thebigger t1_iybcezv wrote on November 30, 2022 at 3:11 AM

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

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.

kemptonite1 t1_iybyl18 wrote on November 30, 2022 at 6:44 AM

So… I’ve never seen someone use so much space to talk about something they don’t seem to understand very well. I’m not an expert, but I do have a degree in physics and have taken several college level thermodynamics classes.

Your last point in particular is contradictory. And really hits the nail on the head.

Heat transfer (in conduction at least) depends on several things - the area of contact between objects, the coefficient of heat transfer, and the difference of temperature between the objects. (Q/t=kA(T1-T2)/d)

We can’t change the area of contact between the ice and the metal tube containing the cream. We can’t change the coefficient either (not meaningfully). What we CAN change is the difference in temperature-by lowering the freezing point of the ice, we lower the temperature the ice can raise to before it begins melting. A mixture that is partially ice and partially water (such as a mass of melting ice in an ice cream maker) will ALWAYS be at the freezing temperature (0 C for normal ice, or -5ish C for salty ice).

Because the salty ice is colder, heat is transferred more rapidly due to the difference in temperature being greater.

So yes, the freezing point is relevant. Ice cannot have a higher temperature than its freezing point. Therefore salty ice having a lower freezing point means greater temperature difference and therefore greater heat transfer and therefore faster freezing ice cream. Does that make sense?

thebigger t1_iybzdxm wrote on November 30, 2022 at 6:54 AM

>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.

thebigger t1_iyc0i8z wrote on November 30, 2022 at 7:09 AM

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.