Freezing Point Depression in Solutions

The freezing point of pure water is 0°C, but that melting point can be depressed by the adding of a solvent such as a salt. The use of ordinary salt (sodium chloride, NaCl) on icy roads in the winter helps to melt the ice from the roads by lowering the melting point of the ice. A solution typically has a measurably lower melting point than the pure solvent.

纯水的凝固点是0°C，但加入溶质如盐后，其熔点会降低。冬季在结冰的道路上撒普通盐（氯化钠，NaCl），有助于通过降低冰的熔点来融化道路上的冰。通常来说，溶液的熔点通常比纯溶剂低得多。

The following figures were found in a published report, but have not been checked out in detail. A 10% salt solution was said to lower the melting point to -6°C (20°F) and a 20% salt solution was said to lower it to -16°C (2°F).

以下数据来自一份已发表的报告，但未详细核实。10%的盐溶液被称能将熔点降低到-6°C（20°F），而20%的盐溶液则被称能将熔点降低到-16°C（2°F）。

A more formal treatment of freezing point depression is given by Ebbing. The freezing point depression ΔT_f is a colligative property of the solution, and for dilute solutions is found to be proportional to the molal concentration c_m of the solution:

Ebbing 对凝固点降低进行了更正式的论述。凝固点降低 ΔT<sub>f</sub> 是溶液的依数性质，对于稀溶液而言，发现它与溶液的molal浓度 c<sub>m</sub> 成正比：

其中 K_f 称为凝固点降低常数。

A pleasant application of the freezing point depression is in the making of homemade ice cream. The ice cream mix is put into a metal container which is surrounded by crushed ice. Then salt is put on the ice to lower its melting point. The melting of the solution tends to lower the equilibrium temperature of the ice/water solution to the melting point of the solution. This gives a temperature gradient across the metal container into the saltwater-ice solution which is lower than 0°C. The heat transfer out of the ice cream mix allows it to freeze.

利用凝固点降低的一个愉快应用是在制作自制冰淇淋时使用。将冰淇淋混合物放入一个金属容器中，容器周围放上碎冰。然后在冰上撒盐以降低冰的熔点。溶液的熔化趋势会使冰/水溶液的平衡温度降低到溶液的熔点。这会在金属容器中形成一个温度梯度，温度低于0°C，进入盐水-冰溶液。热量从冰淇淋混合物中传出，使其冻结。