freezing point

Physical Chem.

the temperature at which a liquid freezes: The freezing point of water is 32°F, 0°C.

[1740-50]

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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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▪ chemistry and physics

      temperature at which a liquid becomes a solid. As with the melting point, increased pressure usually raises the freezing point. The freezing point is lower than the melting point in the case of mixtures and for certain organic compounds such as fats. As a mixture freezes, the solid that forms first usually has a composition different from that of the liquid, and formation of the solid changes the composition of the remaining liquid, usually in a way that steadily lowers the freezing point. This principle is used in purifying mixtures, successive melting and freezing gradually separating the components. The heat of fusion (see thermal fusion), the heat that must be applied to melt a solid, must be removed from the liquid to freeze it. Some liquids can be supercooled—i.e., cooled below the freezing point—without solid crystals forming. Putting a seed crystal into a supercooled liquid triggers freezing, whereupon the release of the heat of fusion raises the temperature rapidly to the freezing point.

      The addition of one mole (molecular weight in grams) of any non-ionic (does not form ions) solute to 1,000 grams of water lowers the freezing point of the water by 1.885° C, and this has been used as an accurate method for determining molecular weights.

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