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

It’s a matter of what properties you care about: making something cold, vs. breaking up ice to stop it from being slippery.

In the ice-cream maker, you’re trying to transfer *coldness* out of the ice and into the ice-cream, to make the water in the ice-cream freeze. It doesn’t matter to you that the ice melts in the process.

On the road, you’re trying to melt and break up the ice so it’s less slippery. You don’t really care about *coldness*.

Most of the energy involved doesn’t go into changing the temperature; it goes into shifting water between solid and liquid forms. This is the [latent heat of fusion](https://en.wikipedia.org/wiki/Enthalpy_of_fusion) … where “fusion” doesn’t mean *nuclear* fusion like the sun, but rather water *physically* fusing into ice.