Winter Weather Awareness Week: Rock salt, why does it melt ice?

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Winter Weather (Photo Credit: Associated Press)

(WVNS) — Every year, from the interstate all the way down to your front porch, you’re throwing salt or some type of pellet down all winter long. But why does tossing all these rocks and pellets help? Have you ever really sat back and wondered what’s going on behind the scenes? Well, it all has to do with some basic chemistry.

When ice forms, on the surface there is a thin film of water that stays in its liquid form. This is why ice is slippery, but it is also why rock salt can help to melt ice. Salt, in this Sodium Chloride, is soluble in water, meaning it can be dissolved in water. Even though the layer of water is super thin, the salt begins to dissolve, and as it does, it separates into two different ions, one positive (Na+) and negative (Cl-).

These ions will bond with the positive and negative ends of the water molecule, a process that releases heat, thus thawing a bit more of the ice. This process happens hundreds of thousands of times over and eventually the ice melts! The ice then stays melted, since saltwater has a lower freezing point than plain water.

This process also works with other types of salts, rock salt (sodium chloride) is one of the most common forms of ice melt used across the country, but other types like calcium chloride are also effective at melting ice. Calcium chloride in many cases is also cheaper than normal rock salt and can lower the freezing point of water, even further than rock salt.

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