Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
8. Chemistry in the World
Why do people put salt on icy roads?
Salt melts the ice. When you read about snow melting in the answer above, you saw that there is a balance between how many molecules froze onto the ice and how many molecules in the ice melted into the surrounding water. You saw that you could add heat or lower the pressure to tip the balance in favor of melting.
There is another way to tip the balance. Salt water doesn’t freeze until it reaches almost -6° Fahrenheit (-21° Celsius). If salt is added to the water at the surface of the ice, the ice still melts at the same rate (that hasn’t changed), but the water no longer freezes onto the ice until the temperature drops to -6° F.
It takes heat to melt ice. The ice can get that heat from the air or water around it, if that is warmer than the ice. The ice stays the same temperature (the freezing point), but the salt water around it gets colder as it loses heat melting the ice. Eventually the salt water reaches -6° F, and we are back in equilibrium again, with water freezing onto the ice as fast as it melts off it.
So if the temperature outside is below -6° F, salt won’t melt the ice. You have to use something that freezes at an even lower temperature. A calcium chloride solution freezes at -20° F (-29° C), so it is used instead of the cheaper salt when the temperature is really low.