How does salt help boil water and help ice freeze?
It’s quite simple. When salt is dissolved in water, the salt ions get in between the water molecules keeping the molecules slightly separated. So when the water is heated as in boiling, the individual molecules, lacking the attractive forces of adjacent molecules, get ejected more easily from the water surface to become vapour (gas). Hence less heat is required and boiling is achieved at a lower temperature. On the other hand, when the water is cooled as in freezing, the individual molecules, lacking the attractive forces of adjacent molecules, are not able to easily bond into a crystal (solid) state. Hence more cooling is required and freezing is achieved only at a lower temperature. Now, to make ice cream, you need very cold water. This you get by adding salt to water, then freezing it. Note that both the boiling point and freezing point are lowered with the addition of salt. Your grandma is a smart lady!
