X-Message-Number: 1805
Date: Tue, 23 Feb 93 11:36:55 EST
From:  (Perry E. Metzger)
Subject: Boiling and condensing

> Message: #1791 - Re: Remarks on Ettinger's Proposal
> Date: 21 Feb 93 22:38:13 EST
> From: Mike Darwin <>
> Message-Subject: CRYONICS Re: Remarks on Ettinger's Proposal
> 
> From: Mike Darwin
> Re: Ettinger Proposal Comments
> To: All
> Date: 21 February, 1993
> 
>      Richard Schropel speculates that the temperature of liquid nitrogen 
> vapor is the same as that of the liquid and asks for clarification.  
> Having considerable first-hand experience with this I can readily back up 
> Bob Ettinger.  The temperature even a few millimeters (indeed even a 
> millimeter) above the liquid nitrogen is greatly higher.  Physics tells us 
> why this should be so, but rather than try to explain thermodynamics 
> (which is by no means my forte) it is perhaps sufficient to point out that 
> if the nitrogen vapor were the same temperature as the liquid nitrogen it 
> would also be liquid nitrogen instead of a gas...  The degree and 
> stability of temperature stratification in a cryogenic dewar refrigerated 
> by liquid nitrogen is truly astounding.  

Far be it for me to correct Mike Darwin, who knows far more on this whole
subject than I can ever hope to, but its rather untrue that vapor and liquid
phases cannot exist at the same temperature, which seems to be the
implication.

Vapor and liquid phases certainly CAN and DO exist at the same temperature 
every day. As an example, the water vapor in the air around you is water vapor
existing at a temperature way below the boiling point of water at 
a pressure of one atmosphere. Liquids, and even solids, exist at equilibrium
with their vapor phases at all times. The reason that water boils at
a particular temperature is that at 100 degrees C the partial pressure of
water vapor is one atmosphere. Simply bringing water to 100 degrees C is
not, however, sufficient to get it to boil. You must add energy to overcome
the van der waals forces holding the liquid together -- this is the
so called heat of vaporization. Thats why, among other things, boiling
distilled water stays at exactly 100 degrees C, boiling liquid nitrogen
stays at exactly 77 degrees K, and why it is that merely bringing a liquid
to its boiling point won't make it boil without adding energy. (Actually,
in practice it will boil slightly and the liquid will cool, the energy to
produce the vaporization having been extracted by cooling the liquid.)

The vapor flying off the surface of a liquid in a properly conducted 
experiment will be measured to be at more or less the same temperature as 
the liquid. Now, in practice, boiling is conducted in an atmosphere thats 
held at a temperature different from that of the liquid -- thus, you will 
measure, in practice, that the temperature over the surface of boiling 
liquid nitrogen is higher, and that over boiling water is lower, than the 
temperature of the liquid. This makes sense -- mixing is occuring with the 
atmosphere, which skews the temperature.

By the way, a similar phenomenon occurs in freezing -- and indeed, most
substances at a particular pressure sufficient for liquid to form
may experience a so-called triple point, where liquid, solid and gas forms
all exist. There is also the interesting phenomenon of solids sublimating.
At a pressure of one atmosphere, carbon dioxide cannot exist as a liquid
because its partial pressure at its melting point is already higher than
one atmosphere. Therefore, it goes from solid phase directly to gas without
a liquid phase.

(BTW, my use of the phrase "partial pressure" is likely an incorrect way
of stating the concept -- its been too many years since high school chem
class.)

Perry

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