Which keeps streets from freezing better?

Question

Would an equal number of sodium chloride, NaCl, or calcium chloride, CaCl2, do a better job of keeping streets from freezing? Explain at a molecular level. Why do most highway departments use both chemicals?

Answer 1

It is really controlled by Raoult's Law, in which the number of ions in solution depress the melting point and raise the boiling point of the solvent.

So, one mole of CaCl2 would be more effective than one mole of NaCl, because the calcium chloride would make three moles of ions (Ca+2 plus two Cl-), while the sodium chloride makes only two moles of ions (Na+ plus Cl-).

Answer 2

no.
Sodium chloride crystals attract birds and mammals, which can contribute to road kills. Sodium-deficient wildlife sometimes travel great distances to ingest road salt. Many animals tend to overshoot their salt deficit and then drink salty snow melt to relieve thirst, which increases salt toxicity in blood and tissues.([xxi])

In contrast, magnesium chloride and calcium chloride deicers do not attract wildlife since the main chemical attractant is sodium. Acute toxicity tests show that there is slight oral toxicity of the chloride deicers to small animals

calcium chloride may contribute to the mobilization of trace metals from the soil to surface and groundwater, though field evidence is limited. The chloride-based deicers have the potential to increase the salinity of the rivers, streams, and lakes. Since the dilution of deicers from the roadways to nearby streams is estimated to range from 100 to 500-fold, salinity increases are only likely to occur in slow-flowing streams and small ponds.

Answer 3

Actually the streets in Vail CO have underground snow melt control - which is a series of pipes run under the concrete that has hot water going through them which prevents ice and snow buildup.

Answer 4

one is used to keep the streets from freezing and the other is used to melt the ice and snow already on the streets...