Why does salt melt ice on roads?

Salt lowers the freezing point of water through a process called freezing point depression. When salt dissolves in the thin layer of liquid water that exists on ice, it disrupts the equilibrium between water molecules freezing and melting. The dissolved sodium and chloride ions interfere with water molecules' ability to form the crystal lattice structure of ice, so the water needs to reach a lower temperature to freeze solid again. This means ice will melt even when temperatures are below 0°C, because the saltwater solution has a lower freezing point than pure water. Regular table salt can effectively melt ice down to about minus 9°C, while other compounds like calcium chloride work at even lower temperatures.