Salt lowers the freezing point of water because it makes it difficult for water molecules to form ice crystals together. Salt is a chemical compound of sodium and chlorine.
When salt comes into contact with water or ice, the salt crystals are dissolved and positive sodium ions and negative chloride ions are released.
These ions mix with the water molecules and ensure that they cannot come so close together that ice crystals form, even though the temperature is below freezing.
Salt disturbs balance between ice and water
There is a balance in a mixture of ice and water around the freezing point: new water molecules always come into contact with the ice. Then they are encapsulated in the ice crystals, causing them to grow.
At the same time there are also water molecules that leave the ice crystals. In an equilibrium situation, as many water molecules arrive at the ice crystals as there depart.
When salt is added, the sodium and chloride ions are not automatically absorbed into the ice crystals, but they spread over the surrounding water, where they displace the water molecules.
That means that slightly fewer water molecules are available to be included in the ice crystals, while the number of molecules leaving the crystals remains unchanged.
The result is that the ice crystals lose more water than they get and the ice melts.