thebigger t1_iy8ussc 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
> 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.
kemptonite1 t1_iyaedhm wrote
Well, you kind of have a thought going here. Applying pressure to ice does indeed melt it (applied force can break those crystal formations just like heat can) but the effect is pretty minimal. It takes a lot of force to melt ice (like, heavy hammer forces, not scattered salt force). The freezing point is indeed lowered, which is why the ice melts faster. A simple google search confirms this. š¤·āāļøIf your thought was true then scattering pebbles on ice would be just as effective as ice, which it isnāt.
thebigger t1_iyagn14 wrote
My understanding is that simply walking on ice is what does it. I learned that from a Feynmann video, not a simple search.
thebigger t1_iyagt17 wrote
Also in the far north we use sand instead of salt precisely because of your last comment.
kemptonite1 t1_iyamgl8 wrote
Yeah, sand works well for roads for that reason - cars provide plenty of pressure for melting ice through force alone (and chunky sand gives a lot of little pressure points for cars to melt bits of ice under their tires).
But that dodges the real point here - the salt added to an ice cream maker isnāt increasing pressure to melt the ice and make the ice cream freeze faster - the salt is lowering the melting point to encourage the ice to melt faster from conduction, and thus stealing heat at a faster rate from the surrounding objects.
Sand/salt on roads is a slightly different beast. Salt is 100% better at melting ice on roads (because it does the pressure thing sand does AND lowers the freezing point) BUT salt corrodes metal. And corroding the underside of cars isnāt great. So sand is often used (Iām also from the north).
thebigger t1_iyancll wrote
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?
thebigger t1_iybb5a3 wrote
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.
thebigger t1_iybcezv wrote
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
Soā¦ Iāve never seen someone use so much space to talk about something they donāt seem to understand very well. Iām 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 canāt change the area of contact between the ice and the metal tube containing the cream. We canāt 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
>We canāt 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
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.
thebigger t1_iybycm4 wrote
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.
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