Maximum Density of Compressed Air

Who told you that a gas cannot be converted to a liquid by just pressure alone. I'll grant you that to condense many gasses to a liquid with just pressure one needs astronomical pressure magnitudes. (Clearly these pressures will not be contained by a Scott pack.) Consider the interior of the planet Jupiter. It is expected to have a liquefied and even a solidified layer of Hydrogen. While the gaseous layers of Jupiter are certainly cold, the solid core in the center has a temperature of 20,000~30,000 K. That's much warmer than room temperature for the lightest gas possible.

Now at 300K, what is the pressure required to liquefy Nitrogen, or Oxygen? I'm sorry to say that I did not find that piece of trivia. I did do a quick search but I failed to find it.

Ok, I just found at Wikipedia that Nitrogen has a critical point of 126.2K and only 33.5 atmospheres. At this point Nitrogen becomes a supercritical fluid where gas and liquid phases exist concurrently. You'll notice that Wikipedia itself makes precisely your statement that above this temperature that air cannot be liquefied. However, it also makes the statement that a supercritical fluid contains both phases of gas and liquid. So to answer your question, the minimum volume is where all of the state is liquid.

_{Almost ran out of editing time above. Part II of my addendum.}

So a supercritical fluid above the critical point of temperature and pressure _{(that is redundant, but I like the emphasis)} contains both states when the only parameters known are pressure and temperature. Your question of what is the minimum volume I think should then be handled this way. The density of liquid Nitrogen at boiling is 0.808 gm/cc. This will be limiting factor of how small of a volume one can have of just Nitrogen. If a supercritical fluid has only the minimum volume of space to occupy, then the only state possible is a liquid.

Now I did not include any other element in my discussion here for fairly obvious reasons. This was complicated enough.

You can liquefy air, Captain - if you make it cold enough.

So you need to consider temperature as well as pressure, as with all gas systems.

Hello Crabtree: Guide moaning to you.

and Redfred: Good morning.

I am not trying to liquify the air. I just want find out what happens to air when it is compressed at very high pressures well above the critical temperature.

If it does not liquify as such it must at some point reach a density equal to the liquid and therefore with no further reduction in volume even though the pressure goes on rising. Is this true?

What is this density? - ie, 0.8g/cc ? - and if taken to an extreme, would enough pressure solidify air.

If have no practical use for this knowledge - just curious about what happens.

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You need to understand about the Critical Point, Horace. If the conditions are adjusted such that the locus moves around the Critical Point, then the substance will go from vapour to liquid, or vice-versa, without a phase change. Above the critical point, the terms "liquid" and "vapour" have no real meaning. Just squeeze away until the limitations of the equipment become apparent. Except - you have got pressure relief on this thing, haven't you?

By the way ,did you hear anytime warm ice about? or Ice I, II, III, and ice IV? Try that info first,after all air or water will be the same for what you are looking for.-

at 100,000 psi and 100F, it can have a density of about 1gm/cc

A compressed volume of gas/liquid will have a temperature change. So the task looks like heat rejection in the most efficient way. Maybe this will help: http://en.wikipedia.org/wiki/Enthalpy_of_formation