Daddy
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Different conditions at high altitude / low air pressure
I began to think about this when trying an experiment which seems to be a classical schoolbook experiment: heating iron powder and sulphur together in
a test tube until it reacts and forms iron sulphide. It did not work: all the sulphur evaporated before it could react.
Now, I live at about 3500 meters (about 11'700 foot) above sea level. Air pressure is here only about two thirds of the "normal" pressure. So, boiling
temperatures are lower. (Water, for example, boils here at 88ºC.)
I suppose flame temperatures are also lower here, because there is less oxygen available. But I have no means to find out how much the difference is.
According to an information I found, an alcohol flame should just be able to heat glass and iron to red heat - well, mine did not. I transformed then
my gas stove into a Bunsen burner by means of an old iron tube. This device could heat an iron wire locally to a dull red, but not more. And for my
iron-sulphur experiment, it yielded the same result as above.
So I have actually two questions:
- I wonder if the failure of that experiment was just due to the combination of these two effects: lower boiling temperature and lower flame
temperature?
- And what other "interesting" effects might high altitude / low air pressure have on chemical reactions?
(Of course, gasses occupy much more volume per mass in these conditions. This is sometimes an advantage: I need less of the reagents for filling a
flask with a gas, for example.)
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vulture
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A few immediate things come to mind;
- A vacuum pump will be more effective than on sea level, because the exhaust pressure is lower
- Substances should dry faster in air
- Reaction which proceed gas should be driven to the right more or faster than on sea level
- You will have a terrible time trying to store iodine or naphtalene 
[Edited on 6-7-2009 by vulture]
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Daddy
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Actually, yes, as for your last two points, with my kids we did the experiment of extracting some iodine from tincture and heating it to see the
vapors, and it sublimated with almost no heat.
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JohnWW
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Someone on another thread said just lately that, on the summit of Mt. Everest (29,000-odd feet), you can drink boiling water. There, air pressure is
only one-third that at sea level. But at that altitude, on the boundary of the stratosphere, you would die from anoxia before long, unless breathing
either bottled compressed air, or better still pure oxygen (which can be at Everest pressure).
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pantone159
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Quote: Originally posted by JohnWW  | | But at that altitude, on the boundary of the stratosphere, you would die from anoxia before long, unless breathing either bottled compressed air, or
better still pure oxygen |
Or be very well acclimitized, in which case you might last 48 hours. 
More OT - I wouldn't expect flame temperatures to change much, though it would be interesting to know. There surely are some differences in some
chemical reactions.
I do like the physical experiment of emptying a water bottle at c. 14000 ft (e.g. summits in Colorado), closing the top, and carrying down to c. 9000
ft, the bottle is very deformed after this.
[Edited on 7-7-2009 by pantone159]
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watson.fawkes
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| Quote: Originally posted by Daddy | | Air pressure is here only about two thirds of the "normal" pressure. [...] I suppose flame temperatures are also lower here, because there is less
oxygen available. |
Basically, yes, if you make the caveat that you're talking about open flame. If anybody
might doubt this, apply the ideal gas law. 2/3 the pressure is 2/3 the moles of oxygen. (Atmospheric gas percentages pretty constant to high up, until
you start getting ballistic gas flow.) Now the actual oxygen supply is likely bigger than 2/3, because less pressure also means higher flow rates, but
this effect doesn't compensate.
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