Oxygen Combustibility

Name: Matthew
Status: educator
Grade: 4-5
Location: IN
Country: USA

Question: At what percentage of oxygen saturation does ambient air
become combustible. Ex: The air hovers around 20.9 percent O2,
however if one was in a hyperbaric chamber, holding a lit match as
the oxygen level was constantly increasing at what point would the
"air" blow up?
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It will not.  Oxygen is reactive, but not inherently unstable. In other
words, oxygen reacts with other substances, not with itself.

Exposing a lit match to increasing pressures of oxygen will make it burn
faster.  Other substances also burn faster in higher pressures of
oxygen, and some things that will not burn in normal room air burn under
increased oxygen pressure.  So nasty fireballs can result from burning
things under high oxygen pressures, but it isn't because the oxygen
itself burns.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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It needs to be clarified that oxygen itself does not burn. It is the 
combination of oxygen and some fuel that burns, that is the fuel combines 
with the oxygen. The most common fuel contains carbon and hydrogen which 
combine with oxygen to form carbon dioxide, and water -- there can be 
other products of the combustion, which is another word for "burning". As 
simple as "burning" sounds, actually measuring the required amount of 
oxygen required is not an easy experiment. In addition to oxygen, the type 
of fuel, and the physical configuration of the flame, moisture content and 
other variables all play an important roles. Very recent experiments 
(Chemical & Engineering News, September 01, 2008 issue, page 12) goes into 
some of the details. Older estimates of the minimum amount of oxygen needed 
were about 12 volume percent. The new work, using carefully controlled 
conditions has raised that lower limit to about 15 volume percent. These 
difficult experiments took 3 years to complete and  cost more than $1.4 
million dollars.

Vince Calder
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