Why Does Salt Melt Ice?

When salt (sodium chloride) is spread on ice, it dissolves in the thin film of liquid water on the ice surface, creating a saltwater solution. This solution has a lower freezing point than pure water — a phenomenon called freezing-point depression. The dissolved salt ions interfere with water molecules ability to form the orderly crystal lattice of ice, so the ice continues to melt even below 0 °C. A saturated NaCl solution freezes at about -21 °C. This is why salt is effective for de-icing roads in moderate winters but becomes less useful in extreme cold. Calcium chloride, which lowers the freezing point further (to about -52 °C), is used in colder regions.