Pure water’s freezing point—the temperature just cold enough to make water harden into ice—is 32 degrees Fahrenheit (0 degrees Celsius). When dissolved in water, salt (which is a compound called sodium chloride) interrupts the freezing process; it slows the action of water molecules trying to join together into crystals to form ice or snow. Experiments have shown that, for saltwater to freeze, it has to be colder than 32 degrees Fahrenheit. So you can say that salt lowers the freezing point of water. The more salt that is added to the same amount of water, the lower its freezing point will be.
If you sprinkle salt on ice, some of the ice will melt because the salt pulls some water away from its crystal form (ice) and also mixes with the thin layer of water on top of the ice. This creates more saltwater, which will melt more ice. The dissolved salt will also prevent the melted ice (now water) from refreezing.
But if temperatures fall to 15 degrees Fahrenheit or below, the salt cannot melt the ice—it has reached its freezing point. Then people often spread icy areas with a type of salt called calcium chloride, which creates a lower freezing point; it can work up to 15 degrees Fahrenheit below 0!
Try a freezing experiment with Salt, Water, and Ice!
Photo © iStock/Michael Krinke