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From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: Daughter needs help...Please
Date: 25 Feb 1996 00:46:02 GMT

In article <dmg-2402961614390001@sos-dialup03.sos.net>, dmg@sos.net (Dan Greenwald) says:
>
>\Please excuse the intrusion but I need some help.
>My daughter brought this home from 6th grade science class. She is
>counting on dad to help her, but dear old dad dosen't have a clue. 
>Pressure? Temperature? Volume?
>For those of you in the know this is probably a no brainer and I would
>greatly appreciate your help....
>Thanks,
>D. Greenwald
>dmg@sos.net
>
>Cold tap water is put into a beaker.  It is placed over an open flame
>until it boils.  The flame is extinguished and a rubber stopper is put in
>the neck of the beaker.  After a few moments of cooling time, the beaker
>is placed under running tap water and it starts boiling again.  This
>happens every time the beaker is placed under the running water, even when
>the beaker has cooled to room temperature. Why?

I think you mean a round bottom flask rather than a beaker. 

The steam from the boiling water purges the air from the flask.
If you stopper it while it is still very hot the gas above the
liquid will be mostly steam with very little air. The steam, 
has a very low heat capacity capacity compared to the much more
massive quantity of hot water and it is in contact with a large
glass surface.

When you run cold water over the flask the steam cools and condenses
on the sidewalls of the flask creating a partial vacuum in the flask.
The water, which cools much more slowly, remains hot enough to boil
at the now reduced pressure in the flask.  Here it is important
to understand that water can boil even at zero degrees C if the 
pressure is low enough. It is only the presence of our high 
atmospheric pressure that keeps the water in our iced tea from boiling.

As the water boils it carries heat away from the liquid which it
loses by condensing on the sidewalls of the flask from whence that
heat flows to the outside world. The process continues until the
water cools to the point where it is no longer above its boiling
point at the pressure inside the flask.

Liquid water always exerts a pressure of vapor against the 
atmosphere above it. At freezing, this pressure of water molecules
is only about half a percent of  atmosphere pressure, the latter 
having a value of about 14.7 psi. At room temperature this pressure
rises only to about 3% of an atmosphere. As you heat the water
the vapor pressure rises exponentially until it reaches whatever
ambient pressure surrounds it - and then it boils. This boiling
normally occurs at 100 degrees on planet earth at sea level. On
a high mountain, the boiling point my drop to about 70 degrees
as the pressure is only about 1/3 of a normal atmosphere on, say
Mt Everest.

Your daughter's experiment could produce pressures as low as a
few percent of an atmosphere so the water might continue to 
reboil right down to room temperature (external water temperature)
if the cork gets slapped into the neck of the flask while a little 
steam is still trying to escape.

A round bottom flask or heavy Erlenmeyer suction flask should
be used for this experiment to prevent the possible occurrence
of a vacuum implosion, which might happen if you use a thin
flat bottom container.

Jerry (Ico)



From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.chem
Subject: Re: [Q] Boiling water at low pressure
Date: 24 Feb 1996 23:14:15 GMT

In article <312F19EE.3377@indiana.edu>, "John C. Huffman" <huffman@indiana.edu> says:
>
>Gerald L. Hurst wrote:
>> 
>> In article <312E9040.7E3B@indiana.edu>, "John C. Huffman" <huffman@indiana.edu> says:
>> >
>> >The answer is in your cookbook.  When cooking at high elevations, most[snip]
>>
>> Yuck. Who would add 50 grams of salt to a liter of water to
>> raise the BP by half a degree C?? It would be easier to lug
>> a pressure cooker to the top of Mt Everest than enough salt
>> to do much cooking.
>> 
>> Jerry (Ico)
>
>How fast does salt diffuse through an egg-shell?  Shucks, I always salt
>my hard boiled eggs anyhow.  And don't forget, the question was " asked 
>us if it was possible to hard-boil an egg on top of Mt. Everst"  He 
>didn't say anything about eathing it :-)  You go ahead and carry your 
>pressure cooker.

Notwithstanding the rhetoric, the practical use of salt to raise
the boiling temperature of water for cooking purposes at 29,000
feet is a joke. Saturated NaCl is about 6.5 molar so the increase
in boiling point would be about 3 deg C. At the top of Mt Everest
the boiling point of water would be around 70 deg C so those 
three degrees would hardly do the trick, would they?

So instead, you decide to use a more soluble salt and finally
succeed in raising the temperature of your fused mixture to
100 degrees. Ooops, the egg cracked and exploded because of
the differential pressure across that salt-proof shell (about
10 psi at 29,000 ft) and you, Dr. H, wound up with egg on your
face :)

Jerry (Ico)



From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.chem
Subject: Re: [Q] Boiling water at low pressure
Date: 24 Feb 1996 23:44:46 GMT

In article <4go647$qv0@geraldo.cc.utexas.edu>, glhurst@onr.com (Gerald L.
Hurst) says:


>Notwithstanding the rhetoric, the practical use of salt to raise
>the boiling temperature of water for cooking purposes at 29,000
>feet is a joke. Saturated NaCl is about 6.5 molar so the increase
>in boiling point would be about 3 deg C. At the top of Mt Everest

Pardon me, I forgot to count my ions. The maximum increase in
boiling temperature should be roughly 7 degrees, not 3 degrees
for saturated salt water - raising the boiling point on Everest 
to the high 70's. Still can't brew a decent cup of tea at that
altitude even if you don't mind a little salt :)

Jerry (Ico)



From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.chem
Subject: Re: [Q] Boiling water at low pressure
Date: 25 Feb 1996 05:11:47 GMT

In article <4gomti$cg4@news.aladdin.co.uk>, tonywilk@aladdin.co.uk
(tonywilk) says:

>"Michael R. Jones" <mrjones@niia.net> wrote:
>
>*snip*
>>...hard-boil an egg on top of Mt. Everst [el. 29 029 ft].
>*snip*
>
>Hmm... seems like a chance to look at it sideways....
>
>The question doesn't _specifically_ state you have to boil it
>in water does it? 
>Why not just boil it in something who's boiling point will be
>around 100C at that altitude ?
>
>Preferably something non-toxic :-)
>
>Would Glycerine (or a mix with water) do the trick ?

It would if you don't mind a face full of hot glycerine
when the occassional egg blows up. With a little bit of
luck you may avoid this if the shell is undamaged, thanks
to the good structural engineering of the shell. But don't
bet on it. I understand they used to boil a lot of rotten
eggs in oil.

Jerry

 



































































































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