Why does salt melt ice?

What is happening?

Even below 0 °C, a thin liquid layer can exist on ice. When salt dissolves in that water, the solution has a lower freezing point than pure water.

If the temperature is within the useful range, some ice melts to establish the new equilibrium. Melting absorbs heat, so the ice-salt mixture can become colder while more liquid forms.

What changes it?

Salt is not equally effective at every temperature. At sufficiently low temperatures, ordinary sodium chloride becomes a poor de-icer, which is why other salts or methods may be used.