as thermal hysteresis.
Freezing resistance of winter annual and biennial legumes at different developmental stages. (Crop Ecology, Production & Management).(Abstract)(Statistical Data Included)
Mar 01, 2002; EXPERIMENTS IN MANY COUNTRIES have shown that the use of cover crops has many potential benefits. Examples of such benefits are...