Why does salt melt ice?

Salt melts ice by lowering the freezing point of water through a physical chemistry process known as freezing-point depression. Under normal conditions, liquid water turns into solid ice at 32 degrees Fahrenheit (0 degrees Celsius). At this temperature, water molecules slow down enough to form a rigid, highly organized crystalline structure. However, even on a freezing day, a microscopically thin layer of liquid water always exists on the surface of ice.

When you spread salt on an icy sidewalk, the salt dissolves into this thin layer of surface water, breaking down into its individual sodium and chloride ions. These dissolved ions physically get in the way of the water molecules, making it much harder for them to bond together and form the rigid crystal structure of ice. Because the salt ions disrupt this crystallization process, the water must be much colder than 32 degrees Fahrenheit to freeze.

The salt does not actually generate heat to melt the ice. Instead, it prevents the liquid water on the surface from refreezing into solid ice while the ice continues to melt naturally. This chemical barrier effectively lowers the freezing point of the liquid mixture, keeping the sidewalk wet instead of icy down to about 15 degrees Fahrenheit, depending on the concentration of the salt solution.

At extremely low temperatures, standard rock salt, which is sodium chloride, becomes ineffective because there is not enough liquid water on the ice surface to dissolve the salt. In those cases, road crews often use other salts like calcium chloride or magnesium chloride. These salts can lower the freezing point even further, keeping roads safe even during extreme winter weather.