Phase Change Cooling and Ice-Cream Making ?

[uart]

Ok, maybe this is a little OT for "highly technical", but it's something I was wondering.

Though I've never done it, I've read that in days of old when "hand making" ice-cream that they used ice to cool it and progressively added salt to the ice in order to reduce the temperture to that required to freeze the ice-cream mix (typically 5 to 10 C below the freezing point of water).

BTW in case you were wondering, the ice is in an outer bucket that surrounds the vessel that contains the actual ice-cream mix, so the salt doesn't get into the ice-cream at all (yuk.)

One thing that is not clearly explained about this process is whether or not the temperature obtained by adding salt is indeed colder than the temperature of the original ice, or whether the original ice is still colder and the salt is merely making the melt (or brine) colder than it would otherwise be.

Certainly the addition of salt will make the melt colder than it would otherwise be, that part is beyond dispute. But can the salt actually lower the temperature to less than that of the original ice. I tend to believe that it could, as the phase change is endothermic I cant see anything wrong with that in principle.

So here's an actual experiment, guess the outcome. If I set a freezer to have a well defined temperature of say -5C and then freeze a bunch of ice in it, so I know exaclty what temperature the ice starts at. Now lets say I take the ice and place it in a thermally insulated container which is also pre-chilled to an initial temperature of -5C (with no heat pump or refrigeration of any kind so that ideally no heat should be flowing in or out). Now if I start adding salt to this ice and mixing it in will the temperatue of the mix drop to below -5C. What do you say, yah or nay ?

 

[AbsolutDealage]

howstuffworks is your friend.

 

[dkozloski]

As a point of interest the definition of 0deg C. is the coldest temperature that can be achieved with an ice and saturated brine mixture. Of course 0deg. F is the freezing point of water.

 

[uart]

Originally posted by: dkozloski
As a point of interest the definition of 0deg C. is the coldest temperature that can be achieved with an ice and saturated brine mixture. Of course 0deg. F is the freezing point of water.

Sorry, but that's nonsense.

Originally posted by: AbsolutDealage
howstuffworks is your friend.

As far as I can tell the page linked merely reiterates what I have already posted but does not answer the specific question.

The specific question is NOT "does salt lower the melting/freezing point or ice/water". It does, I know that, no questions there.

The specific question is NOT "does salt lower the temperature of the melt/brine relative to what it would be if no salt was added". It does, I know that, no questions there.

The specific question is whether or not the addition of salt can reduce the temperature of the brine to less than that of the original temperature at which the ice is at before the salt is added, without any heat extraction by any other means.

Salt or no salt there is nothing to stop ice from existing at temperatures much lower than it's melting point. It's only the melt that, when in thermal equalibrium with the ice, is at 0C. Look at the very specific example and question in the last paragraph of my original post.

 

[ZeroNine7]

Just a few basics for others reading the thread:

1) pure ice can exist at any temperature below the freezing point of water
2) in a water/ice mixture that is well mixed (i.e. stirred) the liquid water will be essentially at the freezing point, however the chunks of ice will have a thermal gradient from freezing point at the surface to some colder temp on the interior.
3) dissolving salt into the water lowers the freezing point of the water

Now since the ice packed into an icecream freezer has many cavaties of air, the only significant place where the heat is transferred from the icecream mixture (metal container) is where the pieces of ice are in direct contact with the container. If you allow the ice to melt so that you have water contacting the surface of the container, you have significantly more surface area for heat transfer, however instead of being contacted by -10C ice, for example, it is mostly contacted with 0C icewater mixture over most of the surface. By adding salt to the water, you create a brine with lower freezing point, thus the ice (which is already at, say -10C) is able to cool the liquid water to below 0C without a phase change, so you get the significant heat transfer of liquid surrounding the container combined with a -5C (for example) medium, instead of 0C. Because the icecream will not freeze at 0C due to impurities in the water (sugar, fat, etc) you must cool it below 0C, which is why you use salt.

There may be more at work here than this, but it is at least a partial explanation.

 

[DrPizza]

Originally posted by: AbsolutDealage
howstuffworks is your friend.

Howstuffworks was somewhat wrong on this one. As already stated by zeronine7, ice cream freezes at a temperature below 32 degrees F. (At 32, ice cream would be well melted.)

A properly working freezer is well below 32 degrees F... closer to 0 degrees F.

When you're cooling the ice cream into a frozen dessert, heat flows FROM the ice cream mixture into the salt-water ice bath. Crushed ice by itself acts somewhat as an insulator and doesn't have as high of thermal conductivity as water. Plus, a liquid will have more surface area in contact with the ice cream vessel.

 

[FrankSchwab]

I'm still waiting to find out what happens if you take a bucket of ice at, say, -2 degrees C (there's no meltwater, assume the cubes are a constant temperature throughout, we don't have to worry about the freezing point of ice cream, etc) and dump salt on top.

Does the brine end up at a temperature below -2 degrees C?

It's been a long time since my college chemistry classes, but it seems to me that the phase change forced by the salt could indeed cool the system below it's initial temperature.

 

[dkozloski]

Sorry uart. I got it backwards. O deg. F is the lowest temperature that can be achieved by a brine/ice mixture. O deg. C is the freezing point of pure water. The last time I made home made ice cream I used ice that I had to carry a long ways from the store. By the time I got it home it was melting pretty well. I drained off the water and put it in the ice cream freezer and started adding table salt(works much faster). I didn't have a thermometer but the mixture was sufficiently cold to freeze the ice cream. It looked to me that I was going to run short of ice so I added ice from the ice maker in my fridge. The freezer compartment is set for 0 deg. F. It seemed to me like the temperature of the mixture dropped some more. The results were somewhat subjective but I would have to say that the ice cream would not have started to freeze at all unless the initial addition of the salt to the melting ice had caused the temperature of the mixture to depress.
Why don't you get a bag of ice, some salt and a thermometer and do some experimenting. When you give us the results it will make you a peer reviewed scientist.

 

[Mark R]

It's been a long time since my college chemistry classes, but it seems to me that the phase change forced by the salt could indeed cool the system below it's initial temperature.

I don't see why it shouldn't - by displacing the equilibrium in favour of the liquid phase, there should be a net cooling of the mixture - potentially to below the initial temperature.

 

[rjain]

dissolution is an endothermic process. The entropy of a solution is higher than the entropy of the solvent and solute separately. For those not familiar with thermodynamics, it's the same as the reason why aerosol cans get cold when they spray. The contents go from a small volume to a large volume, just as the salt goes from a small volume of the solid to the large volume of the solution, getting dispersed throughout it.

 

[NickE]

O deg. F is the lowest temperature that can be achieved by a brine/ice mixture

The lowest you will get with ice/salt is -21°C which is -6°F. Using other salts will give different temperatures - such eutectics are widely used for ice cream distribution, the choice being made dependent on the required temperature:-
Sodium Chloride: -21°C
Urea: -12°C
Sodium Acetate: -17°C
Calcium Chloride: -50°C

There are many others, including various mixtures (usually patented) to give a desired temperature.

The benefit of using eutectics over other phase change materials is that the temperature remains constant until all the ice has melted - obviously this is important for ice cream storage.

Adding salt to ice will lower the temperature until the mixture of ice/salt/water (assuming good mixing) reaches equilibrium. The temperature falls as the latent heat of fusion of the ice that melts is extracted from the remainder of the material. The reason that you continue adding more salt as time passes is that as more ice melts. so the brine becomes more dilute and you need to get it back to its optimum concentration.

 

[dkozloski]

Originally in the seventeenth century 0 deg.F was established as the freezing point of sea water. This was adjusted to be the freezing point of a saturated salt, brine/ice mixture because ocean salinity and the composition of the solutes is not constant worldwide. This was adjusted later to be the freezing point of equal parts of salt, ice, and water. There are so many variables here that it looks like the modern standard verges on being arbitrary.

 

[WobbleWobble]

Originally posted by: dkozloski
As a point of interest the definition of 0deg C. is the coldest temperature that can be achieved with an ice and saturated brine mixture. Of course 0deg. F is the freezing point of water.

At sea-level... depending on your elevation, the freezing point of water will vary.

 

[Trianon]

Originally posted by: FrankSchwab
I'm still waiting to find out what happens if you take a bucket of ice at, say, -2 degrees C (there's no meltwater, assume the cubes are a constant temperature throughout, we don't have to worry about the freezing point of ice cream, etc) and dump salt on top.

Does the brine end up at a temperature below -2 degrees C?

It's been a long time since my college chemistry classes, but it seems to me that the phase change forced by the salt could indeed cool the system below it's initial temperature.

I would think it would stay at -2C and liquid, as when they use salt on the roads in the winter. I am sure there are several superimposed TD processes going on when you mix salt with ice, so to isolate whether the temp of the brine drops from adding salt would be a pretty complex experiment in physical chemistry.

 

[rjain]

The thermodynamic processes are simply the conversion of ice and crystalline salt to a salt solution. There is no change in covalent bonding. The entropy of the mixture goes up quite dramatically, as we go from two solids to a liquid. Therefore, the process should be endothermic.

 

[Mday]

Originally posted by: NickE

O deg. F is the lowest temperature that can be achieved by a brine/ice mixture

The lowest you will get with ice/salt is -21°C which is -6°F. Using other salts will give different temperatures - such eutectics are widely used for ice cream distribution, the choice being made dependent on the required temperature:-
Sodium Chloride: -21°C
Urea: -12°C
Sodium Acetate: -17°C
Calcium Chloride: -50°C

There are many others, including various mixtures (usually patented) to give a desired temperature.

The benefit of using eutectics over other phase change materials is that the temperature remains constant until all the ice has melted - obviously this is important for ice cream storage.

Adding salt to ice will lower the temperature until the mixture of ice/salt/water (assuming good mixing) reaches equilibrium. The temperature falls as the latent heat of fusion of the ice that melts is extracted from the remainder of the material. The reason that you continue adding more salt as time passes is that as more ice melts. so the brine becomes more dilute and you need to get it back to its optimum concentration.

i cant believe you used the term eutectic with icecream =D

 

[Mday]

Originally posted by: NickE

O deg. F is the lowest temperature that can be achieved by a brine/ice mixture

The lowest you will get with ice/salt is -21°C which is -6°F. Using other salts will give different temperatures - such eutectics are widely used for ice cream distribution, the choice being made dependent on the required temperature:-
Sodium Chloride: -21°C
Urea: -12°C
Sodium Acetate: -17°C
Calcium Chloride: -50°C

There are many others, including various mixtures (usually patented) to give a desired temperature.

The benefit of using eutectics over other phase change materials is that the temperature remains constant until all the ice has melted - obviously this is important for ice cream storage.

Adding salt to ice will lower the temperature until the mixture of ice/salt/water (assuming good mixing) reaches equilibrium. The temperature falls as the latent heat of fusion of the ice that melts is extracted from the remainder of the material. The reason that you continue adding more salt as time passes is that as more ice melts. so the brine becomes more dilute and you need to get it back to its optimum concentration.

i cant believe you used the term eutectic with icecream =D

 

[NickE]

i cant believe you used the term eutectic with icecream =D

Why?

 

[ZeroNine8]

to answer the question about endothermic reaction, yes dissolving salt in water is endothermic and will result in a net decrease of the system temperature when salt is dissolved in water. The reason that salt is used in ice cream, however, is the primarily the freezing point depression that occurs by mixing salt in icewater. A quick googling yielded this handy little lab that explains it more.

http://www.iplp.org/Syllabi/GenEd/Chem110/Fpdep.doc

 

[rjain]

Yeah, ZeroNine. The OP recognized that the FP depression was the reason, but wondered if there were further effects. It's good to hear that I'm not totally off my rocker, tho. Thanks.