Temperature Limits on deicers-why?

While following news coverage on the Puget Sound snowstorm, one report on road cleaning efforts claimed that chemical deicers used by the roads agency wouldn’t work below 15 degrees F. My question, assuming the claim is true, is why a deicer has such a small temperature limit. Are there other deicers with larger ranges, and if so why would you choose a deicer with a smaller range?

By “larger range” I am assuming you mean works in colder temperatures? I can’t imagine something that generates no heat melting ice below 15 degrees, although I guess it’s possible. I know that in most of the US they salt the roads because it’s cheaper than using chemicals. Sometimes when prolonged subzero temps are expected, they use sand, which doesn’t melt but unlike salt it does provide traction.

I’m not sure what you expect. Deicing compounds work because salty water freezes at a slightly lower temperature than fresh water, so if you drop some salt on a chunk of ice, if it’s not too cold you’ll end up with some salty water.

But there’s a limit: salt water will freeze, too if it gets cold enough. Some kinds of salts can lower the temperature a little more than good old sodium chloride, but only so far. If it gets cold enough, water will be a solid; that’s just chemisty and physics. And alternatives to ordinary sodium chloride are expensive.

By the way, sand isn’t really considered state-of-the-art these days. Absolutely useless at providing traction at even moderate speeds (might help on a driveway, but not a highway), and it’s a pain to clean up.

I thought roads were salted with calcium chloride at least as much as NaCl.

Is there a limit to how cold a water solution can get? I could imagine it having to do with the latent heat of fusion.

Surprised nobody has mentioned other deicers that easily work at colder temperatures (not necessarily road deicers but still). I have used chemical (alcohol-based) deicers on my windshield that are rated to 0 F or even below zero.

Start with a beaker of liquid helium.
Add atoms of water, until such point as you are willing to call it a solution.
Unless that point is some ridiculously high number like 1 or 2% H[sub]2[/sub]O, you will have a very cold solution of water. Of course, over time any water you add to liquid helium will crystallize out. In the meantime, you have a very cold solution.

Freezing point depression in water depends on the concentration of the added solute, but not much on the identity of the solute. Ethanol, or urea will work just as well as ethylene glycol, iff you can get their concentrations as high as you can ethylene glycol. Freezing point depression is thus one of the Colligative Properties.
When an antifreeze is said to be good only to 15°F, that’s a measure of the stuff’s probable concentration as dispensed on the sidewalk, or airplane wing, or whatever.

Our county highway department has different chemical formulas for different temperatures, and they include sand in the mix for some purposes. The highway dept guy makes it sound like a science.

Now if only the weather forecasting could be accurate enough so they know which formula to use.

I know many places use waste molasses left over from beet sugar production as an additive. It’s supposedly cheaper and less corrosive/toxic than the various salts they use.

Our local highway department has said in the past that calcium chloride costs about 10X as much as road salt, so they keep only a small supply and use it sparingly.

Sure, but eventually you get to temps cold enough that they refreeze almost immediately, too. It’s kind of pretty to watch - sort of like seeing Jack Frost do his work - but it ends up being more of a pain in the butt than simply scraping the ice/frost off would have been. This is probably a problem most every where temps drop below -10F, which is about where I’ve noticed this happening.