thebigger t1_iybycm4 wrote
Reply to comment by kemptonite1 in Eli5: Some ice cream recipes put ice + salt outside the recipient to make it cool faster. But in the winter, salt is put on snow on the street to melt faster. Why one make cool and other melt? by zimobz
Upon further thought I think the idea I'm trying to convey has to do with something such as, "phase states."
Ice is very cold, and it can only get warmer. So by adding salt to ice we make ice begin to melt quicker, which facilitates heat transfer in the case of making ice cream.
Water is not very cold. It can only get colder before it becomes ice. Adding salt to it will lower the freezing point, but this really doesn't matter because cooling water takes a lot of energy, and we aren't trying to cool the water, we are trying to cool the ice cream, which is most efficiently done by using ice, and adding salt.
The reason salt makes the ice melt faster is complex, and that it lowers the freezing point of water is relevant, it really isn't the actual answer here as far as I know, but I don't know much.
This is a mechanical and chemical physical question, and is fairly complex I suspect, but as far as I know from Feynman (what did he know besides winning a Nobel prize?) the reason is that the salt adds weight to the ice, and then that creates salt water, which further adds weight, and then you have this process almost like burning a candle where an ice cube will melt into water.
The maximum coldness of that ice cube is not increased by adding salt, but by adding salt it does increase the maximum coldness of the inner shell of the ice cream machine, because it helps the ice melt faster, which helps transfer the heat more efficiently. The 'grit' of this salt as it rubs into the salt is a major factor here, but more generally speaking if you were to simply drop salt onto ice in a very cold environment it will melt the ice a bit because simply dropping the salt on the ice is going to melt it.
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