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Name: Scot
Status: student
Grade: 6-8
Location: VA
Date: September 2007

Question:
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.



Replies:
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,

Burr Zimmerman


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 various substances.

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.

Vince Calder



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