Both #NaCl# and #CaCl_2# are used to melt ice on roads and sidewalks in winter. What advantages do these substances have over sucrose or urea in lowering the freezing point of water?
1 Answer
Both salt and calcium chloride are cheap. In terms of colligative properties (i.e. freezing point depression), they also give more bang for the buck, because they give more than the one particle in solution.
Explanation:
Sucrose is a molecular species. It has a higher molecular mass than do the salts, and thus it would not decrease the melting point as much as the others. Sucrose would also probably encourage vermin (rats and mice are things that I really find unacceptable!). Urea is also highly water soluble (more so than the salts), so it probably would not stick around as much grit salt. On other hand, when grit salt is deployed, it gives 2 ions to decrease the melting point; calcium chloride gives 3 ions.
I note that the downside of salting the roads, is that when cars use the roads they often splash salty water onto their chassis. This is the reason why you see so many relatively new cars on North American roads that are absolute rust buckets - you look at some of them and the only thing holding them together is the rust. Salt on the chassis facilitates corrosion.
