Compressing Air to Liquid Density
Date: September 2007
I am doing a demonstration on air compression and I need
to compress air so that it will have the same density as liquid. I
don't know how I would do that, if the air would turn into a liquid
itself, how I could harness that pressure, and how the air would
react. I know my question is complicated but no one else can help me.
Actually, it is not a complicated question -- you just may not like the
answer. :) Unfortunately, this is not something easily done.
Usually these gases are liquefied by lowering their temperature and
pressurizing them. There are sites on the Internet that describe how to make
liquid oxygen and liquid nitrogen, but I have not evaluated them for safety,
so I cannot recommend any. Nitrogen is much easier than oxygen, as I am sure
you have guessed.
Oxygen is also very hazardous to work with because things have the tendency
to burn easily when a lot of oxygen is around. I cannot emphasize enough the
hazardous nature of working with pure oxygen (unless you very much know what
you are doing, I do not recommend it).
However, even if you could pressurize them that much, the density of liquid
oxygen is a little higher than water, but liquid nitrogen is lower. A liquid
mixture of the two in the same proportions as air would be roughly the same
density as water (within 10%), but still would not be as dense as water if
you want to be exact. Depending on the type of demonstration you want to do, 10%
error might or might not be close enough.
And, if you *only* want to compress it (as your question says) -- e.g. not
cool it -- you will need around 500,000 psi of pressure. That is quite
challenging to generate, to say the least. I do not have any suggestions for
any easy way to do that.
Hope this helps,
To compress a gas to a density equal to the density of the liquid phase of
the same substance, it is necessary to compress the gas to a pressure that
exceeds the temperature and pressure called the "critical" temperature and/or
"critical" pressure. That is, the "critical temperature" is the temperature
above which a gas cannot be liquefied, no matter how high the pressure, and
the "critical pressure" is the pressure is the lowest pressure which will
liquefy the gas at the critical temperature. These temperatures and pressures
are typically outside the range that most laboratory setups can be used safely.
That is why you cannot find a lot of demonstrations of the critical behavior of
With the proper equipment the behavior of fluids is very interesting because
then the fluids have very unusual properties. If you search the term
"supercritical fractionation" for example, you will find a lot of information
on the solubility properties of such substances. Nonetheless, it is not easy
or safe to attempt these conditions without specialized equipment.
Click here to return to the General Topics Archives
Update: June 2012