This is probably a dumb question but can anybody tell me if the freezing point of water changes if the water is under pressure ? If yes, what is the effect (i.e upwards or downwards and by how much ?) Thanks in advance david

This is probably a dumb question but can anybody tell me if the freezing point of water changes if the water is under pressure ? If yes, what is the effect (i.e upwards or downwards and by how much ?) Yes, it does.  The volume of a given weight of ice at 0 C is larger than the volume of water at the same temperature.  The expansion against pressure is work, which takes energy.  Therefore, pressure lowers the freezing point. That is what happens in ice skating.  The pressure exerted on ice where the skate blades touch lowers the freezing point, which gives a flim of water between the skate blade and the bulk of the ice.  The skater glides on that water film. Similarly, water vapor occupies the larger volume than liquid water at 100 C, so pressure raises the boiuling point. Marvin Margoshes

effect (i.e upwards or downwards and by how much ?) Thanks in advance david If one compresses ice, it melts.  So the higher the pressure, the lower the freezing point.  I think water is the only substance that does this - all others freeze more easily at higher pressures.  It's certainly (perhaps an unwise word to use, as in wades some Phys. Chem. Prof to flame me) the only *common* substance that does so. Rob Rob Blundell, School of Chemistry, University of Leeds, Leeds LS6 1HP, ENGLAND. Tel: (+44) (0)113 233 6526 Just in case I'm talking out of my butt, The opinions and comments within are my own, and should not be construed as being those of the University of Leeds or any other member of it.

the lower the freezing point.  I think water is the only substance that does this - all others freeze more easily at higher pressures.  It's certainly (perhaps an unwise word to use, as in wades some Phys. Chem. Prof to flame me) the only *common* substance that does so. Rob Oh yeah - forgot - how much?  It's a *very* small effect.   We're talking huge increases in pressure (don't know exact values). Rob Rob Blundell, School of Chemistry, University of Leeds, Leeds LS6 1HP, ENGLAND. Tel: (+44) (0)113 233 6526 Just in case I'm talking out of my butt, The opinions and comments within are my own, and should not be construed as being those of the University of Leeds or any other member of it.

That is what happens in ice skating.  The pressure exerted on ice where the skate blades touch lowers the freezing point, which gives a flim of water between the skate blade and the bulk of the ice.  The skater glides on that water film. I know that this is commonly understood, but I think that this should soon be placed in the scrap heap along with hot water freezes faster and glass flows . Science News had an write-up in the 2nd half of last year (insert pictures and audio tracks of me kicking myself for having not copied it at the time) that there appears to be an extremely thin _layer_ of liquid on the surface of ice. Now for the anecdotal evidence: The arguement that Marvin made can be made mathematically and I recall my P-Chem prof doing it in class. The contact area that he assumed was too small to play any effect, so he waved it all aside by mentioning that blades are hollow ground in order to decrease the contact area. While this is true for hockey skates, speed skates are ground flat. In addition, the rock on the blade (radius of curvature along the blade) is much less for speed skates than for hockey skates. This further increases the contact area. Despite these handicaps, speed skates easily outglide hockey skates. John

This is probably a dumb question but can anybody tell me if the freezing point of water changes if the water is under pressure ? If yes, what is the effect (i.e upwards or downwards and by how much ?) Clausius-Clapeyron equation, standard example in any PChem text (the how much ).  Or look at the phase diagram of water, ditto. Mp of H20 Ice I drops with increasing pressure to about 2000 atmospheres - obvious because it expands on freezing - until you touch the transitions to Ice II and Ice III.