Why salt makes ice melt

Salt makes ice melt because dissolved ions get in the way of water molecules joining the crystal, so the balance between ice and water shifts. Ice then melts at a temperature where pure ice would have stayed solid.

Salt is not hot, and it does not melt ice by warming it the way a flame would. The mixture often gets colder as the ice melts, because melting takes in heat, which is why an old ice-cream churn packed salt around the ice.

The mechanism is more particular than the slogan. At the ordinary freezing point, molecules leave the crystal and join it at the same rate, so a mixture of pure ice and pure water holds steady. Add salt to the liquid and fewer water molecules are available to attach, melting wins, and the temperature of the slush falls until a new balance appears.

Follow it one step further. The effect depends on how many dissolved particles are present, not on salt as a uniquely magical substance. Sugar lowers the freezing point too, though more mass of sugar is needed for the same shift, because each salt unit splits into more than one dissolved ion.

The case worth keeping in mind is specific. Road salt scattered on a thin ice film near the usual freezing temperature clears the film into brine. The road can end up wet and colder than the ice was, which is a nuisance for the next night's refreezing and still the reason the first film disappeared.

That case has a boundary. There is a floor to the trick, a lowest temperature the salt-and-water mixture can reach before everything is solid again. Ice already colder than that floor does not clear just because salt was thrown on it, which is why arctic roads are not saved by the same sprinkle that works in a mild winter.