Why Does Salt Melt Ice on the Roads in Winter?

Road salt is simply halite — rock salt — which is table salt (NaCl) in its natural form. The difference is table salt goes through a lengthy purification process, while rock salt does not. And because rock salt still has impurities, it's brown or gray in color.

Some cities use other types of salt, like potassium chloride or calcium chloride, to keep their roads ice-free.

Ice forms when the­ temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius), and that includes ice on roadways. Road salt lowers the freezing point of water via a process called freezing point depression.

The freezing point of the water is lowered once the salt is added, so it the salt makes it more difficult for water to freeze. A 10-percent salt solution freezes at 20 degrees Fahrenheit (minus 6 Celsius), and a 20-percent solution freezes at 2 degrees Fahrenheit (minus 16 Celsius).

The key is, there has to be at least a tiny bit of water on the road for freezing point depression to work. That's why you often see trucks pretreat roads with a brine solution (a mixture of salt and liquid water) when ice and snow is forecast.

If the roads are dry and the DOT simply puts down road salt, it likely won't make much of a difference. But pretreating with a brine solution can help ice from ever forming and will help reduce the amount of road salt trucks will need to spread to de-ice later.

Rock salt is one of the most widely used road de-icers, but it's not without critics.

For one, rock salt does have its limits. If the temperature of the roadway is lower than about 15 degrees F (minus 9 C), the salt won't have any effect on the ice. The solid salt simply can't get into the structure of the frozen water to start melting ice. In these cases, the DOT typically spreads sand on top of the solid ice to provide traction.

Rock salt also has major environmental issues, including the sodium and chlorine that leaches into the ground and water. And as we mentioned earlier, because rock salt isn't purified and contains contaminants — including lead, iron, aluminum and phosphorus — when it's spread, these are spread as well.

However, rock salt still remains the most widely used and affordable de-icers available. And while there are other chemical de-icers, too, none are 100 percent risk free.