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Thread: Freezing Point vs Pressure

  1. #1 Freezing Point vs Pressure 
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    I do not believe I have seen this addressed. F.P. and B.P. in my studies of Physics and Chemistry only detailed changes due to mixtures.

    How does the pressure present affect the freezing point of water? Water is my main concern because of "busted pipes".

    If this is a commonly considered thing, though I have no recollection of having studied it, shame on me. Otherwise, ?


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  3. #2  
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    Quote Originally Posted by jocular View Post
    I do not believe I have seen this addressed. F.P. and B.P. in my studies of Physics and Chemistry only detailed changes due to mixtures.

    How does the pressure present affect the freezing point of water? Water is my main concern because of "busted pipes".

    If this is a commonly considered thing, though I have no recollection of having studied it, shame on me. Otherwise, ?
    Since water expands when it freezes, a pressure increase makes it harder for the water to freeze. In other words, increasing pressure lowers its freezing point. This is highly anomalous: most materials contract when they freeze, rather than expanding.

    The unusual behaviour of water is the reason why ice is slippery. When something presses on it, some melting of the surface occurs, creating a thin film of water that lubricates the ccontact.

    The reason why ice occupies a larger volume than liquid water at the same temperature is due to hydrogen bonding, which causes water molecules to take up a relatively "open" structure in the solid.


    Last edited by exchemist; October 16th, 2013 at 03:08 AM.
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    As well as ocean water at great depth will not freeze, even when below zero degrees centigrade- it's under great pressure.
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    Look at the phase diagram of water here,

    Triple point - Wikipedia, the free encyclopedia



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  6. #5  
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    Quote Originally Posted by Harold14370 View Post
    Look at the phase diagram of water here,

    Triple point - Wikipedia, the free encyclopedia



    Yes. The interesting thing about this is the decline in freezing point with increasing pressure is only very gradual.

    This implies that to make ice slippery, very high local pressures must exist in the contact, in order to achieve the required reduction in FP and thus cause liquifaction. But this is in fact what happens. When you consider any contact between two surfaces at the microscopic level, the load is taken on the tips of the asperities that inevitably exist. So even a light object can locally melt microscopic amounts of ice in the contact.
    Last edited by exchemist; October 16th, 2013 at 04:19 AM. Reason: word corrected
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    Asperity. I learned a new word.
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  8. #7  
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    Quote Originally Posted by Harold14370 View Post
    Asperity. I learned a new word.
    Oh sorry. After 30 years in the lubricants industry I use it unthinkingly. All specialisms have their jargon, I suppose.
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    No need to apologize. Learning a new word is a good thing.

    An interesting point is that the freezing point never goes below about 250K (-23.15C, -9.67 F) which I guess is why snow gets crunchy and less slippery when it's really cold.
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    As well as ocean water at great depth will not freeze, even when below zero degrees centigrade- it's under great pressure.
    This might go off-topic, but my husband asked me a climate question I couldn't answer today.

    OK, oceans are warming and that's one of the main reasons for sea level rise as the water expands. Buuuuuut, most of the ocean's waters are below the thermocline and therefore, less than 4C. So far, so good. Water warming from 0C actually contracts (from the larger volume at freezing point) until starting to expand at 4C.

    Mr's question. Does the pressure below 4kms offset, enhance or have no effect on the expansion - contraction usually seen when water warms at normal atmospheric pressure? I'd expect there to be an effect but can't even estimate a net result.
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    Quote Originally Posted by adelady View Post
    As well as ocean water at great depth will not freeze, even when below zero degrees centigrade- it's under great pressure.
    This might go off-topic, but my husband asked me a climate question I couldn't answer today.

    OK, oceans are warming and that's one of the main reasons for sea level rise as the water expands. Buuuuuut, most of the ocean's waters are below the thermocline and therefore, less than 4C. So far, so good. Water warming from 0C actually contracts (from the larger volume at freezing point) until starting to expand at 4C.

    Mr's question. Does the pressure below 4kms offset, enhance or have no effect on the expansion - contraction usually seen when water warms at normal atmospheric pressure? I'd expect there to be an effect but can't even estimate a net result.
    OK. Each 10m of water adds a pressure of about 1 bar, right? So what you are in effect asking is for the coefficient of thermal expansion of water at a pressure of >/= 400 bar. I confess I was not able to find this easily, but perhaps others might know.
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