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Pure water doesn't freeze at 32F ?

  1. Pure water doesn't freeze at 32F ??

    This has me curious. I am reading a Scientific American article, which the following is from(emphasis mine):

    In flight icing is where the airplane is flying through clouds made up of small liquid water droplets. These liquid water droplets can be sustained as liquid below the freezing point. Everybody knows that 32 degrees Fahrenheit (0 degree Celsius) is where water freezes. It turns out that if the water is very pure—if it is condensed out of the atmosphere—and there is nothing for that water to freeze on, it can be sustained below the normal freezing point. What we find in the wintertime is clouds that are made up of small water droplets where the water temperature can be as low as negative 40 degrees C. Here comes this plane flying through the cloud, and the water droplets impact the airplane and then freeze because now they have a surface to freeze on.

    Source: http://www.scientificamerican.com/article.cfm?id=ice-flight-3407

    Anyway, I'm confused. Can water remain liquid at -40 C. ?
     
  2. jcsd
  3. Re: Pure water doesn't freeze at 32F ??

    Yes it can.

    This is what is important from the statement you posted. Evaporated water droplets hanging in the atmosphere, have nothing to "grab" to so they turn into ice.

    You have the same effect with vapor in your shower, evethought you don't shower at more than 100°C (hopefully for you, anyway). In your shower, you will have some water vapor that is haning in the air. If an object passes through this vapor, condensation will form on the object.

    Cheers
     
  4. Re: Pure water doesn't freeze at 32F ??

    The keyword phrase you'd want for a search is supercooled water.
     
  5. Borek

    Staff: Mentor

    Re: Pure water doesn't freeze at 32F ??

    You may also want to check for superheated water - same problem, although on the other end of the scale.

    Note, that fact that supercooled and superheated water exists is not in conflict with the statements "water freezes at 0 deg C" and "water boils at 100 deg C". Both superhated and supercooled water are metastable.
     
  6. Mapes

    Mapes 2,532
    Science Advisor
    Homework Helper
    Gold Member

    Re: Pure water doesn't freeze at 32F ??

    On a related note, pedants will eschew the term "freezing temperature" because of this issue: freezing involves an energy barrier, so this temperature can vary with cooling rate, impurities, container internal smoothness, etc. "Melting temperature" is better defined; there's little or no activation energy associated with melting.
     
  7. Re: Pure water doesn't freeze at 32F ??

    Thanks. This is making more sense now.

    It's funny, for all the years I've involved myself with physics(as a layman) every once-in-a-while something pops up that surprises me and I need clarification.

    And Mapes, I like your "related note" Thanks.
     
  8. Re: Pure water doesn't freeze at 32F ??

    There's an interesting experiment you can do with this - if you take high end pure bottled water and put it in a freezer for awhile, you can take it out and it wont be frozen. But, if you tap the bottle all of the water will freeze at once. It's very cool to watch.
     
  9. Re: Pure water doesn't freeze at 32F ??

    Indeed, those are great demonstrations.

    However, the "freezing" does not occur all-at-once. In fact, there is a substantial lag.
     
  10. DaveC426913

    DaveC426913 16,178
    Gold Member

    Re: Pure water doesn't freeze at 32F ??

    I've had this happen inadvertantly. It's cool. The bottle goes from liquid to solid in about five or ten seconds. In terms of something that usually happens too slow to be easily observed with the eye, ten seconds is pretty close to all-at-once.



    BTW, I have some heat pads that do this. They are liquid-filled bags about the size of your hand. You press hard on a button on the bag (the pressure somehow provides a nucleus for crystalization) and the liquid turns solid in about ten seconds and heats up. You can watch the crystals grow. They're reusable too. Pop em in boiling water and the solid returns to a liquid.
     
  11. Borek

    Staff: Mentor

    Re: Pure water doesn't freeze at 32F ??

    Sodium thiosulfate pentahydrate.
     
  12. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    Re: Pure water doesn't freeze at 32F ??

    Do we allow that sort of language on pf?

    You can easily do the opposite experiment (superheating) with a cup of coffee, a microwave and a clean white shirt.
     
  13. Re: Pure water doesn't freeze at 32F ??

    I'm pretty sure this is a joke, but I wouldn't recommend trying this! I had it happen with water once; the water looks all peacefull and calm, until you try to take the cup out. Then it starts to boil and the water sprayed over my hand giving me a pretty bad burn!
     
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