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I Freezing due to ice with salt

  1. Jul 4, 2017 #1
    I have seen this experiment with cooling down a beer: My question is why is cooling so effective?
    I was thinking about it and my guess is:
    Salt breaks down a bond in ice crystals, but this reaction needs energy. It took a energy from environment and all around is cooled down.
    Are my ideas fine?
     
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  3. Jul 4, 2017 #2
    That's pretty much it. My understanding is that the water molecules are more strongly attracted to the salt molecules than to each other. Putting salt on ice will pull water molecules out of the ice crystal structure onto the slat; forming a solution. Breaking these bonds requires energy, which is quantified by the heat of fusion of ice, about 333.5J/g. This energy will be drawn from the surroundings, cooling the beer.
     
  4. Jul 4, 2017 #3

    russ_watters

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    Ehh, kind of. The key issue is that salt changes the freezing temperature of the water. And since the water and ice can only be at equilibrium at one temperature, that lower temperature is the new freezing/melting temperature.

    ....though for this experiment, just the fact that the beer is submerged in cold water makes it beat the convection method of cooling.
     
  5. Jul 6, 2017 #4
    Thank you for your response. Maybe for me is also important, why ice (or generally materials) "freeze". Again I suppose:
    1) It depends on molecules (material) and their energy (temperature, mean velocity of molecules). If temperatu is high enough, there are no bonds and it's gas. If temperature is a bit lower there arise some bonds among few moleculs but these "bunches" are "free" - liquid. And if temperature is more lower there start appear of crystal.

    2) The temperature of these changes depends on moleculs and types of bonds which they create. And it is reason why materials vaporate and freeze in different temperature.

    3) A mixure of water and salt create bonds of crystal in lower temperature than water, and if I have ice with salt in 0 C, may I say that I have somthing like "a over heat ice"? Something as "a under cool water" (liquid water with temperature under 0 C)? (I'm not sure with terminology in english for these things). And a mixure of water and ice is starting to be a liquid and during this reaction, ice destroys bonds using surrending energy.

    Is it right?
     
  6. Jul 14, 2017 #5

    462chevelle

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    This is used a bit loosely for my tastes. Are there not 2 hydrogen atoms and 1 oxygen atom bonded together in water vapor?

    Also consider how pressure effects it as well. Think of the boiling point of water at high altitudes.

    I would recommend Tro's general chemistry book, you would probably enjoy that piece.
     
  7. Jul 15, 2017 #6

    symbolipoint

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    Keep in mind that when ice is made and stored, and very shortly after supplied, it is generally at BELOW 0 degrees C. It will melt at 0 degrees C.

    As a simple qualitative statement, adding salt to water will lower the freezing point of that water. This means, add salt to water, dissolve, then add very cold ice (LESS than 0 degrees C), you can make the water with salt in in, become lower than 0 degrees C.
     
  8. Jul 15, 2017 #7

    Orodruin

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    You can make salt water at lower than 0 °C using ice at 0 °C. The reason is not necessarily that the ice is colder. See, e.g.:

    Note that the original mixture in the video is already a stable mixture of water and ice that, unsurprisingly, has the temperature 0 °C.

    Yes, adding the salt lowers the freezing temperature, but melting the ice requires energy.
     
  9. Jul 15, 2017 #8
    But some of the ice, towards the center of the cubes, could still be several degrees below 0 C. A home freezer is typically ~ -10F/-23C, so ice is typically this temperature as well. As long as there is enough water to melt the surface of the cubes w/o freezing, you have 0 C water. An ice cube doesn't melt instantly, it is large enough, and insulative enough to retain the lower temperature internally. This can keep chilling the water, which can now be below 0 C with the addition of salt.

    I think there is an explanation for how salt can actually lower near 0 C ice in 0 C water to below the temperature of the ice. But the experiment shown doesn't really prove this, as the ice itself could be cold enough to give these results. I'd like to see that done with ice that was carefully frozen and kept at -1 C all the way to the center.
     
    Last edited: Jul 15, 2017
  10. Jul 15, 2017 #9
    Poorly done experiment though. I'd like to see a comparison between ice water without the added salt as well - is it worth it to waste all that salt? I'd bet that at 10 minutes it wouldn't make all that much difference, and just a little movement of the can in the ice water would make a big difference. You tend to get a boundary layer forming around the can, and even a little motion will keep pulling the heat away much faster. It is why we have fans mounted to the radiators of ICE cars.

    And what's the point of wrapping a wet towel around the can and putting it in the freezer? The water will be absorbing heat and preventing the surface of the can from getting below 0C (until it freezes, but that takes time). You'd be better off with the naked can in the freezer, at least the aluminum would be in direct contact with -23 C air. But again, a water slurry in the freezer would be better than a bare can. The freezer would help keep the water near 0 C.

    If the towel was in the freezer and already frozen, then maybe just enough water added to make it flexible, wrapping it around the can might help a bit. But I dunno, the towel is an insulator, even when wet.
     
  11. Jul 15, 2017 #10

    Orodruin

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    Yes it does if you do it correctly. The correct way is to mix it in a thermally isolated container and let it sit for some time to equilibrate the temperature. This is the only reasonsble way to do it. Try it!

    If the ice was below zero, heat from the surrounding water would dissipate into the ice, resulting in more freezing. If your mixture is stable, the temperature of the ice is zero.
     
  12. Jul 15, 2017 #11
    Oh, I believe that it does! I've read the explanation somewhere, and though I have a little trouble fully understanding the exact science behind it, I can accept it.

    My real point was that we can't really know in that experiment if there was time for the ice to be just a few degrees below 0 C through and through, so it isn't really a definitive proof of the concept that adding salt to the mixture lowers the temperature of the entire mass. It really only proves that the freezing point of the water was lowered, which maybe only allowed very cold ice to bring the temperature of the salt water solution lower.
     
  13. Jul 15, 2017 #12

    Orodruin

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    You could fake anything in a YouTube movie. I only provided it as an illustration. If you want to convince yourself, it is a very easy experiment to carry out.
     
  14. Jul 15, 2017 #13
    I'm not insinuating anything was faked in that video. I'm just saying that demo-wise, it's not taking into consideration the possibility that the ice itself could still be very cold internally.

    I watched again, they were zoomed into the thermometer and not the ice. For the temp drop to be due to just lowering the freezing point and having ice that cold, it seems that you'd need to melt a lot of the ice very fast to get to that colder interior, but we can't see the ice. But it doesn't seem likely that could happen that fast. And it doesn't seem likely you could get as low as they did (-13 C?) starting with -23 C ice. I'm sure an experiment with -1 C ice would show me that the salt-water solution would go below -1 C.

    But since I can't easily stick a probe into the center of an ice cube, it would be a little tough to be sure my ice was only just below freezing, so I'll probably pass on the experiment.
     
  15. Jul 15, 2017 #14

    Orodruin

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    So what? Given the heat conductivity of ice it is just a matter of waiting for some time before you can be sure that your ice is sufficiently close to zero. Still you talk about the video and not the experiment itself. I would say you are focusing on the entirely wrong thing.

    Also, the ice will not be below freezing. It will be at the freezing temperature and so will the water. They must be at the same temperature or heat will flow between them. This is basic thermodynamics. If the ice was colder, all you would need to do is to wait while the heat released as the water freezes heats up the ice. This is a rather rapid process. Do the experiment yourself if you are worried about such things. It is extremely simple to ensure that the water and ice both start out at zero degrees unless you want to argue against basic thermodynamics.
     
  16. Jul 15, 2017 #15
    Sorry, I must be doing a poor job of communicating. Couple that with my lack of some specific knowledge (but curious none the less), and we seem to have a gap of understanding here.

    You say "Given the heat conductivity of ice...". OK, but I was thinking that ice isn't so very conductive (sorry for the imprecise, subjective terms). I live in a cold climate, and snow on the ground is known to insulate the ground to reduce how deep the frost goes. I think ice on a lake helps keeps the water warmer, it is not exposed to blowing sub zero (F) winds.

    And I have observed that ice added to a room temperature beverage does not melt in seconds, or even minutes, yet the beverage is not so very cold (not within a few degrees of 0 C). So to my thinking, ice isn't so very thermally conductive, or it would melt quickly until the beverage was very near 0 C.

    I think you over-estimate how simple the experiment would be. I really shouldn't assume anything. So I would want to go to some fairly extreme measures to assure that my ice cubes were just a few degrees below 0 C. That isn't easy without assuming they have good thermal conductance. My freezer is ~ -10 F (-23 C), my fridge is ~ 36 F (~ 2.2 C). So getting my ice to sit for hours @ ~ -1 C isn't so trivial. Maybe it makes no difference, but I should not assume that in an experiment.

    You say "If the ice was colder, all you would need to do is to wait while the heat released as the water freezes heats up the ice. This is a rather rapid process.". But I've also seen ice in a cooler, in a warm room, stay frozen for days (floating in the ice melt). So, experimentation-wise, I really don't want to assume how 'rapid' rapid is.
     
  17. Jul 16, 2017 #16

    Orodruin

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    Ice is quite conductive. The exact time depends on the size and shape of the ice, but leave the mixture for an hour and you can be quite sure your mixture is fairly homogeneous.

    Snow is not ice. It has ice crystals, but also contains a lot of air.


    You are mixing two things now, the heat of fusion and its time scale and the time scale of heat conduction. As has been stated countless times, melting ice requires energy, quite a lot of it. The heat of fusion is around 300 kJ/kg. Compare this to the specific heat capacity of water (4.2 kJ/kg K) and ice (1.9 kJ/kg K) and you will soon come to a few conclusions:
    • Ice has less than half the heat capacity of water. If you had equal amounts, lowering the water temperature by one degree would heat the ice by more than two degrees.
    • What really makes the water cold is the process of melting the ice. By comparison, raising the temperature of the ice by 20 degrees requires around 40 kJ/kg, which is still an order of magnitude lower than the energy required to melt it.
    The same goes for the water. It will relatively easy go down to the freezing point, but it will not freeze unless additional heat is removed. This is unlikely to happen because once the water gets down to zero, the ice is already at that temperature.

    This is just getting silly. You are not listening to what I say. You do not need to keep the mixture in the freezer or fridge. Just keep it in a thermally isolated container. The ones you use for keeping beverage hot when you go on an excursion will do perfectly fine. The ambient temperature will then not matter.

    Also, it does not need to sit for hours. A single hour is significantly more than enough.

    Again, you are mixing concepts here. The ice-water mixture equilibrates rather quickly to zero but it takes much more energy to melt the ice than to heat it to zero degrees. Once the mixture is equilibrated, the heat transfer from the surrounding air is pretty inefficient.

    In conclusion, I would say that your objections seem grounded on not having learned or ignoring the entire concept of heat of fusion. What kind of background in thermodynamics do you have?
     
  18. Jul 16, 2017 #17
    Ironically, you seem to be getting pretty 'heated' for a thread on ice. :smile: We seem to be talking in circles, so I'm going to drop any further discussion with you on this topic in this thread, after a (hopefully) brief closing statement.

    I think we are talking past each other. I have a fairly good grasp of some of the basics of thermodynamics (you jump to some conclusions that I don't know some things that I do know), but I'm no expert and there are limits and gaps in my knowledge. But I fully understand it takes far more energy ( ~ 144x IIRC) to move water/ice through 1 degree F and a phase change, than just moving it 1 degree F. That's not the point though.

    My point was, if someone is doing a demo like this, it is obviously for people who don't have a solid grasp on the subject. Experts in the field don't need to be shown this, anymore than I need to be shown that E= I * R with a video of a resistor, wires, source and meters to know how to apply that knowledge to some circuit analysis or design.

    And if you are demonstrating an effect to someone, I'd say it is best to eliminate other effects, or explain them. (edit, longer than needed :) )

    Bottom line, all I was trying to say, (and I didn't think it would be controversial or I would not have bothered) is that I feel the demo would be more effective and would have more impact (but not 'wrong' or 'faked' or anything of the sort') to the intended audience if they started with ice @ ~ -1 C, and showed that the salt addition brought the temperature below the temperature of the ice. That's all. Then they wouldn't also have to explain that the phase change has a larger effect. One step at a time, seeing the temp drop below the ice would just be a more direct way to demo this. Sorry if that got you worked up.

    And if I did get anything wrong in that, I would appreciate corrections from anyone else interested in the topic.
     
    Last edited: Jul 16, 2017
  19. Jul 16, 2017 #18

    Orodruin

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    They are starting with ice at 0 degrees. I do not see why this is a difficult concept to grasp. I think a beginner in the field would accept this if given and if not the reasons can be easily explained.

    Yes, it is the point. Based on your previous posts it seems that you do not fully understand the implications of this statement. The entire point of adding the water is to make sure that the thermodynamical equilibrium between the two phases is reached. The ice is at 0 degrees whether you accept it or not.

    I believe I have explained to quite some extent why the ice is at zero degrees in this experiment. It follows from basic thermodynamics as long as you wait long enough for the ice and water to reach equilibrium. To reach this conclusion, you do not need a thermometer, just basic knowledge in thermodynamics.
     
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