Why Salt Melts Ice

Salt melts ice because it lowers the freezing point of water, not because it adds heat. On an icy surface, salt dissolves into the thin film of liquid water that is always present on ice, forming brine. That brine cannot freeze at 0 °C, so the ice keeps dissolving into it, gradually turning solid ice into a slushy saltwater solution.

The mechanism is freezing-point depression

Ice and liquid water are in constant exchange at the surface. Water molecules detach from the solid and reattach to it. In pure water at 0 °C, the two rates balance. Dissolved salt changes that balance: ions in the liquid make it harder for water molecules to rejoin the crystal. The system has to cool further before freezing can happen. The more dissolved ions, the lower the freezing point. This is a colligative property: it depends on the number of dissolved particles, not on what they are. Each mole of particles in a kilogram of water lowers the freezing point by about 1.86 °C. Sodium chloride contributes two particles per formula unit, so a one-molal salt solution depresses the freezing point by roughly 3.7 °C.

A worked example

Seawater contains roughly one mole of dissolved ions per kilogram of water. That predicts a freezing point around -2 °C, which is why seawater does not freeze at 0 °C. Road salt is much more concentrated. A sodium chloride brine near saturation contains about five or six moles of salt per kilogram of water, meaning more than ten moles of dissolved particles. The ideal formula gives a depression above 20 °C, which matches the practical limit: around -21 °C, the brine reaches its eutectic point. At that temperature, no more salt can dissolve, and the brine itself freezes solid.

The common misunderstanding

Salt does not work by physically wedging into the ice lattice and cracking it apart. The ions stay in the liquid layer and disrupt the equilibrium between solid and liquid. The ice melts because the salty liquid is more stable than the solid at the same temperature. Salt also does not melt ice by heating it. In fact, melting ice absorbs heat. When salt causes ice to melt, the mix can become colder. A salt-and-ice bath gets below 0 °C precisely because the melting ice pulls heat out of its surroundings. That is why rock salt is used in hand-cranked ice cream makers: the brine can get far colder than freezing water alone.

When salt does not apply

Below the eutectic temperature, salt becomes useless because the concentrated brine freezes anyway. Sodium chloride will not melt ice at -30 °C, no matter how much is spread. Calcium chloride is used in colder regions because its brine can stay liquid at much lower temperatures. Salt also needs some liquid water to dissolve in. On extremely cold, dry surfaces, salt crystals can sit on ice for a long time doing nothing until moisture forms. The ice must have a surface layer of liquid, even microscopic, for the salt to change its freezing point.