KING%KESTREL@sri-unix.UUCP (01/03/84)
From: Richard M. King <KING@KESTREL> I can think of a few reasons why soda should suddenly freeze when the cover is removed. I will list them in what I judge to be the order of importance. 1> A solvent's freezing point is lowered in direct proportion to the amount of solute. This is a large effect which is used to protect car radiators at temperatures of -40 or less. When the cap is removed the CO2 bubbles out, leaving behind a more dilute solution that suddenly finds itself below the freezing point. 2> Dissolution of an acid-forming solute is exothermic; therefore "precipitation" of that solute would be endothermic. (Anybody remember the fancy term for a gas bubbling out of solution?) The soda froze because it got colder. 3> Pressure is a factor, but a minor one. I seem to remember that pressure changes the freezing point by less than .1 C per atmopsphere. (Isn't the triple point of water 0.015 C at some very low pressure?) If you had tried to go ice skating on that day, where your blade would put your (say) 150lb weight on a blade (say) 1/16 in by 8 in for a pressure of 200 atmospheres, you wouldn't have gotten very far. Even that very high pressure couldn't melt ice below about -15 C (5 F). (I concede that this last "measurement" was made when I was a kid, say 100lb, but even this "modest" pressure of 130 atm. is MUUCCCHHHH higher than that of a soda bottle. (Do not protest that skate blades are thicker than 1/16 in. They are deliberately "hollow ground" as shown below to make the area of contact small. | | |<---------- 1/8 - 3/16 in ---------------->| | _____________________ | | __------ ^ ------___ | | / | \ | | / .04in or so \ | |/ ..............V. \| The depth and curvature of the hollow grinding have been exaggerated by the primitive nature of the "graphics". Dick -------
jlg@lanl-a.UUCP (01/04/84)
I think that the freezing of soda mentioned is from adiabatic cooling when the bottle was opened. When the pressure on a substance is lowered suddenly (not enough time for heat to flow), the substance will cool off. If the soda was very near its freezing point when it was opened, this temperature drop would have been sufficient to start the freezing process. Most of the time when I have been unfortunate enough to leave a soda bottle in a cold place, it has frozen. Often breaking the bottle. It is my impression that supercooling a fluid is very difficult and can usually only be performaed under laboratory conditions.