Freezing Point Depression

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This is a photo of damp brick pavement on which a white crystalline material has been spread.
Figure 1. Rock salt (NaCl), calcium chloride (CaCl2), or a mixture of the two are used to melt ice. (credit: modification of work by Eddie Welker)

Freezing Point Depression (OpenStax Chemistry 2e)

Solutions freeze at lower temperatures than pure liquids. This phenomenon is exploited in “de-icing” schemes that use salt (Figure 1), calcium chloride, or urea to melt ice on roads and sidewalks, and in the use of ethylene glycol as an “antifreeze” in automobile radiators. Seawater freezes at a lower temperature than fresh water, and so the Arctic and Antarctic oceans remain unfrozen even at temperatures below 0 °C (as do the body fluids of fish and other cold-blooded sea animals that live in these oceans).

The decrease in freezing point of a dilute solution compared to that of the pure solvent, ΔTf, is called the freezing point depression and is directly proportional to the molal concentration of the solute

where m is the molal concentration of the solute and Kf is called the freezing point depression constant (or cryoscopic constant). Just as for boiling point elevation constants, these are characteristic properties whose values depend on the chemical identity of the solvent. Values of Kf for several solvents are listed in Table 1.

Boiling Point Elevation and Freezing Point Depression Constants for Several Solvents

SolventBoiling Point (°C at 1 atm)Kb (ºCm−1)Freezing Point (°C at 1 atm)Kf (ºCm−1)
water100.00.5120.01.86
hydrogen acetate118.13.0716.63.9
benzene80.12.535.55.12
chloroform61.263.63−63.54.68
nitrobenzene210.95.245.678.1
Table 1. Source: OpenStax Chemistry 2e

Source:

Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/books/chemistry-2e

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