thebigger t1_iybb5a3 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
Ice is a solid, so adding salt to it cannot lower the freezing temperature of water, which is a liquid. The only way this can work is if the ice melts.
Why does ice melt when you put salt on it? Well, ice is already melting because, in this example with an ice cream machine, it is at room temperature. But by adding salt to it you are also adding weight, you are increasing the 'grain' of the mix, which becomes relevant as the machine spins, and grinds. This facilitates it melting faster.
It has nothing to do with lowering the freezing point as far as I can see. It does lower the freezing point, but the ice cream machine is operating at a temperature above freezing, and spinning (adding energy) which increases the temperature. The explanation is that it helps it melt faster, which then allows the coldness to transfer to the inner shell of the machine more efficiently.
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