For the first time in years Lowcountry residents have experienced something that is routine in northern climes: salt on roads and sidewalks to melt ice.
But does it really work?
Yes, as long as it doesn't get too cold.
Anyone who has made homemade ice cream can see it work. Salt lowers the temperature at which water freezes. So by mixing salt and ice into a brine around the liquid ingredients for ice cream, which won't freeze at 32 degrees Fahrenheit, the temperature drops enough to make ice cream.
On roads it works the same way.
Water normally freezes at 32 degrees Fahrenheit. When salt is spread on roads in freezing conditions, it can prevent ice from forming, and can help melt existing ice. However, it it's too cold, the salt won't work. Generally salt is effective at melting ice down only to about 15 degrees F.
So why does salt work so well on roads but not bridges?
Because the bridges are exposed to the outside cold air from all sides, and roads are exposed only on the surface, allowing them to radiate heat longer. Also, roads typically are paved with asphalt, which is not a good conductor of heat, meaning the heat stays in longer. Bridges typically are made of steel and concrete, both of which lose heat rapidly.
Source: About.com Chemistry