Freezing Point of Water

  1. Why do we define the freezing point of pure water as 0°C when most pure water does not freeze at that temperature?

    Instead, it can and does remain in the liquid phase down to temperatures as low as -48°C [Molinaro & Moore, 2011].

    Cold-weather clouds throughout the Earth’s atmosphere consist primarily of water droplets at temperatures tens of degrees below 0°C. These droplets of liquid water are quite stable. They condense, collide, agglomerate, fragment, and vaporize—all without freezing. Most cumulonimbi (thunderheads) do not form their icy “anvil” until updraft ambient air temperatures approach -42°C.

    I am well aware that the abundance of exotic (non-water) icing nuclei on the Earth’s surface brings about surface freezing in the vicinity of 0°C, but such water is hardly “pure”. Water uncontaminated by such nuclei does not freeze at 0°C.

    Why then should the generally accepted scientific definition of water’s freezing point be 0°C?
     
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  3. phinds

    phinds 9,053
    Gold Member

    I think it's because that's the highest temperature at which it will freeze. Everything thing you say is true as far as I know but which of those various temperatures at which water CAN freeze would you pick as being the temperature to say that it is where water DOES freeze? You really can't pick any particular temperature in the range of 0 degrees to something like minus 50 degrees. SO ... if you don't like zero, what would you recommend?
     
  4. PURE water at the surface of the earth does freeze at zero Celsius. Although I can't explain what goes on in the clouds because I don't know enough about those systems, I can assure you that zero Celsius is not a mistake.

    With that said there are plenty of factors which can change the physical properties of water like solutes for example, which will lower the chemical potential of liquid water thereby lowering the freezing point. I would not expect water droplets in the atmosphere to be pure, there's gotta be some 'stuff' in them.

    There is also the phenomenon of supercooled water which is a metastable state in which water can exist as a liquid below its freezing point.

    Not trying to completely solve the dilemma here. Just trying to say that the freezing point of pure water is not a mistake. In fact water has been measured so thoroughly that temperature scales were built around things like the freezing/boiling/triple point of water for a long time.

    I also don't understand what you mean by 'icing nuclei' and would like an example of such.
     
  5. phinds

    phinds 9,053
    Gold Member

    Hm ... I though it was possible to cool water below zero if it is kept very still. I've seen videos of so called "supercooled" water that freezes instantly if agitated. Perhaps that was not pure water and had suspended particles.
     
  6. Yes you are correct, I mentioned supercooled water in the third paragraph of my reply. Supercooled water can exist at temperatures below the freezing point provided that the container is very clean and, I believe, the water has minimal contaminants. Supercooled liquids will freeze when a site of nucleation is provided, such as a crystal of the already frozen compound. In undergraduate chemistry labs students are instructed to gently scrape the beaker containing the liquid to provide nucleation sites for freezing of supercooled fluids. Supposedly it is the tiny chunk(s) of glass that are scraped off which do this in the latter case.

    Metastability is analogous to a ball teetering on the edge of a cliff or top of a hill. Thermodynamically disfavored yet kinetically slow to reach equilibrium.
     
  7. AlephZero

    AlephZero 7,298
    Science Advisor
    Homework Helper

    Yup, I accidentally did the experiment once. It was an unopened bottle of distilled water, bought for topping up car batteries (which is showing my age!), left on a shelf in the garage at sub-zero temperatures. it was definitely still liquid until I took it off the shelf and tried to open it, and the expansion of the suddenly-freezing ice cracked the bottle.

    I don't know how pure it was, but it don't expect it contained the amount of gunk that is dissolved and suspended in tap water.
     
  8. I have no problem with saying that water freezes at a variety of temperatures, depending upon a variety of contributing factors. That's what I used to tell my classes.

    By the way, 0°C is not the maximum temperature at which water freezes. Plant scientists have long reported icing in plant tissues at air temperatures as high as 4°C, and very stable water-ice clathrates (ice mixtures that are neither compounds nor solutions) form under pressure in pipelines at temperatures as high as 18°C.

    The natural world is not as simple as we might wish.
     
  9. phinds

    phinds 9,053
    Gold Member

    Very interesting. I didn't know any of that.
     
  10. russ_watters

    Staff: Mentor

    The fact that you can sub cool water below 0C does NOT mean it doesn't still freeze at zero: when the sub cooling breaks, the temperature almost instantly rises to 0C and remains there while it freezes.
     
  11. How do you define "pure" water. All naturally-occurring water is contaminated to some degree, with various substances in solution and others in suspension. As a general rule, cloud droplets are much "purer" than surface water (the overwhelming bulk of which is saline).

    There is--as mentioned above. However, Most of this "stuff" does not depress the freezing point significantly.

     
    Last edited: Nov 30, 2013
  12. I did not read about any such warming in the study by Molinaro & Moore; nor have I read about any such warming in cold air clouds. There is a good deal of literature on the "icing point" in thunderheads that refers to freezing occurring at or near the -40°C level. I must admit that I do not know of any researcher brave (or foolish) enough to personally measure the temperature of these freezing cloud droplets in cumulonimbi. I suspect that it was done by remote sensing.

    I have no doubt but what you say is true under controlled laboratory conditions. I just wonder how well these conditions apply to the free and often violent atmosphere.
     
  13. russ_watters

    Staff: Mentor

    I'm sorry, but you taught them wrong.
    That's wrong too. The air might be at 4C but the water is not. Through evaporation, it cools.
     
  14. russ_watters

    Staff: Mentor

    0C is for atmospheric pressure. Which also applies to clouds: you'll have to check the freezing point at altitude based on the pressure.
    But simpler than you are making it.
     
  15. The studies clearly state that the plant tissues are at the same temperature as the air. The water that freezes is in the cells of the plant tissues. It is not exposed to the air and does not evaporate.

    As for the clathrates, I was given to understand that increased pressure depresses the freezing point, not elevates it. You have not given an explanation of water ice forming clathrates in pipelines at temperatures well in excess of 0°C.
     
  16. russ_watters

    Staff: Mentor

    It should - I was wrong about that. I don't know what is going on there. But:
    Without seeing such studies, I decline to speculate further. In any case, this is off track from the OP which was about normal water at atmospheric pressure.

    You should read the wikis on freezing point and supercooling:
    http://en.wikipedia.org/wiki/Freezing
    http://en.wikipedia.org/wiki/Supercooling
     
    Last edited: Dec 1, 2013
  17. I should have been more careful with my phrasing. By "the surface" I simply meant under the conditions of a laboratory, at the surface of the Earth. As opposed to the clouds. By pure water, I mean water that has been run through some sort of purification protocol.

    I never claimed that colligative properties are the sole factor responsible for such behavior. I'm just offering some phenomena up to show you that the freezing point of water can change and that those changes can be accounted for by known factors, instead of "Oh the freezing point of water is just wrong."

    Then they are, by definition, thermodynamically unstable. Think diamond/graphite, same kind of thing. Graphite is the stable form yet you need not fear that your diamonds will disappear any time soon as the conversion is kinetically disfavored.

    Please elaborate. Numerically the temperature scales were either identical or had very minor corrections. Certainly not anywhere near the error you are suggesting in your post. Regardless of all that, you do realize that the triple point of water is still used to set up the thermodynamic temperature scale?

    Yes it does, unless the fluid supercools but, once again, this is a metastable state which is not favored thermodynamically. Hence supplying a site of nucleation will help the kinetics side of it, but the freezing point of pure water is still O° C.

    There is always some amount of auto-ionization in water which is described by the value Kw. I don't know what you mean by electrostatically unsaturated.

    Have you ever learned thermodynamics? I think learning some of the core principles will help you greatly. I wish I knew more about atmospheric chemistry so I can help you out with your dilemma but I don't. I only want to get it through to you that the freezing point of pure water is not wrong and isn't a lie or whatever. There are plenty of factors which can explain liquids existing below their freezing points without resorting to such beliefs as all of the measurements were simply wrong.
     
  18. Plant tissue - Are we talking about why you have to bring your tomatoes indoors on a calm clear night where the air temperature can fall to somewhere in the vicinity of 4C. The plants radiate out to space and the air temperature is too low to supply enough heat via conduction to prevent freezing.

    Clathrates - that is a different ice structure from that of what we consider where the melting point of normal ice at 0C. Apparantly clathrate formation is thermodynamically favourable above certain pressures and below certain temperatures. Clathrate is actually classified as a hydrate since the ratio of water molecules to the interstial molecule is variable. The water will form around the methane molecule, for example, with hydrogen bonds caging the methane molecule inside. No methane ( or other stabilizing molecule that can fit within the space - no cathrate. )

    As you know liquid water molecules are always forming and unforming hydrogen bonds with related neighbors, and I suppose the methane molecule provides a location whereupon which the kinetic energy of other liquid molecules does not interupt the already hydrogen bond formation and thus other low kinetic energy water molecules can attach themselves into favourable locations to form the structure.
     
  19. fluidistic

    fluidistic 3,314
    Gold Member

    I agree with russ watters when he says that the water, despite freezing at lower than 0°C temperatures, will reach a temperature of 0°C almost instantly after freezing.
    I've played a lot in the lab with small drops of distilled water over a Pelletier cooler which was itself inside a refrigerator.
    The smaller the drops were, the lower the temperature would be before freezing. I could measure temperatures as low as -18°C, but as soon as they froze (which was instantly for the human eye), they'd reach a temperature of 0.0°C instantly.
    Water can reach temperatures below 0°C without freezing because it isn't an equilibrium state (it's metastable equilibrium). Some people say that with a minor "bump" of energy the water would freeze, but this is not what I've experienced with small drops of distilled water.
     
  20. I do not dispute this. This is explained by the release of the enthalpy of freezing (what used to be called "latent heat"). I note that you agree that (in ordinary English) water freezes at temperatures lower than 0°C and at temperatures higher than 0°C.

    When a mass of rising air containing water droplets at temperatures approaching -40°C freezes to become a mass of rising air containing ice crystals at temperatures approaching -42°C, I do not think that I am misleading my students in telling them that many cloud droplets freeze at around -40°C.

    By the way, since the triple point of water is 0.01°C and not 0°C, shouldn't we be using the former as the freezing point and not the latter?
     
  21. Are you saying that tomatoes are such effective radiators of EM that this radiation drops the temperature of the tomato below the temperature of the ambient air? I don't think so.

    The ice in clathrate lattices is chemically identical to ordinary ice. It is H2O. Yet, it is a solid at temperatures above 0°C.
     
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