Buzz
Numerous cities and municipalities rely on rock salt to clear ice from their roads during inclement weather. Image credit: Rupert Hitchcox Photography Limited/Getty ImagesIf you live in an area prone to heavy snow and ice, you're probably well-acquainted with road salt. Your local government most likely deploys de-icing trucks that spread salt across highways, streets, and sidewalks to help melt the ice. But what is it about salt that makes it so effective at melting ice?
Road Salt vs. Table Salt
Road salt is essentially halite — rock salt — which is the natural form of table salt (NaCl). The primary distinction is that table salt undergoes an extensive purification process, whereas rock salt retains impurities, giving it a brown or gray hue.
Some cities opt for alternative salts, such as potassium chloride or calcium chloride, to keep their roads free of ice.
How Salt 'Melts' Ice
Ice forms when the water temperature drops to 32 degrees Fahrenheit (0 degrees Celsius), including the ice that forms on roads. Road salt lowers the freezing point of water through a process known as freezing point depression.
When salt is added to water, it lowers the freezing point, making it harder for the water to freeze. For example, a 10-percent salt solution freezes at 20 degrees Fahrenheit (minus 6 Celsius), while a 20-percent solution freezes at 2 degrees Fahrenheit (minus 16 Celsius).
For freezing point depression to take effect, there needs to be at least a small amount of water on the road. This is why trucks often treat roads with a brine solution (a mix of salt and liquid water) when snow or ice is expected.
If the roads are completely dry and the DOT applies only road salt, it probably won’t make much of a difference. However, by pre-treating with brine, ice formation can be prevented, reducing the need for spreading additional road salt to de-ice later.
Pros and Cons of Road Salt
While rock salt is one of the most commonly used de-icers for roads, it does have its drawbacks.
Rock salt has its limitations. If the temperature drops below about 15 degrees F (minus 9 C), the salt won’t affect the ice. The solid salt cannot penetrate the ice’s structure to begin melting it. In these situations, the DOT usually spreads sand over the ice for better traction.
Rock salt presents significant environmental challenges, primarily due to the sodium and chlorine that seep into the soil and water. Additionally, as noted earlier, rock salt is not refined and contains impurities such as lead, iron, aluminum, and phosphorus, which are also dispersed when the salt is applied.
Despite its environmental concerns, rock salt continues to be the most commonly used and cost-effective de-icer. Other chemical de-icers exist, but none are completely without risks.
