Salt on the roads and very low temps.

Chem guy could you explain in a easy way why the salt the road crews usually dump on our MI roads don't work when it gets too cold? We have been in blizzard warnings the last few days and they say a big part of it, besides the blowing and drifting snow and white outs is because the salt usually dumped on the roads won't work in the low temps. I have some driveway melt stuff that must have more than just salt in it because it melted the ice on the stairs and walk way. But it wasn't cheep. Blizzard temps in west MI is unusal even with our usual lake effect snow. If the salt can't work what does? Other places used to continuous weather must do something? When I briefly lived in a farm with a wood stove we used ashes in the driveway sometimes, I think...Or maybe somone else can explain why and what they do? Thanks, Kimrst in Buurrry West Michigan

The addition of salt doesn't actually melt the ice in a direct fashion (i.e. by heating it) though some salts (such as calcium chloride) do actually produce some heat when they dilute in water. Technically, the addition of any substance that dissolves in water will lower the temperature at which water freezes (this is called "freezing point depression"). There is nothing special about using salt (sodium chloride) for this purpose. Salt is used because it is inexpensive and dissolves readily in water. So when the salt is added to snow or ice, it begins to dissolve into it and that lowers the temperature at which the snow/ice freezes so if it's not too cold, it will melt and stay as a liquid.

The amount that the freezing point of water is lowered is 1.86 C/m (3.35 F/m) which basically means that adding more of a substance to water will lower its freezing point more. Of course, only so much salt can dissolve in water so the lowest possible freezing temperture for water when using salt (sodium chloride) is -21.1C (-6.0F), but in real-world environments (i.e. for pavement or roads), the practical limit is -9C (15F). This link has a table of different de-icing compounds and their lowest practical temperatures (and pros/cons). You'll note that Calcium Chloride will melt ice down to -29C (-20F). This is the same calcium chloride you can get at pool stores to add calcium to your pool (i.e. to increase Calcium Hardness), but note that surfaces are slippery below -18C (0F). I don't know what is in your expensive compound you used on your walkway, but you are right that it could have been something that dissolves into more parts than salt in water.

Richard

Richard, thanks for the link, I bookmarked it for future reference.

There was a substance I used to use for melting ice that wasn't on the list - cal-hypo - granular chlorine. It worked like a charm in the ~warmer~ central Va climate when ice storms would coat everything with 1/4 - 1/2" ice. (I used it because I had ~ 50lbs of the stuff on my service truck, at first all I wanted was traction, but the stuff literally 'ate' the ice)
We had a gravel driveway and only a small concrete walkway to it, so I'm not sure of the damage done to surfaces it's applied to, but the grass was all gone near the walkway come spring! (however, when I lived in NH, one could see the effects of 'road salt' on the smaller pines along the interstate - the ones closest to the highway were brown in the spring (on the highway side, yet green on the other). I just wanted to mention the damage of (some) ice melters to the local flora. Any insight as to the 'killing power' of salt, et al, on the botanicals?

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

Kim, I'm in Grand Haven. Where are you? :)

I very much prefer winter as it was in December instead of this!! I saw 3 cars fly off the road on my 2 mile trip home for lunch. People don't realize that just because a road doesn't look icy, doesn't mean it isn't!

The UltraMelt I got at Meijers lists Calcium Chloride(CAS#010043-52-4) as its main ingredient with a use of 2-4 oz per square yard applied from the shaker container. It was just over $5.00 a 7.5lb jug. I also got Road Runner Ice Melt 20 lbs for $7.00 from Walmart that has Soduim Chloride first and then Calcium Chloride as its second ingredient. They both work fairly well and I sprinkle them from a big shaker or a cup and don't get the stuff on my hands as its not good. Both recommend wearing rubber gloves minimually, and washing hands after use. So I'm on the right track...

We live near Kalamazoo, MI. The weather here has been challenging to say the least. Monday on the way home from Grand Rapids we were plassed by a car flying down 131. My husband wanted to go faster too but I convinced him to slow down. The car flew by and about 5 mins latter we saw him climbing out of his car in the middle of the ditch where his car went sliding off the road. His flashers were flashing as I called 911 to have him rescued. The guy looked ok but his car was deep into the snow. DH kept his speed down the rest of the way home. It pays to be slow and carefull. :-) Kimrst