31 ms·

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

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

itronitron 4y ago
Thank you, your explanation is much clearer than the article.
fsckboy 4y ago
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)
- dcow 4y ago
  I understand Fahrenheit now.
  - kimixa 4y ago
    Unfortunately for this theory, brine freezes around -6f
    - aeternum 4y ago
      You need something colder than brine to freeze brine. Fahrenheit didn't have that so the theory/story is still plausible.
  - Maursault 4y ago
    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.
    - dlgeek 4y ago
      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.
- Taniwha 4y ago
  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
  - fsckboy 4y ago
    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.
foobarian 4y ago
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.
- floren 4y ago
  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).
  - foobarian 4y ago
    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.
    - bobthepanda 4y ago
      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)
      - robotresearcher 4y ago
        > 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.
        4y ago
iamkroot 4y ago
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.
- Taniwha 4y ago
  That's true of the surface of the ice, but the core is colder
  - iamkroot 4y ago
    For a little while, yes. Ice has middling thermal conductivity, it'll eventually homogenously warm to the melting point.
    - Taniwha 4y ago
      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
      - topaz0 4y ago
        "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.
        4y ago
topaz0 4y ago
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.

itronitron 4y ago

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

fsckboy 4y ago

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)

dcow 4y ago
I understand Fahrenheit now.
- kimixa 4y ago
  Unfortunately for this theory, brine freezes around -6f
  - aeternum 4y ago
    You need something colder than brine to freeze brine. Fahrenheit didn't have that so the theory/story is still plausible.
- Maursault 4y ago
  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.
  - dlgeek 4y ago
    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.
Taniwha 4y ago
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
- fsckboy 4y ago
  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.

foobarian 4y ago

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.

floren 4y ago
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).
- foobarian 4y ago
  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.
  - bobthepanda 4y ago
    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)
    - robotresearcher 4y ago
      > 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.
      - 4y ago

iamkroot 4y ago

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.

Taniwha 4y ago
That's true of the surface of the ice, but the core is colder
- iamkroot 4y ago
  For a little while, yes. Ice has middling thermal conductivity, it'll eventually homogenously warm to the melting point.
  - Taniwha 4y ago
    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
    - topaz0 4y ago
      "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.
      - 4y ago

topaz0 4y ago

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.