Testing for melting point depression is a common diagnostic method used in chemistry to check for impurities. Pure compounds melt at known temperatures, and typically do so within a narrow range (+/- 0.5 °C). Impurities almost always lower the melting point and widen this band. I spent a lot of my undergrad chem courses packing my products into capillary sized test-tubes and watching them slowly melt.
that's funny, i literally just grabbed a melting point apparatus out of storage to get rid of, because I have too much stuff laying around I probably won't use again
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
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
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.
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.
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.
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).
Now check out Eutectic mixtures ... old-timers may remember soldering with 63-37 tin/lead solder.
The reason? With any other mixture of lead/tin, the liquid solder freezes over a temperature range, often resulting in what very-old-timers called a "cold solder joint". For example, 50-50 tin/lead mixture starts melting at 183C and is fully melted at 214C.
Using Eutectic Solder, the phase transition happens at exacctly 183 C ... the lump is solid at 182C and liquid at 184C.
Geologists take advantage of this: when non-eutectic mixtures of lava freeze (say, a basalt flow in Hawaii or on the moon), different minerals will be found in the rocks. Analyzing the minerals, and assuming equilibrium, you can understand temperatures and pressures in the origination magma.
(ps - yep, new ROHS rules have largely eliminated lead based solder)
Without lead you get tin whiskers. I wonder how the math works out in terms of what’s better for the environment if electronics break a lot more often…
A giant pile of scrap electronics is probably easier to deal with than a very large pile of scrap electronics that's leaching toxic heavy metals into the ground and runoff water.
Most electronics these days seem to get upgraded before tin whiskers can cause problems anyway.
Given the explanation above, this now makes a lot more sense: if the (non-eutectic) solder has cooled gradually, some the higher melting component will freeze first at the coolest location, and things will move around, leaving less of the "low-melting" components in the remaining molten solder. Once it's all solidified, some areas will have more tin and other areas will have more of the other elements. This will induce some weirdness, maybe the tin will want to diffuse to other parts where there is less (is this possible?) or maybe just there will be areas of relatively pure tin, perhaps more susceptible to whisker formation?
On the other hand an evenly-frozen mix of lead and tin (eutectic) wouldn't have these non-uniformities as there would be no driving force for it. One location freezing before the other wouldn't change the composition of the remaining melted solder.
> It turns out, yes! What happens is that when the salt is added some of the ice melts – pulling heat from the system – until the temperature has reached the new, lower equilibrium point.
Correction, or addendum here: the actual dissolution of the salt is an endothermic process, so even if there was no ice, the temperature of water decreases when salt is dissolved.
>Correction, or addendum here: the actual dissolution of the salt is an endothermic process, so even if there was no ice, the temperature of water decreases when salt is dissolved.
That's technically true, but it's a rather negligible amount.
Salt has an enthalpy of dissolution of +3.9kJ/mol (1) and a molar mass of 58.44g/mol (2), for roughly 67J/g.
For comparison, water=ice has an enthalpy of fusion of 334J/g (3), and you'll be adding at least three times more ice than salt (as max salt concentration is around 25% g/g (4) ). When you take this into account, it's a whole order of magnitude of difference, so for practical purposes you can outright ignore the heat being consumed by the dissolution of the salt.
But which has a greater effect on removing heat from the cream-containing vessel: the decrease in temperature from the dissolution of salt, or the more efficient thermal coupling to the vessel provided by the salt/ice slurry (versus the original solid ice chunks)?
The goal is to remove heat from the cream faster than the system as a whole warms up due to room temperature. I thought the value of salt was to help the cream win that race by making a better heat sink.
If the primary benefit of adding salt was improving thermal coupling through liquid by melting some of ice then you could achieve the same effect by adding some tap water. Which in my opinion would be a lot simpler and less messy than getting salt involved. Some energy would be lost to cool down tap water, but as mentioned in the article phase transition takes a lot more energy than changing temperature of water.
Presumably that is less significant a drop than the equilibrium melting freezing point being 5 degrees lower as even if endothermic it won’t be much will it and will just return back up when you add the warmer mixture bowl?
Always heard about this but never tried it. Sounds like fun. Great description, very clear and much better than just saying it lowers the temperature! Nice writing.
It's been bouncing around in the back of my brain for a long time.
I couldn't find any clear and concise explanations about what really happens when salt is added to ice, so I did some research and wrote it out myself :D
Thanks for this - I was watching a video of ice cream making with my son the other day, and the guy making the ice-cream said how it lowered the temp, and I totally didn't believe it was correct and started to explain my theory before realising I had no idea. Great to see it laid out so clearly!
More importantly, how was the ice cream you made? Apparently liquid nitrogen ice cream makes smaller crystals or something and tastes better? That could be the sequel…
When you do the math or work it at the bench, the difference is quite remarkable in the number of kilos of ice needed (per kilo of ice cream) when starting with 0C ice versus -20C ice.
Hint: start with rock salt at -20C also.
Edit: and the prepared cream premixed and chilled to 0C.
You can quickly experience it yourself by holding an ice cube in each hand but pouring salt on one of them. While both cubes contain approximately the same deficit of energy, the salted one with the lower equilibrium temperature will pull heat out of your hand much faster and feel much colder. I guess you could always put it in two cups too.
I thought it was going to about adding salt to the ice cream but was not :/
I have a compressor so I have no use of a salted ice bath but I find that using salt in the mixture will make the ice cream not as hard when left overnight or longer in the freezer.
Right, alcohol, sugar and salt all change the hardness of the ice cream but you can only add so much until the flavor isn't what you want anymore. I don't think the ice cream texture is due to the melting point, though, that the post talks about, it's also whether large enough ice crystals can form.
It feels like cheating but adding stabilizers (gums, mostly) was really a game changer for our homemade ice cream.
It's easier to think of a closed system and what the temperature of water vs. ice would be with just a phase change.
1 kg of ice turning into 1 kg of water requires 333,550 J.
1 kg of water require 4184 J to warm up 1 C.
So ignoring all the physical constraints, if you were to turn 1kg of ice "magically" into liquid water, keeping the total energy of the system the same, you'd end up with 1 kg of water at -80C (yes, I know I'm ignoring entropy).
Or put the simpler way: salt makes the icewater colder than 0C, and cream needs to be about 0F to freeze.
The ions from the salt get in the way of water molecules aligning to crystallize into ice. When salted ice melts, the water can't refreeze as readily because the saline isn't pure water anymore and because the freezing point is colder. As more ice melts, more heat is absorbed, bringing the temperature down even lower. (https://www.thoughtco.com/how-cold-does-ice-get-with-salt-40...)
Readit News
https://www.mt.com/us/en/home/applications/Application_Brows...
Some companies leverage this effect to make non-reversible temperature indicators that change color at specific temperatures.
https://www.mcmaster.com/temperature-indicating-stickers/
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
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)
Naturally there's some small local variations, but if you let the system come up to steady state, that's what will occur.
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.
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.
The reason? With any other mixture of lead/tin, the liquid solder freezes over a temperature range, often resulting in what very-old-timers called a "cold solder joint". For example, 50-50 tin/lead mixture starts melting at 183C and is fully melted at 214C.
Using Eutectic Solder, the phase transition happens at exacctly 183 C ... the lump is solid at 182C and liquid at 184C.
Geologists take advantage of this: when non-eutectic mixtures of lava freeze (say, a basalt flow in Hawaii or on the moon), different minerals will be found in the rocks. Analyzing the minerals, and assuming equilibrium, you can understand temperatures and pressures in the origination magma.
(ps - yep, new ROHS rules have largely eliminated lead based solder)
Most electronics these days seem to get upgraded before tin whiskers can cause problems anyway.
On the other hand an evenly-frozen mix of lead and tin (eutectic) wouldn't have these non-uniformities as there would be no driving force for it. One location freezing before the other wouldn't change the composition of the remaining melted solder.
Correction, or addendum here: the actual dissolution of the salt is an endothermic process, so even if there was no ice, the temperature of water decreases when salt is dissolved.
That's technically true, but it's a rather negligible amount.
Salt has an enthalpy of dissolution of +3.9kJ/mol (1) and a molar mass of 58.44g/mol (2), for roughly 67J/g.
For comparison, water=ice has an enthalpy of fusion of 334J/g (3), and you'll be adding at least three times more ice than salt (as max salt concentration is around 25% g/g (4) ). When you take this into account, it's a whole order of magnitude of difference, so for practical purposes you can outright ignore the heat being consumed by the dissolution of the salt.
Sources:
1. https://chem.libretexts.org/Bookshelves/Physical_and_Theoret...
2. https://en.wikipedia.org/wiki/Sodium_chloride
3. https://en.wikipedia.org/wiki/Enthalpy_of_fusion
4. https://en.wikipedia.org/wiki/Saline_water
The goal is to remove heat from the cream faster than the system as a whole warms up due to room temperature. I thought the value of salt was to help the cream win that race by making a better heat sink.
Hah, that's true, but I didn't want to mention it as it's not entirely in the aim of the essay :)
It's been bouncing around in the back of my brain for a long time.
I couldn't find any clear and concise explanations about what really happens when salt is added to ice, so I did some research and wrote it out myself :D
1. To make ice cream, we need to cool milk/cream below the freezing point of water (because milk/cream contains water).
2. To cool things, liquids have good thermal conductivity properties, so we would prefer to use a liquid.
3. We need some substance which is still a liquid at slightly below freezing.
4. It happens that salted water has this property and is relatively cheap.
Hint: start with rock salt at -20C also.
Edit: and the prepared cream premixed and chilled to 0C.
I have a compressor so I have no use of a salted ice bath but I find that using salt in the mixture will make the ice cream not as hard when left overnight or longer in the freezer.
Yes, though you have to be careful. If you add too much alcohol you'll prevent your mixture from properly freezing.
David Leibowitz, author of "The Perfect Scoop" recommends no more than 45ml of 80 proof liquor per 1 liter of ice cream mixture.
It feels like cheating but adding stabilizers (gums, mostly) was really a game changer for our homemade ice cream.
1 kg of ice turning into 1 kg of water requires 333,550 J.
1 kg of water require 4184 J to warm up 1 C.
So ignoring all the physical constraints, if you were to turn 1kg of ice "magically" into liquid water, keeping the total energy of the system the same, you'd end up with 1 kg of water at -80C (yes, I know I'm ignoring entropy).
The ions from the salt get in the way of water molecules aligning to crystallize into ice. When salted ice melts, the water can't refreeze as readily because the saline isn't pure water anymore and because the freezing point is colder. As more ice melts, more heat is absorbed, bringing the temperature down even lower. (https://www.thoughtco.com/how-cold-does-ice-get-with-salt-40...)