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.
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
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...)
Also in the recipes I googled right now, salt is also an ingredient for icecream
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'.)
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.
[0] https://www.amazon.com/Nostalgia-WICM4L-Electric-4-Quarts-Mi...
I've seen ice cream served with parmesan crisp though, which is probably fairly salty too, but that's close.
