Why do we salt roads in the winter?

Discovery Place Science

Salt Truck

Every winter, Charlotte only sees an average of about 4 inches of snow per year, but we also receive the most ice per year.

What happens when Charlotte weather calls for black ice, snow and slippery roads? Well, it’s a salty solution!

In anticipation of winter weather, Charlotte’s Department of Transportation will spray a wet mixture of two types of salt — sodium chloride and magnesium chloride — along with water to make a "salt brine" on key streets. This method is called anti-icing, or pre-wetting, and is a technique used by many U.S. cities as preventative measure before inclement weather arrives.

Once there is an accumulation of precipitation, you may also see salt trucks spreading hard rock salt over snowy and icy roads.

So why do they do this? Why does salt melt ice and snow?

Salt causes a phenomenon called freezing point depression, which means it lowers the freezing point of water. Water normally freezes at 32 degrees Fahrenheit, but when you add salt, that threshold can drop as low as 20 degrees Fahrenheit. This means if a snow storm in the area has an outside temperature of 30 degrees Fahrenheit, surfaces treated with a salt solution would only freeze if the temperature outside dropped another 10 degrees. If you sprinkle salt on ice, it will melt for the same reasons.

As the ice melts into puddles of salty water, it will spread to the surrounding roads, mixing with other nearby ice. Eventually, the whole surface of ice will become a salt slushy that won’t melt unless the temperature outside drops significantly. How fast the ice melts and how low the freezing point of that puddle gets will depend on how much salt you add.

Just as salt lowers the freezing point of water in cold temperatures, it also raises the boiling point in hot temperatures. You can see this principal in action when cooking pasta.

With a little help from science, you can now enjoy your snow days with safely salted walkways and a steamy pot of pasta!

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