Why do we salt roads in the winter?

Discovery Place Science

Charlotte’s Department of Transportation will be salting key streets this Saturday in anticipation of snow on Sunday. Why do they do this? And what do icy roads and cooking pasta have in common?

It’s a salty situation of colligative proportions!

If the road and the air around an icy patch stay below the freezing point of water (32° F), the ice won’t melt and it could create a slippery hazard for drivers. That’s where salt comes into play.

Whether it’s salt on ice, sugar in tea or air freshener in a room, it’s all a solution. A solution is a mixture of two or more substances that are combined so well that it can be hard to tell a difference between the solute (stuff present in smaller amount) and solvent (main ingredient). Solutions can be in solid, liquid or gas form depending on the main ingredient.

The property that we see when salt causes ice to melt is its colligative property. That’s a whole range of characteristics about a solution that determine what will happen to a solution when you add stuff to it, such as salt or sugar. That affects things like vapor pressure (how easily a solution will evaporate), boiling point and freezing point.

In winter, we’re really concerned about changing the freezing point of that icy patch on the sidewalk or roads. For example, if the temperature of the air and sidewalk stay at 20° F, it will stay solid water—ice.

But what if you add a little salt? The salt will mix with the ice where the crystals touch the water. That tiny mix now has a freezing point that isn’t 32° F or even 20° F. It’s likely much lower at maybe 15° F. So what happens to a solution that won’t freeze at the surrounding temperature? It melts!

Those tiny little puddles spread as the salty water now mixes even more with the ice around it. Eventually, the whole section of ice becomes a slush or complete liquid puddle of salty water that won’t melt unless the weather get significantly colder.

How fast the ice melts and how low the freezing point of that puddle gets will depend on how much salt you add. It could get as low as about -6° F.

How does this relate to cooking pasta? Some cooks like to add a little salt to the water for taste and a little olive oil for health, flavor and to help the pasta not to clump together. If you’re in a hurry to cook dinner, don’t add salt or olive oil to the pot of water until after it boils. Otherwise, it would make a solution and raise the boiling point of the water, meaning you’d have to wait that much longer for it to boil.

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