鹽水why可以解凍?

When the salt is dissolved into the ice, the ice point or freezing point would be lower. Then the ice would melt to water although it is zero degree.
It is a good method for dissolving the snows which block the roads and airport in snowing countries as it is a common method too.

I think your question is asking why salt can help to melt ice, right? In the ice, due to the water molecules, all the water molecules are binded tightly together by intermolcular force. If we want to melt the ice, we need to provide energy to weaken the intermocular force to make the solid state to liquid state. However, when we add salt to the ice surface, salt will melt and release sodium and chlorides ions, therefore, it will interfere the force between water molecules and less energy to melt the ice, then temperature to melt the ice will higher than normal. That is why help to melt ice>