Salt on the roads and very low temps.

[chem geek]

Yes, salt, specifically sodium chloride, can kill plants. Both the sodium and the chloride in high concentrations are detrimental to plants, but it is the sodium that is worse. When total salt concentration is high, then this can dehydrate the plants as the salinity of the water outside (fresh-water) plant cells is higher than that inside the cells so the water leaves the cells in a process called osmosis. But even smaller levels of salt are a problem because the sodium replaces potassium and magnesium that plants need to manufacture chlorophyll. The chloride ion isn't good either, but is not as desctructive (in the same quantity) as the sodium.

So using calcium chloride would be better than sodium chloride, both because it is more effective at de-icing and because it is less harmful to plants, but it is still harmful in concentrated quantities. Urea (fertilizer) would be safer for plants, but isn't as effective at de-icing, though is somewhat comparable to sodium chloride (urea is still somewhat corrosive to non-plant materials such as some stones and concrete).

As for Cal-Hypo, it is an effective de-icer because it is rather soluable in water at around 21g/100ml (of 65% Cal-Hypo; compared to about 36g/100ml for sodium chloride and 75g/100ml for calcium chloride) and it produces 3 ions per molecule just like calcium chloride (sodium chloride produces 2 ions per molecule). This link gives some more info on anti-icing though they do not mention the use of Cal-Hypo, probably because using a chlorine product would be very corrosive and bad for the environment. Note that calcium chloride has a high heat of dilution so that it does literally melt more ice as it dissolves, not just by lowering the freezing point but by increasing temperature of the ice/water. Urea, just like ammonia, also has a relatively high heat of dilution. I'm not sure if Cal-Hypo does.

Richard