Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: latent heat of evaporation



RC has described an xpt. similar to what I was going to suggest (to L).

Boil warm water in a vacuum chamber; weigh before and after, and
continuously monitor the temp. (thermocouple).

Blow warm air (dry will be more effective) across a wide mouth Dewar.
Weigh before and after monitoring temp. A variation would be to bubble
the air (using a chemist's bubbler) * .

This is a difficult xpt, as the air's temp must be the same as the water's.

A variation of the first xpt. would be to evacuate slowly, so no
boiling, and rapidly w/ boiling and compare. If one could weight
continuously, one could also determine the change in L (w/ temp.)

* this reminds me of how I (and many others, I'm certain) would cool lN2
so it wouldn't boil thus not obscuring helium demos, i.e. fountain
effect, floating magnet, etc. bubble helium thru the Nitrogen. It
cools it mightily.

bc

p.s. there may be a problem w/ the syringe xpt. (someone who can think
e.g. JD could help me here), as the evaporant will be in thermal contact
w/ the liq., while in the usual xpt. the gas is not. This reminds me of
a complication w/ the usual condensation xpt. i.e. the gas at 100 C is
cooled to the water sink's temp. i.e. 540 + SH (100 - whatever). not
just 540. (I suspect this problems is only in my mind and the analysis
includes this addnl. factor.



Robert Cohen wrote:

Ludwik Kowalski wrote:


What would one need "to measure the cooling?"



I'm not sure. As JD wrote, though, you'd have to do it
very precisely to convince someone of a non-null result
and I don't have the expertise to come up with a way to do
that.



I was thinking about a student experiment.



Again, I don't know of a simple student experiment. My
suggestion was more of a "thought" experiment. However,
perhaps you can take a large syringe filled with
water (no air). Insert a temp probe, seal the tip, and then
quickly pull the plunger out. The water will then "boil"
until saturation is reached. Compare that temp drop
with what happens if you do the same thing but don't seal
the tip and instead allow the syringe to draw in very dry
air. Just keep in mind that there is air in the second
case, which will lead to less overall cooling since the air
will cool also.

Warning: My "back of the envelope" calculation predicts
the change in temperature would be very small unless the
volume of air is much greater than the volume of liquid.
And, given the precision of the experiment, I doubt you'll
be able to show the difference in L, if any exists. I
much prefer the thought experiment. :-)

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