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Comment by Taniwha | Hacker News Reader

Surely part of the issue is that the ice at < 0C while the liquid portion is at 0C (because of the equilibrium thing) - but it's the liquid portion, not the ice, that's most physically connected to the inner container you're trying to freeze (this is the important point).

If you add ice you reduce the equilibrium temp and as a result the < 0C ice temp can be passed to the liquid phase and as a result on to the inner con tain er where you're making the ice cream

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it's not just the coldness of the ice, salt dissolving in water actually decreases the temperature, it's an endothermic reaction.

the salt dissolving into the water brings the water down to 0... omg time for Farenheit to shine... brings the water down to 0F without freezing it (because of the lower equilibrium temp), which is -17.8C

(Farenheit uses this endothermic salted water temp as its definition of 0, I think because it was the coldest thing Dr. Farenheit knew how to produce in the lab)

That's why I said it was "part of the issue" - but once the salt has dissolved the system is kept at -5 for a period of time because inside of the ice cubes are < 5C

yes but as long as there is more undissolved salt (and when making ice cream you use a lot of salt), the melting ice will continue adding pure water to be chilled/saturated with salt.

I understand Fahrenheit now.

I thought I did, but just thinking about what temperature is, i.e. the degree of hotness or coldness something is, so it relates to heat, which is the energy transferred from something hot to something cold, so it relates to energy, the more energy something has the hotter it is, the less energy the colder. So I am utterly confused now why wind cools or even can be cold, and wind chill should be a contradiction, and still stuffy air should be cold. Wind has more energy than still air, so wind should be hotter and still air cooler. Wind turbines deployed on a global scale should solve Global Warming even if no energy is generated, because they should slow down the energetic and hot wind, cooling the atmosphere and the planet with it.

The air around you is (usually) colder than your body. That means your body is transferring energy to the air. When the wind blows, convection causes you to transfer more energy to the air than you do when the wind doesn't blow. That increased transfer of heat away from you makes the wind seem colder/creates wind chill.

Unfortunately for this theory, brine freezes around -6f

You need something colder than brine to freeze brine. Fahrenheit didn't have that so the theory/story is still plausible.

The ice actually comes up to the temperature of the water while it's melting. That's what the equilibrium temperature is: the temperature of the entire ice / water system until it's been converted to all liquid or all solid.

Naturally there's some small local variations, but if you let the system come up to steady state, that's what will occur.

That's true of the surface of the ice, but the core is colder

For a little while, yes. Ice has middling thermal conductivity, it'll eventually homogenously warm to the melting point.

yes - then it wont be ice any more - but while there's still ice the temp of the liquid will stay at the equilibrium point - after that it will start to warm

"then it won't be ice anymore". Not so: the freezing point is the temperature at which ice and water are in equilibrium. If you take a bowl of ice to its freezing point it will still be entirely frozen. Then if you continue adding energy, it will remain at its freezing point while progressively more of it melts, i.e. going from 100% ice to 50% ice/50% water to 100% water. All at one temperature. In other words, if you are at the freezing point, then all of the energy you add goes to melting the ice (and none of it goes to increasing the temperature), until all of the ice is gone. That is the case even if you wait to true equilibrium, e.g. for all temperature gradients to go away.

> and none of it goes to increasing the temperature

You’re clearly more knowledgeable than I am, but this strikes me as probably wrong? The temperature at the surface has raised, which is how some of the surface ice melted. The temperature on average has to raise because thermodynamics. If I understand the article, the core temperature of the remaining ice can decrease and localities can decrease with them. But the energy is increasing temperature in every other area at a higher rate, because it has to (conservation of energy) and because it has to (more ice melts than freezes).

At melting, the energy is going into the state change.

And the state change occurs when the portion of solid ice becoming liquid water warms however slightly, even if there’s a cooling side effect on the remaining ice as more energy is dissipated during that warming. Or am I still misunderstanding?

Yes. It is literally an isotherm. Energy is added without changing the temperature.

>> If you take a bowl of ice to its freezing point it will still be entirely frozen. Then if you continue adding energy, it will remain at its freezing point while progressively more of it melts [...] if you are at the freezing point, then all of the energy you add goes to melting the ice (and none of it goes to increasing the temperature), until all of the ice is gone.

> this strikes me as probably wrong? The temperature at the surface has raised, which is how some of the surface ice melted. The temperature on average has to raise because thermodynamics.

What do yo you think is wrong in the quoted passage?

The "temperature on average" of the system described is the temperature of every part of it: the temperature where the liquid and solid phases co-exist. When you heat that system (i.e. add energy) the temperature doesn't have to increase "because of thermodynamics". The ice melts and everything remains at the same temperature. Only when the ice is gone the water starts to warm up.

Right on. It’s the same for boiling, isn’t it? The water “can’t” get above 100 degrees C - added energy just goes into making steam. Right?

That's right (well... assuming constant pressure lower than the critical pressure.)

To be fair to GP, it does take some energy to heat the ice from freezer temp to 0C (or the new, depressed freezing point), part of which will come out of the ice cream. It's just that that amount of energy is very small compared to the other energies we're interested in here (as you pointed out elsewhere).

This effect should be negligible. The whole point here is that it takes way more energy to take a chunk of ice and turn it into water (at fixed temp -- namely the freezing point) than it does to heat that chunk of ice a few degrees (below the freezing point). And remember that you don't just have to cool the cream to its freezing point, you also have to remove enough energy to overcome its latent heat of fusion. If you were doing this just with the heat capacity of ice from like -20 to -5 C, you would need many times more ice than you could make ice cream. Like tens to hundreds of times. The blog discusses some related facts a bit near the end.

This was a bit hard to spot in the writeup, as I have no clue about how ice cream making and machines work. Otherwise we could just use ice, which will be as cold as the refrigerator can get it. (I doubt the endothermic reaction of dissolving the salt contributes very much to the cooling).

Now that I think about it, if I were doing this I would use antifreeze for the coolant instead of wasting salt. Bonus, I can store the antifreeze when done, but the salt water is wasted unless I'm going to use it to make some kind of soup or similar.

Salt's way cheaper than antifreeze, and I'd be a lot happier about getting a little stray salt in my ice cream than getting a little stray ethylene glycol (with bittering agents, since 2010).

I suppose I could store the salt water solution instead of throwing it away. Assuming I made ice cream often this would be acceptably frugal for me.

Also, if you really want to recover the salt, you’d just have to boil away the water.

Alternatively, pickling is a perfectly good use of brine used for frugal purposes (extending the shelf life of produce, eggs, and what have you)

> Also, if you really want to recover the salt, you’d just have to boil away the water.

This is probably the point the poster was making: this requires a lot of energy. There's a lot of NaCl in the world, but getting it out of solution is expensive.

If that’s a major concern, you can literally leave it in the sun like the other person suggested.

Boiling is quick and convenient, but just evaporating the water out with ambient air is all but free.

You could use propylene glycol that is Generally Recognized As Safe.

What's in the blue liquid in those ice cream making bowls that is normally sealed but sometimes people report it leaking. The manufacturers say it is nontoxic.

It’s propylene glycol. The same stuff they fill “ice packs” with. And it’s also the thickening agent in coffee syrups. Supposedly food safe. I wouldn’t season my food with it though

It’s also what’s used in vape cartridges.

Author here

Surface contact is one reason you want an ice/water slurry instead of just ice, but the real reason is that ice melting consume a lot more energy than just ice being warmed up to it's melting point.

The ice will quickly come up to it's melting (equilibrium!) point, without cooling the ice cream mixture very much. Remember, we're trying to freeze the ice cream (not just cool it down), which is proportionally just as thermodynamically expensive as melting ice. Bringing the ice up to it's melting point alone won't suck enough heat out of the ice cream mixture to freeze it.

This right here is the explanation that clicks for me. It’s not enough to say “salt makes the ice colder than 32”. Which might cause one to wrongly assume you do it so the ice cream freezes “faster.”

What you say here is the reason WHY that is needed in the first place, and you say it very clearly

Remember, we're trying to freeze the ice cream (not just cool it down), which is proportionally just as thermodynamically expensive as melting ice.

Is it just proportional, or is it actually pretty close to 1:1? That is, how accurate is the view that if you want to freeze 1L (or kg) of ice cream you need to melt 1L (or kg) of ice? Although I guess ice cream is not just frozen water, so perhaps that forms a fixed proportion. Alternatively stated, how much ice do you need to start with to freeze a given quantify of ice cream?

According to some random post on google, ice cream needs about 2/3 as much energy per gram to melt or freeze.

Of course you also have to worry about losses, extra margin, and the heat of stirring.

You're right, I should have said equivalently :D

antifreeze is really really really toxic for mammals so there's that, while ingesting bit of salt never hurt anyone.. maybe diabetics

I think the ice cream would be a bigger concern for diabetics.

Thank you, your explanation is much clearer than the article.

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