Ask A Scientist Chemistry Archive

name         Jamal
status       student
age          14

Question -   In my experiment I have succeeded in making water boil at
70 degrees Fahrenheit using a bell jar with a vacuum pump. My question
is: At what altitude will water boil at 70 degrees Fahrenheit Ex. at
63,000 ft the boiling of water is 102 Fahrenheit, at 70,00 ft the boiling
point is 92 F. My father and I have exhausted all means to find the
answer.The closest we have come is 18.8 mm hg. What would the altitude be
if the atmospheric pressure was 18.8 mm Hg? Thank You.
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The vapor pressure of water at 70 degrees Fahrenheit is 0.363 psi.  That
means that water will boil at 70 F if the pressure is 0.363 psi or below.

0.363 psi is equivalent to 0.0247 atmospheres, or 25.0 millibars, or 18.8 mm
Hg.  So all we need to do to find the answer to your question is find the
altitude at which the air pressure is 18.8 mm Hg.

There is no simple equation to determine the air pressure at a given
elevation.  The air pressure is complicated by the fact that the atmosphere
is divided into several layers with different behavior.  So, the best thing
to do is just look up the profile of the "standard atmosphere", which is a
compiled average designed to be as close to the truth as possible for as
much of the time as possible.  The listed air pressure from the "standard
atmosphere" is 25.5 mb at 25,000 m and 21.9 mb at 26,000 m.  So the pressure
should be around 25 mb at around 25,140 m.  That's about 82,500 ft.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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This is not such an easy question to answer. The altitude at which water boils depends upon 
the temperature and the pressure of the atmosphere at that particular altitude. That temperature 
and pressure will vary from day to day, even hour to hour depending upon atmospheric conditions 
-- cold fronts, warm fronts and so on. The temperature does not always just decrease as you 
increase altitude. One way to get a handle on your question is to calculate the 
temperature/pressure vs. altitude for what is called the 1976
Standard Atmosphere. This is a kind of "average" atmosphere engineers and scientists use to make 
sure everyone who is doing some sort of calculation is at least using the same model. You can 
find an "atmospheric calculator" at the following site. It is kind of fun just to play around 
and see how things change.
http://www.digitaldutch.com/atmoscalc/index.htm

Vince Calder
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