You do realize that in the ocean, when ice forms, that the ice is salt free because the ice is 'expelled' from the molecules that form the ice, and the lattice structure?

Ice by definition is essentially salt free.

When you make ice cream you rotate a bowl inside of another bowl, and there is fiction between these bowl. This friction creates energy, and when you put ice in between these bowls it will "grind" the ice, which helps the ice melt more quickly.

>We can’t change the area of contact between the ice and the metal tube containing the cream

We absolutely can, we can add salt. I have studied advanced physics, but more importantly I have been a professional chef and I understand how ice cream is made.

You can absolutely do this. You add salt.

That's literally why we add salt.

>Because the salty ice is colder

Ice does not get salty. Ice is a solid, and it is a unique solid which expands when it cools. Because of this property when you add any weight to it, it will compact, and melt.

If you don't agree with this, or can disprove this in some capacity then feel free to do so.

A cube of iron that is chilled to absolute zero, if stepped on, isn't going to compact any further. A cube of ice will.

>So yes, the freezing point is relevant.

How? Salt's affect on ice diminishes as the temperature drops. By the point we're talking about absolute zero the affect of salt on ice would be almost zero.

edit: I suppose the iron would compact slightly, but not to the magnitude of ice, and the energy transfer relative to the melting point would be negligible for these purposes, but if you were to step on ice with boots covered in salt, and push the salt down into the ice... it will melt faster, and increase the surface area of the inner shell, which has nothing to do with the freezing point. The ice is already melting, you're just helping it melt faster by introducing water which has a lower freezing point than the solid you're trying to melt.

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.

https://www.youtube.com/watch?v=MO0r930Sn_8

An ice cream machine requires an inner shell to rotate or 'churn' around an outer shell. The space between the inner and the outer shell is filled with a cold substance.

Liquid nitrogen would work well here, but that is a bit expensive, and dangerous for making ice cream, so we use H2O.

Well what kind of H2O should we use?

We could use H2O in a liquid form, and get it perfect. We would need a controlled environment below freezing to achieve the same results. This would cost money, and be more difficult. I imagine this would work as well as using ice, except it would warm up more quickly instead of 'melting' as ice would.

So the solution is to use ice.

Well why wouldn't we add salt to the mix? It isn't to lower the freezing temperature, because we are not interested in that. We are not in an environment which is below freezing. The ice is already melting, but by helping it melt faster, especially within the confines of the mechanics of the machine (i.e. being more gritty) we are therefore transferring more 'coldness' to the inner shell.

I surmise that if you were to add salt to an ice cream machine that was inside a room at, or near absolute zero, that the salt would have no affect at all. Then again, the ice wouldn't either. The inner shell would be cold enough to simply spin around on it's own and create ice cream.

I might be wrong here, but I don't see how the freezing point of water is particularly relevant here. The main point is heat transfer.

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.

How does it lower the freezing temperature before the ice melts? The salt makes it melt because you are adding weight, and the salt decreases the viscosity in the chamber making it easier to 'grind', which in turn facilitates the melt process because it adds energy.

No?

Also in the far north we use sand instead of salt precisely because of your last comment.

My understanding is that simply walking on ice is what does it. I learned that from a Feynmann video, not a simple search.

> As people have stated, salted ice melts faster than non-salted ice because the freezing point is lowered.

Is that true? The freezing point isn't lowered, per se. Adding salt to ice is adding weight to ice, just like stepping on ice. When you step on ice you compact it, and a bit of it melts. That bit that melts then mixes with the salt and the process begins.

Certainly the freezing point is lowered, but I'm not sure that is relevant here, and it isn't why the ice melts AFAIK.

Are you sure about that? Certainly if you're comparing ocean water to lake water you are correct, but adding salt to ice would be adding energy and slightly increase the temperature if I'm not mistaken.

The role the salt plays is that it facilitates the melting of the ice more quickly, which is colder, but I'm not sure that adding salt makes it colder.

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.