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)
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.
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.
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).
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).
>> 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.
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?
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.
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?
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).
> 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.
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
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.
I would also imagine the very quick solidification also prevents larger crystal formations. Crystal growth itself can also lead to separation of the metals and the extra few seconds or more of a half-liquid state solder could represent decades or more worth of crystal growth in ambient temperatures.
> 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.
If you had really cold ice cubes, already tightly packed, then the water you add would freeze, making a solid ice sheath around the cream-containing vessel, and yes, that would work great.
But with too much space around the ice cubes, or ice cubes that aren't cold enough, adding water will just give you more cold (but not freezing) water.
I think people have converged on adding salt to ice because it's so forgiving (for a variety of ice cube temperatures and geometries), and the salt itself doesn't appreciably heat anything (unlike your added water). Other comments here quantify this better than I can.
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…
And yes, the other commenter is correct. LNO2 works so well because it freezes the ice cream so fast that the crystals don't have time to grow very large, which produces a nice and smooth texture in the final product.
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.
In addition to the ones mentioned above we've also tried powdered milk (cribbed from Salt&Straw, a local favorite) and this thing called "Avacream Ice Cream Stabilizer". That product works great as well but not sure I can really tell the difference, maybe the xanthan/dried milk is a little less smooth.
It's amazing how potent that stuff is. You only need 1/8th of a teaspoon of the powder per drink. I tried using a whole teaspoon once but it was so thick that I couldn't get my coffee through a straw.
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...)
Adding salt to an ice and water mix in a cooler is a known trick in hotter countries. Makes your beer cool down faster, and can get it down to -2C, the sweet spot for light Pilsner.
Creating a saltwater slush and packing this around our ice cream base allows us to cool the base enough so that it starts to thicken and freeze before the ice melts completely.
I always wondered why not other things with lower freezing points, like alcohol/vodka? (Not to add in the ice-cream, but to immerse the container that the ice-cream mix is in.)
Spoiler: "Salt added to an ice / water slurry in an ice cream machine lowers the temperature of the mixture beyond the typical freezing point of water."
We add some salt to most deserts as well, to make them more tasty (I am no expert so I don't actually know how it makes them "more tasty"). I highly doubt the salt in ice cream is for melting point, and probably it is also for taste reasons. OP is probably overthinking this
Alternatively (edit - in the product itself): because it's delicious.
If you like 'salted caramel' ice cream, try sprinkling some salt on vanilla ice cream. (I bet you'll find it's the 'salted' you like more than the 'caramel'.)
The article is specifically talking about salting the ice in a homemade ice cream maker like this[0]. The ice is used to reduce the temperature of the milk/sugar/etc. to "freeze" the ice cream. None of this salt gets into the ice cream.
I agree with you - people should try salting their ice cream. But the article is about a different part of the ice cream making process.
Yes, that's why I said 'alternatively'. I anticipated something more along the lines of reasons for salt in the product going in. TFA was much more interesting, frankly, just thought I'd offer the other use in comments.
No, sorry, I've never used it (as an artificial/extracted additive I mean) - not opposed to it, I'm curious to experiment with it vs. 'natural'/more traditional ingredient sources.
I've seen ice cream served with parmesan crisp though, which is probably fairly salty too, but that's close.
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/