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A question about water and freezing

  1. May 8, 2006 #1
    First please note that I have never taken a physics class in my life. The only reason that I have any clue whatsoever about what went on is because of SCUBA equipment and paintball markers. I have completed a brief search in the forums here and have not found a sutible answer. We have come upon an instance where one liter of water at approx 35-40 deg F will "spontaneously" freeze in a 120 deg F enviroment when pressure is removed from the water... is this a known....um...theory or something? I've looked around online for a little bit and may have found the answer but didn't understand it. Could anyone help? My guess is that during decompression the air that is dissolved the water cools. This may bring the water temp down to a freezing temperature...does that sound dumb? I really need "closure" on this one. Thanks :smile:
     
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  3. May 8, 2006 #2

    Mk

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    This is called supercooling. Its not a dumb question! Stay inquisitive.

    Supercooling is the process of chilling a liquid below its freezing point, without it becoming solid. Every few months somebody comes by and asks about the same situation. After the liquid is disturbed it spontaneously freezes, pretty much all in a jolt. The liquid must be very pure, and the sides of its container very smooth. Once disturbed, I think nucleation sites form, and combined with the lower-than-freezing-point-temperature, extremely fast leading to flash freezing.

    See http://en.wikipedia.org/wiki/Supercooling
     
    Last edited: May 8, 2006
  4. May 8, 2006 #3
    thank you.
     
  5. May 8, 2006 #4

    DaveC426913

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    The freezing/boiling temperature of a substance is dependent on its pressure.

    - Water freezing at 32F is only true at Sea Level.
    - Water at high altitudes will boil at a lower temperature than water at sea level (as anyone who goes mountain climbing and likes hot coffee in the morning will attest to - the water boils while the temp is still too low). - Water in space (zero pressure) will boil away regardless of how cold it is.

    Likewise, the reverse is true. If water is put under pressure, that lowers its freezing point. It stays liquid even as its temperature drops.

    The corollary is true as well.
    - If you apply pressure to ice, it will liquify. This is how ice skates work - the pressure of the blade causes a tiny bit of ice to liquify, allowing the blade to slide smoothly over it.
    - Same with curling stones and the brooms.
    - This is also why the Earth has a mostly liquid interior - under a huge amount of pressure, the rock remains molten i.e. liquid.


    The upshot is, the water in the container is - as Mk says - supercooled. It remains liquid while under pressure. But the moment the pressure is released, the water can freeze.

    Look at the attached diagram. Find the point at -5C and 1000Atm - you'll see that it is just barely liquid (I am picking greatly exagerrated values to make it easier to see). Now, reduce the pressure from 1000Atm to 10 atm (basically, draw a line straight down the chart from your first point). You can see that the water will spontaneously change from ice to liquid without any change in tempreature.
     
    Last edited: Jul 20, 2007
  6. May 9, 2006 #5
    Thank you, both of you, these are exactly the answers I was looking for. My mind is at peace now. :smile:
     
  7. May 9, 2006 #6

    Art

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    This is a common but flawed misconeption. The blades do not exert sufficient pressure to raise the temp by even a fraction of 1 deg.
     
  8. May 9, 2006 #7

    ZapperZ

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    Er.. the pressure is not supposed to raise the temperature. It doesn't have to to make the ice melt, because a higher pressure will cross the phase boundary at that same temperature (see the attached figure posted earlier).

    Zz.
     
  9. May 9, 2006 #8

    DaveC426913

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    The only thing flawed here is your reading... :rofl: You were overeager to find flaw. I didn't mention temperature at all.

    The whole point of the ice skate example is that we get liquifaction without raising the temperature. The liquifaction occurs directly from increased pressure. As Zz points out.
     
    Last edited: May 9, 2006
  10. May 10, 2006 #9

    Art

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    No you don't. The slope of the boundary line is -(1.2x10^7 N/m^2/C) It would take an increase of 120 atmospheres to lower the melting temperature 1 Celsius degree.

    For the typical skate blade, the area is 27 cm x 4 mm = 11 cm If the full weight of a skater with mass 65 kg is exerted on one blade, the increased pressure would be about 6 atm. Sharpening the blades does not decrease the contact area appreciably, since the blade sinks down into the relatively soft ice.

    At the interface between ice and air there is a fhin film of water. The thickness increases from monomolecular to several hundred molecules as the temperature rises from -10 to 0 C this is what allows skaters to skate. The ideal temp being -7 C. :biggrin:
     
    Last edited: May 10, 2006
  11. May 10, 2006 #10

    ZapperZ

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    But one doesn't have to change the melting temperature that much! At 0K and 1 atm, I only need to CROSS the phase boundary by the tiniest amount. Since at that condition I am practially on the phase boundary already, I don't have to lower the melting temperature by 1 Celsius to do that. That is why even a "small" pressure would be sufficient.

    Zz.
     
  12. May 10, 2006 #11

    Art

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    My point is you don't have to lower the melting point at all. There is already a thin film of water to skate on. That is why pucks (which can hardly be said to exert a great deal of pressure) also glide easily on ice.

    Also At 0 C ice would be very soft allowing the skates to sink which would reduce the ability to glide which is why -7 C is the ideal temperature for speed skating.
     
    Last edited: May 10, 2006
  13. May 10, 2006 #12

    ZapperZ

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    No, there is a difference here. A hockey puck glides easily in ALL directions. This is because of the surface smoothness between the ice and the puck. An ice skates doesn't. If it is simply a matter of surface smoothness, then an ice skates will also slide easily perpendicularly.

    In a real-world study, there are many factors that affect things like this (the same way a simple buoyant effect cannot be the sole explanation for how an airplane can take off). The pressure induced melting (in addition to friction induced melting when the skates are in motion) plays an important role.

    Zz.
     
  14. May 10, 2006 #13

    Art

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    I agree in a real world study there are many factors at play but to weight their importance the phenomenon of a thin layer of water appearing at the interface of ice and air is by far the biggest factor, friction from the leading edge of the skates may also play a part but the effect from pressure would be at best negligible if present at all.

    So returning to my original point liquifaction of ice through pressure is NOT what allows skaters to skate.
     
    Last edited: May 10, 2006
  15. May 11, 2006 #14

    DaveC426913

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    Hang on.

    If there were an ever-present layer of water for the blade to skate on, then there would be no reason for making the blades narrow - it would actually be more efficient to make them wider so as to spread the weight over a larger surface (like a hovercraft on a cushion of air).

    If you are about to protest about a larger surface area creating more friction, forget it. If you wanted to reduce friction, then you could reduce the surface area any way you want - you could just as easily reduce the *length* of the blade, while keeping the width. That would proportionally reduce friction while keeping the layer of water distributed. They don't do that.

    No, the only reason for making the blades as *narrow* as possible is because it actually *concentrates* the weight - *more* pressure per unit area.

    So, the question is, why would one deliberately design a *high* pressure-per-unit-area under the contact area of a skater's blades?
     
  16. May 11, 2006 #15

    Ich

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    To flatten obstacles and to carve the edges into the ice.
    I´d say that Art is basically right.
     
  17. May 11, 2006 #16

    Art

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    Dave do the maths and explain how ice at -7 C can be liquified by a pressure of 6 atm (3 atm really as most skaters skate on 2 legs :smile: )
     
    Last edited: May 11, 2006
  18. May 11, 2006 #17

    vanesch

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    I have to agree with Art here - although I was also convinced that it was the pressure that made the ice melt - so I learned something here (it seems indeed to be a very common misconception because my thermodynamics professor gave exactly this example!). But as per the attachment of Dave, giving us the phase boundaries in the T/P plane, indeed, when you go up from about -7 degrees, you arrive at about 100-200 atmospheres (which is certainly not the pressure under the skate, unless you are a slightly overweight sauropode).
     
    Last edited: May 12, 2006
  19. May 11, 2006 #18

    Bystander

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    --- and, to support this argument,

    http://www.me.utexas.edu/~bryant/courses/me383s/DownloadFiles/Sleds/GaneshLitReview.pdf#search='tribologyice'
     
  20. May 14, 2006 #19

    rcgldr

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    Figure skates are not made as narrow as possible. The current trend for figure stakes is for the blade to be thicker in the middle (front to back), apparently it helps with the turns.

    Speed skates are longer, but are also narrowerer than figure skates, and don't have the bulge in the middle. I don't know if it's by rule or by physics, but they don't make speed skates as narrow as possible.
     
  21. May 15, 2006 #20

    DaveC426913

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    Well, now we're getting into finer details of design. Clearly, there are diminishing returns and competing factors. I don't mean literally as narrow as possible, I mean narrower is better than wider.

    Again, if the physics principle behind skates were that "they ride on an existing layer of water" (as someone else stated), then there'd no reason why skates should be narrow. I'm saying the narrowness is critical to the creation of the water layer.
     
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