[maccha]

I'm confused about the compressibility factor. My textbook says it increases as pressure increases because at high pressures intermolecular interactions cause the volumes of gases to be greater than ideal gases. However, I thought that pressure and volume were inversely proportional? What am I missing? Thanks!

[Borek]

What am I missing?

Do you remember definition of ideal gas? What's are limitations of the ideal gas approach?

[Enthalpy]

The compressibility factor is also often confused with its reciprocal, the bulk modulus, which has the unit of a pressure. This confusion is rather common in the case of liquids.

It's the modulus that increases with pressure, or rather with density, because molecules add their volume to the vacuum volume (perfect gas) occupied by the thermal movements, and molecules are stiffer.

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This added volume appears in Van der Waals equation of state. It uses to be similar to but somewhat smaller than the volume of the liquid. So corrections are necessary when the gas' density approaches the liquid's one.
http://en.wikipedia.org/wiki/Van_der_Waals_equation

I wanted to use virial's equation for helium at 90K and 30b to 350b, and it was brutally false. This equation looks like an academic amusement, because thermodynamic values can be deduced from it, but with no practical use. Adding just a dead volume gave me good results.
http://en.wikipedia.org/wiki/Equation_of_state#Virial_equations_of_state

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Liquids and solids have a compressibility as well, from below 1GPa bulk modulus for silicone oil to about 300GPa for steel and much more for ceramics like SiC. It isn't widely documented; at 1 bar (where bubbles and dissolved gas influence strongly), it can be inferred from sound velocity and density. Bulk modulus is near to Young's modulus for metals but very different for elastomers, whose bulk modulus resembles any polymer.

This compressibility must result at low pressure from remaining voids between the molecules. At high pressure, it can only result from forcing bond angles and lengths, and from compressed orbitals themselves: iron and nickel are some 30% denser at Earth's centre, as is reproduced in a diamond anvil.

For liquids and polymers at technological pressures (300b-3000b), compressibility relates with the temperature compared to the melting point, and to the atomic composition. Hydrogen and fluor as external atoms (the ones that make a plastic slippery) give compressibility, while naked -OH functions make stiffer. I couldn't make good relationships nor predictions.

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I suppose pressure during sonoluminescence deforms the orbitals so much that new orbitals and bonds are created, possibly among deep levels. This would explain any non-thermal ultraviolet radiation. For those interested, here's my proposal for a sonoluminescence generator or actuator relying on instant vaporization by an electric pulse, with more uses like lithotriptors or safer detonators (hi Anders Hoveland, money to make here):
http://saposjoint.net/Forum/viewtopic.php?f=66&t=1940#p22060
with deep variations at subsequent posts, for instance
http://saposjoint.net/Forum/viewtopic.php?f=66&t=1940&start=20#p24157

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I'm still looking for experimental data about the density of helium (and hydrogen) at 90K (and 112K and 20K) above 100b. I had to extrapolate to 300b, which I frankly dislike. Purpose is to pressure-feed propellants in rockets and boosters designed to sail back for reuse:
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2554
please, thanks!

Marc Schaefer, aka Enthalpy

[Enthalpy]

Found very nice data about helium (only up to 100 atm, alas) and hydrogen at Nist:
http://www.bnl.gov/magnets/staff/gupta/cryogenic-data-handbook/Section2.pdf (Helium, 6MB)
http://www.bnl.gov/magnets/staff/gupta/cryogenic-data-handbook/Section3.pdf (Hydrogen, 7MB)
for more interesting data for cryotechnology, look at the index, it's Section1.pdf

Grateful thanks to Nist!