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Does putting salt in ice make it colder?

post #1 of 23
Thread Starter 

I saw a show on this once- can someone explain the principle to me?

post #2 of 23

Salting ice is abusing a property called "colligative properties". Salting ice does not make it "colder". Salting ice lowers the freezing point of water. Since there are charged atoms called "ions" in salt, these ions interfere with the normal attractive forces in water. In other words, its hard for the water to sit still and hang onto eachother (freeze) with all of these pesky, yet beatifully beneficial, ions in the way. So, if you don't want ice on your bridge, salt it. Salt will lower the freezing point, which means that it will have to be colder to make it freeze.

post #3 of 23

Also, while I have my soap box handy, colligative properties are used ubiquitously in the kitchen. Brines have high osmotic pressure allowing us to facilitate ion transfer (salting) and, when properly equilibrated, retains more moisture than salting something on the exterior. Same thing with pickling and so on and so on. I must also add that the colligative properties are directly proportional to the number of ions present not the weight of solute added. Please keep in mind that salting your water will not raise the temperature at which it boils significantly. I recall calculating it to be an extremely large amount of salt to raise the boiling point of water by 1*C.
 

post #4 of 23

Yes, it makes it colder. It's the whole concept behind salt in the ice for older ice cream makers.

 

It's all about the phase change of ice to water.  This is not itself a temperature change. Water at 32 degrees can be both solid (ice) or liquid. Liquid is just a higher energy state than ice. Salt forces the phase change in ice. Since the ice is becoming water, this phase change has a certain energy gain associated with it. Normally, this occurs at the speed that heat from the room is absorbed into the ice. But salt forces the issue.

 

So the energy for the phase change must come from somewhere. In this case, it comes from the ice itself. The ice gives off it's own very low heat to the water that formed from the ice. In giving this heat (energy) to the water, the ice becomes colder.

 

Liquid water is a higher energy state matter than ice. It will usually be the same temperature as the ice it formed from until it gains or loses more energy itself.

Palace of the Brine -- "I hear the droning in the shrine of the sea monkeys." Saltair
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Palace of the Brine -- "I hear the droning in the shrine of the sea monkeys." Saltair
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post #5 of 23
Thread Starter 

Thank you for an excellent explanation.  Does the salt also make the ice more dense?

post #6 of 23

No, it doesn't make the ice more dense. That's more of a factor of how much dissolved air there is in the water and  the method of freezing and so on.

 

In fact, freezing water tends to force the salt out of the water and is one method considered for de-salination. But it's pretty high energy so works best in cold climes so you can use the ambient temps for free as it were.

Palace of the Brine -- "I hear the droning in the shrine of the sea monkeys." Saltair
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Palace of the Brine -- "I hear the droning in the shrine of the sea monkeys." Saltair
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post #7 of 23
Thread Starter 

Thanks Phatch

post #8 of 23

OK. So this guy Fahrenheit is making this temperature guide/gage.  He takes normal plain water and freezes it.  He then starts adding salt (ammonium chloride) to the water turning it into brine, while watching the temperature guide/gage go down.  By-the-way ... he doesn't have any number scale on this guide/gage yet.  He keep adding salt, and the temp keeps going down, until it stops.  The salty water freezes. No more salt changes anything.  He then tags this temperature with the degree number "0".  It all goes up from there.  How?  I don't know.  I stopped reading at that point.  It was my appointment time.  That particular science comic wasn't there the next time I had an appointment. 

post #9 of 23
Thread Starter 
Salt makes the ice colder
post #10 of 23

I have to disagree ... salt doesn't make ice colder ... the salt does however dissolve the ice and allow the water to get to lower temps before it freezes ... as for making ice cream I think the biggest advantage that the salt gives you is surface area, the more surface area you have the quicker your ice cream base will become cold and solidify, but putting ice in a bucket with salt doesn't make it colder at least to the best of my knowledge.

post #11 of 23

What purpose does lowering the freezing point of the water around the ice cream serve?  That doesn't change anything inside the ice cream churn.

 

Ah, I think we're saying the same thing in different ways. By forcing the water out of the ice into a salted slush, the water can absorb more energy out of the ice and still not freeze, thus lowering the temperature of the system as a whole  and speed up the transfer of heat out of the churn.

Palace of the Brine -- "I hear the droning in the shrine of the sea monkeys." Saltair
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Palace of the Brine -- "I hear the droning in the shrine of the sea monkeys." Saltair
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post #12 of 23
Quote:
Originally Posted by phatch View Post

What purpose does lowering the freezing point of the water around the ice cream serve?  That doesn't change anything inside the ice cream churn.

I believe it makes the temperature of the water (therefore the inside of the ice cream churn) drop while keeping the water in a liquid state (which is much more efficient to lower the temp of the ice cream). 

 

I worked in a restaurant and they used that technique to quickly cool down bottles in a pinch: put in a bucket with water, ice and salt. 

post #13 of 23
Thread Starter 
Ice cream is dense
post #14 of 23

OK. So this guy Fahrenheit is making this temperature guide/gage.  He takes normal plain water and freezes it.  He then starts adding salt (ammonium chloride) to the water turning it into brine, while watching the temperature guide/gage go down.  By-the-way ... he doesn't have any number scale on this guide/gage yet.  He keep adding salt, and the temp keeps going down, until it stops.  The salty water freezes. No more salt changes anything.  He then tags this temperature with the degree number "0".  It all goes up from there.  How?  I don't know.  I stopped reading at that point.  It was my appointment time.  That particular science comic wasn't there the next time I had an appointment. 

The salt we're talking isn't ammonium chloride, but sodium chloride.  While salt does lower the freezing point of water that doesn't have a great deal to do with what's at work in the churn.  You need to start reading comic books with more of a culinary focus.  

 

I believe it makes the temperature of the water (therefore the inside of the ice cream churn) drop while keeping the water in a liquid state (which is much more efficient to lower the temp of the ice cream).

No as to dropping the temperature of the water.  The temperature of ice doesn't drop when you put salt on it.  Besides, we're not trying to lower the temperature of the ice.  We're trying to lower the temperature of the base inside the churn.  If salt -- by itself -- made the difference you thought it did, we'd all be eating salty ice cream.  But... 

 

Yes as to efficiency. 

 

By forcing the water out of the ice into a salted slush, the water can absorb more energy out of the ice and still not freeze, thus lowering the temperature of the system as a whole  and speed up the transfer of heat out of the churn.

Damn close, but not quite there. 

 

At least you're talking about speeding up the transfer of heat.  But (1) You can't force water out of ice; and (2) Besides the normal processes of energy transfer to the environment and entropy the temperature of the system as a whole is NOT lowered.  The temperature of the system as a whole remains the same, but the temperature of the different parts of the system change as the system comes into equilibrium. 

____________________________________

 

Anyone who cares should go to Wiki or some other basic science site and look up the concepts of Zeroth Law of Thermodynamics and latent fusion.

 

Just as a sort of practical quick and dirty... the Zeroth (prounounced zero - ith) Law of Thermodynamics says that when a hotter thing touches a colder thing, the hotter thing transfers energy to the colder thing and the hotter thing gets colder while the colder thing gets warmer.  Salting the ice in an ice cream churn allows the icy slush to absorb heat energy (through the walls of the churn's container) from the base more efficiently.  

 

So, the next question...

Why is that important?

The faster the base goes from liquid to solid, the better textured (fewer and smaller ice crystals) the ice cream.  That's why -- with most churns -- it's a good idea to get the base as cold as possible before putting it in the churn. 

 

Hope this clarifies,

BDL


Edited by boar_d_laze - 3/27/13 at 7:36am
post #15 of 23

x

post #16 of 23
Quote:
Originally Posted by phatch View Post

What purpose does lowering the freezing point of the water around the ice cream serve?  That doesn't change anything inside the ice cream churn.

 

Ah, I think we're saying the same thing in different ways. By forcing the water out of the ice into a salted slush, the water can absorb more energy out of the ice and still not freeze, thus lowering the temperature of the system as a whole  and speed up the transfer of heat out of the churn.

 



Agreed with FF. Salt will facilitate in melting the ice. The whole idea is that you take a solid at its freezing point and convert to a liquid as close to its freezing point as possible. The  effect upon your icy bucket is that your water is in the liquid state at a lower temperature than if it were pure. This works towards your advantage in making ice cream because a liquid will cover more surface area of the container than the odds, edges and flat surfaces of solid ice cubes. This will give a more uniform temperature along the surface of the container.

 

Now, most importantly, salting and ice/water bath will lower the temperature. I said earlier in this thread that it would not, I didn't think it would be very measurable. I was wrong. The reason why it makes it colder is that the dissolving of salt in water is endothermic. This means it absorbs energy in the form of heat, making that liquid colder. This is due to the fact that ions are breaking hydrogen-bonds in water which requires energy. "Where does this energy come from to break those hydrogen-bonds?" - Good question, it comes from intrinsic thermal energy simply by the object existing at a temperature above absolute zero.

 

 

I'll be the first to cite a reference: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-cools-icewater.shtml

post #17 of 23
Quote:
Originally Posted by boar_d_laze View Post


Anyone who cares should go to Wiki or some other basic science site and look up the concepts of Zeroth Law of Thermodynamics and latent fusion.

 

Just as a sort of practical quick and dirty... the Zeroth (prounounced zero - ith) Law of Thermodynamics says that when a hotter thing touches a colder thing, the hotter thing transfers energy to the colder thing and the hotter thing gets colder while the colder thing gets warmer. 

 


 

This is not what the 0th law of thermodynamics states. You might be confusing it with the 1st law of thermodynamics. The 0th law of thermodynamic states if A is in equilibrium with B and B in equilibrium with C then A must be in equilibrium with C. You can find this by wikipedia.

post #18 of 23
Thread Starter 
Quote:
Originally Posted by thecytochromec View Post

This is not what the 0th law of thermodynamics states. You might be confusing it with the 1st law of thermodynamics. The 0th law of thermodynamic states if A is in equilibrium with B and B in equilibrium with C then A must be in equilibrium with C. You can find this by wikipedia.

That's the transitive property.

post #19 of 23

I always thought that the "transitive property" was all the grassy area around the train and bus stations. 

 

 

 

Fahrenheit

From Wikipedia, the free encyclopedia

 

Fahrenheit proposed his temperature scale in 1724, basing it on three reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt, at a 1:1:1 ratio. This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point, at 32 degrees, was a mixture of ice and water without the ammonium chloride at a 1:1 ratio. The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat".

 

 

LOL.   In regards to the comic books I used to read ... HEY, we're talking over 40-years ago here. Little kids sometimes gotta just take what they're given. 

post #20 of 23
Thread Starter 
Quote:
Originally Posted by IceMan View Post

I always thought that the "transitive property" was all the grassy area around the train and bus stations. 

 

 

 

Fahrenheit

From Wikipedia, the free encyclopedia

 

Fahrenheit proposed his temperature scale in 1724, basing it on three reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt, at a 1:1:1 ratio. This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point, at 32 degrees, was a mixture of ice and water without the ammonium chloride at a 1:1 ratio. The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat".

 

 

LOL.   In regards to the comic books I used to read ... HEY, we're talking over 40-years ago here. Little kids sometimes gotta just take what they're given. 

That's the transient property....

post #21 of 23

I was wrong when I wrote Zeroth Law,  I should have written First Law and specifically referred to adynamic processes.  Considering the gravity of my error, seppuku seems appropriate. Irrespective of my leaky memory the point still stands.

 

Here's a more complete and coherent explanation of how salt works in the ice-packed ice cream churn system:

 

Given:

A:

  • The melting point of ice made from pure water is 0C.
  • With enough salt present, the melting point can drop below -20C.
  • While the freezing point of pure water is 0C, ice can get considerably colder -- basically as cold as its environment.

 

B:

  • The lower the freezing/melting point, the more quickly the ice will melt.
  • When ice begins to melt, the remaining ice and resulting water form a "slurry."
  • As long as the slurry contains a fairly high percentage of ice, the slurry will be the same temperature as the ice's melting point. And yes, it could be as cold as -20C. Thus, if the ice in the churn was colder than the 0C, and it was salted, the resulting slurry will also be colder than 0C.

 

C:

  • The more quickly the ice melts -- the lower the melting point, the quicker the melt -- the more quickly a slurry forms, and the more quickly the liquid in the slurry rises to up the sides of the churn's base container.
  • A water/ice slurry will transfer heat more efficiently than ice cubes (mostly because liquids "fill in the gaps," making immersion-contact generally more efficient than mere contact).

 

And D:

  • The more quickly that the ice cream base goes through the transition from liquid to solid, the smaller the ice crystals in the ice cream and the better its mouth feel.

 

Then QED:

  • A salted ice-water slurry will work more quickly than either ice cubes alone, or an unsalted slurry; consequently it will make better ice cream.

 

Note: Any work done by the breaking of atomic bonds is limited to changing the BP/MP of ice, and consequently the temperature of the slurry. The increased efficiency of salted ice vs pure ice has nothing to do with "sucking" heat energy out of the system.

 

Note also: An ice packed ice cream maker is not a completely closed thermodynamic system. Nevertheless for our purposes, it acts like one.

 

Hope this helps,

BDL


Edited by boar_d_laze - 3/28/13 at 8:27am
post #22 of 23

OK.  So it's the early 90's when I taught in a small building with a short hallway; three(3) classrooms each side.  We found ourselves a 5# coffee can along with a 3# coffee can.  We filled the smaller between 2/3 - 3/4 full with ice-cream stuff, sealed it up and stuck it in the larger can.  We packed the surrounding space with really well crushed ice that was heavily salted; a thick heavy slurry, as it was.  Six(6) students took turns rolling this ice-cream  machine back and forth down the hallway.  When the slurry had melted itself all down to liquid, we removed the smaller can and put it in a small refrigerator/freezer in the teacher's lounge.  At the end of the day we had some really nice finished product. 

post #23 of 23
Quote:
Originally Posted by thecytochromec View Post

 



Agreed with FF. Salt will facilitate in melting the ice. The whole idea is that you take a solid at its freezing point and convert to a liquid as close to its freezing point as possible. The  effect upon your icy bucket is that your water is in the liquid state at a lower temperature than if it were pure. This works towards your advantage in making ice cream because a liquid will cover more surface area of the container than the odds, edges and flat surfaces of solid ice cubes. This will give a more uniform temperature along the surface of the container.

 

Now, most importantly, salting and ice/water bath will lower the temperature. I said earlier in this thread that it would not, I didn't think it would be very measurable. I was wrong. The reason why it makes it colder is that the dissolving of salt in water is endothermic. This means it absorbs energy in the form of heat, making that liquid colder. This is due to the fact that ions are breaking hydrogen-bonds in water which requires energy. "Where does this energy come from to break those hydrogen-bonds?" - Good question, it comes from intrinsic thermal energy simply by the object existing at a temperature above absolute zero.

 

 

I'll be the first to cite a reference: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-cools-icewater.shtml

Do I sense a fellow biochemist in the house, Mr. Cytochrome? :D

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