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.

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).

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:

Where K_f is called the freezing-point-depression constant.

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.