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?

Question

Edit: thank you all for the explanations, I now have understood much more!

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Answer 1

Important distinction: salt lowers the temperature at which water freezes, it doesn’t lower the temperature of the water itself.

For example: water with a low salt content will freeze at 32F. However, typical seawater freezes at 28F. If you add enough salt to 30F ice, ideally you’ll end up with 30F water.

To your question: you want to melt the ice so you have liquid in both cases. For your iced cream, you want better contact with the aluminum container to freeze the iced cream faster. For the roads, liquid water runs off, where ice/snow does not.