Most of us know that water freezes at a temperature of 32 degrees. But, if you add something to the water, it can change the water’s molecular structure and consequently change its freezing point.
Salt is very good at lowering the freezing point of water.
With salt, water might not freeze until it gets to about 20 degrees.
Salt, then, effectively lowers the freezing temperature of the water.
So, if you put salt on ice (which often has a thin film of water on it), you lower the freezing point from 32 to about 20.
That’s why salt melts ice. It lowers the freezing point of water.
However, salt loses its effectiveness at temperatures below 20.