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Chemistry Explains Why We Use Salt on Ice in Wintry Weather

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Welcome to January, when the worst of the winter weather usually occurs, leaving cold snow and ice in its wake. That means that many city and state agencies will be using salt to melt ice on the roads, but how does salt melt ice?

Thanks to chemistry, now we know. Salt doesn't melt ice. That's right, there is absolutely no melting going on. Instead, salt creates a chemical reaction inside ice. This video by the American Chemical Society explains the process.

As we know, the freezing point of water is 32 degrees Fahrenheit. At this temperature, water's molecules organize themselves into crystal structures, becoming ice. Salt, though, interrupts this crystallizing process because it breaks up into separate molecules of sodium and chloride when it hits water.

Sodium and chloride move throughout the water and take up space within the structure of molecules in the water. This pushes the water molecules apart and disrupts the ice-forming process. This, in turn, lowers the freezing point of water.

However, if temperatures get below 16 degrees Fahrenheit, salt is useless. There are also some drawbacks to using it. When the weather gets cold in the U.S., we use over 20 million tons of salt a year on our roads. However, once the salt has done its magic, it still stays on the roads. The chloride molecules, which remain separated after exposure to water, often have a negative effect on the environment and are corrosive to metal, as well as the wheels of cars.

This is why sand is often used instead of salt, but sand isn't as effective and it takes a lot more sand to de-ice roads than salt.

When temperatures drop below 16 degrees, there are chemicals that work well for de-icing, particularly potassium acetate, which is effective for temperatures as low as -75 degrees Fahrenheit. However, these chemicals also affect the environment and cost a lot more money than simple salt.

Considering that much of the U.S. is now suffering from severe wintry weather, it's a sure bet you'll see salt on the roads. Now you'll understand the hows and whys of why we use salt this time of year.

[Photo Credit: Alexis O'Toole/Flickr]

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