Edit: thank you all for the explanations, I now have understood much more!
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I’m no expert, but I believe salt makes the freezing point lower, this allows the water the fills the whitespace in between the ice cubes to be colder. The water is more efficient at changing the temperature of the internal wall of the ice cream container in the middle because it presses up against it and the ice over a wide area, where the ice cubes only touch the inner wall at points.
Water transfers heat better/faster than ice. Also, this may seem counterintuitive, but it when water freezes, it actually gives off some energy, warming up things around it. Not enough to melt the ice back into water, but just a bit. Salt water also has a much lower freezing point than “fresh” water.
So if you salt the ice on your driveway, you’re lowering the freezing point so that the salt + ice will melt back into saltwater and flow away from wherever you’re putting the salt.
If you put salt on ice for a dessert, you’re letting the water stay liquid at lower temperatures while the remaining ice brings the temperature down. This draws more heat from the dessert, keeping/making it cold.
The sodium ions and chloride ions in common salt interrupt the water molecules bonding together to form ice and so lower the freezing point of water. https://youtu.be/oVJAZ002v5k
Good answers here, but I think the main point has been lost.
You add salt to ice *to make it melt faster*, which in turn will cool the dish that has the ice cream in it.
If you add water to the outside shell, it would cool down quickly, but not get that cold, because water is warmer than ice.
However if you put ice in the shell and add salt, it will begin melting faster, which will allow the cooling process inside the shell to occur more rapidly.
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.
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.
The salt makes the ice melt at a lower temperature so the water transfers heat faster cooling the ice cream faster. On the road the salt still makes the ice melt at a lower temperature which means it melts off the road instead of staying ice.
It’s been described accurately here, but maybe a way to visualize it could help too.
As people have stated, salted ice melts faster than non-salted ice because the freezing point is lowered. Why is quickly melting ice good for making ice cream? Well, in order for ice to melt (salted or non-salted) a lot of heat needs to enter that ice to break the crystal water formations. Like, a LOT of heat.
That energy has to come from somewhere, and unlike outside (where the sun can help), your ice cream maker ice has only one option – steal it!
So the energy hungry salted ice gobbles up all the heat from every nearby object – and freezes your ice cream solid.
We use it to melt ice in both cases.
Salt lowers the freezing point of water. Fresh water freezes at 0°C. With enough salt, you can lower that quite a bit. At 10% salt concentration, you’re down to about -6°C, and at 20%, the water freezes at about -16°C.
So, suppose you fill a bucket with ice that is -12°C, and you add 20% salt. This lowers the freezing point to -16°C, but the ice is warmer than that, so it melts. And the action of melting actually absorbs quite a bit of energy, so this will lower the temperature. So you get a bucket full of ice water (i.e. a mixture of liquid water and chunks of ice) at a temperature of -16°C. Stick a metal bowl in there and you can make ice cream in it.
Why do you want this and not just ice? Well, solid ice cubes or shavings don’t make good contact with the bowl that contains the ice cream ingredients, so the heat doesn’t transfer very quickly from the bowl to the ice. Liquid water, on the other hand, makes *great* contact with the bowl, but normal liquid water isn’t cold enough to freeze the ingredients. So the perfect combination is liquid water that is also below the freezing point of your ice cream ingredients.
Note that we added salt to make the ice in the bucket melt. This is no different from putting salt on roads to melt snow or ice.
Salt makes the melting point of water/snow lower.
So salt water needs to be less than 32f/0c to freeze.
You put salt on snow to melt it because cold water is less dangerous than snow.
You do the same icecream makers on the outside with ice. This is because it’s better to have the colder than ice water than ice which has air pockets between the ice and the bowl
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