Freezing point depression

The phenomenon in which the freezing point of a solution is lower than the freezing point of the solvent. It is also called freezing point depression. In general, when a non-volatile second substance is dissolved in a pure liquid, the vapor pressure decreases. In other words, the vapor pressure of the solution is lower than the vapor pressure of the solvent at each temperature, so the freezing point of the solution becomes lower than the freezing point of the solvent.

In the case of a dilute solution (close to an ideal solution) in which a non-volatile substance is dissolved, the degree of freezing point depression is proportional to the number of moles. If the difference between the freezing point of a solution in which W grams of solute are dissolved in 100 grams of solvent and the freezing point of the pure solvent is ΔT , and M is the molecular weight of the solute, then ΔT =kW/M holds. Here, k is a constant specific to the solvent, and is called the molar freezing point depression of that solvent. Molecular weight is often measured using water, acetic acid, benzene, naphthalene, etc. as solvents. As it is necessary to measure the temperature precisely, a Beckmann thermometer is used as the device.

[Toshihisa Yoshida]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

溶媒の凝固点よりも溶液となったときの凝固点のほうが低くなる現象。氷点降下ともいう。一般に、ある純粋な液体に不揮発性の第二の物質を溶かせば蒸気圧は低下する。すなわち、溶液の蒸気圧は各温度において溶媒の蒸気圧より低いため、溶液の凝固点が溶媒の凝固点より低くなる。

　不揮発性の物質を溶かした希薄溶液（理想溶液に近い）の場合、その凝固点降下の程度はモル数に比例する。溶媒100グラムに溶質Wグラムを溶かした溶液の凝固点と純溶媒の凝固点との差をΔT、溶質の分子量をMとすれば、ΔT=kW/Mが成立する。このときkは溶媒に特有な定数で、その溶媒のモル凝固点降下という。水、酢酸、ベンゼン、ナフタレンなどを溶媒にして分子量を測定することが多い。装置としては温度を精密に測定する必要があり、ベックマン温度計が使われる。

［吉田俊久］

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