Why is salt used in making ice cream?

Question

Recently I did an ice cream-making experiment. We added ice cubes & salt into a large ziploc bag and in a smaller bag, milk, sugar, & vanilla were added. then the small bag was placed into the larger bag, sealed, and after some time, ice cream was formed within the smaller bag. Why was salt needed in this experiment? How did it allow (on a molecular basis) the milk solution to turn into ice cream?

Answer 1

This is an example of a "collative property" of ions in water - that they depress the freezing point. We use this idea routinely in our cars with antifreeze, so the car's cooling system does not freeze over in the winter. The salt allows water to exist in the liquid state at a sub-freezing temperature, and the milk solution is bought to a lower temperature at which it can freeze.

For small scale operations, table salt is works, is cheap to acquire, and can be cleaned up after the experiment so there isn't corrosion. In a full-scale operation, refrigerant is used.

Answer 2

You can think about this from this point of view.

Ice is a solid, crystalline structure. But water is a liquid. To change from a solid to a liquid requires addition of heat energy - actually quite a lot of heat has to be added to melt ice.

OK, so when you add the salt, it interferes with the solid structure of the ice and breaks it apart. The ice is going to change into liquid, but it needs heat to do that. So where can it find some heat? Oh yeah, the milk in the other bag is warmer than the ice, so the ice will suck up some heat from the milk and use that heat to convert itself to liquid water. As it sucks up the heat from the milk, the milk is lowered in temperature and so it solidifies. The amount of heat that the milk loses is roughly the same (considering heat leaking out of the bags, etc) as the amount of heat that the ice needs to become salt water. This might be hard to think about but although the TEMPERATURE of the salt water is colder than regular ice, the amount of HEAT in it, is larger. Heat and temperature are not the same thing. If you can get that, you have really learned something important about how chemistry and physics work.