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Re: latent heat of evaporation



The question, it appears to me, is "Does L depend on the whether the
evaporation occurs at the surface of the liquid vs. inside the liquid?"
After all, bubbles can occur inside the liquid at ANY temperature --
they just get "squashed" if the water pressure is greater than the
saturation vapor pressure.

So, perhaps the answer to your question is to measure the cooling when
20C water evaporates into a specified volume of 0% relative humidity air
at 1 atm and compare that cooling to when the 20C water "boils" into the
same specified volume of a vacuum.

I don't expect any difference.

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301


-----Original Message-----
From: Forum for Physics Educators
[mailto:PHYS-L@list1.ucc.nau.edu] On Behalf Of Ludwik Kowalski
Sent: Sunday, September 26, 2004 9:52 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: latent heat of evaporation


As far as i know no bubbles are formed when evaporation takes
place without boiling, for example at 90 C. Molecules escape
from a flat surface overcoming surface tension forces. That
is when, I suppose, the value of 540 cal/g is valid. This
topic is new to me. I do not know what to expect when bubbles
are formed. Should L become larger or should it become
smaller than 540? Perhaps someone will supply the answer
based on thermodynamics. Or on laboratory data. I am sure
that the topic has been studied in the past. Ludwik Kowalski