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Critical Points for some Substances

Critical points of some common substances like air, argon, helium and more

At the critical point there is

  • no change of state when pressure is increased or if heat is added

At the critical point the liquid and gas phase can't be distinguished and there is no point referring to liquid or gas.

At the critical point the physical properties of the liquid and gaseous states are identical.

For states above the critical point the gas is supercritical.

Critical Points for some Substances
SubstanceTemperature
(K)
Pressure
(atm)
Specific Molar Volume
(cm3/gmol)		Compressibility
Z = Pv/RT
Acetylene - C2H2 309.5 61.6 113 0.274
Air 132.63 37.36 92.35
Argon - Ar 150.72 47.99 75 0.291
n-Butane - C4H10 425.17 37.47 255 0.274
Carbon Dioxide - CO2 304.20 72.90 94 0.275
Carbon Monoxide - CO 132.91 34.529 93 0.294
Ethane - C2H6 305.48 48.20 148 0.285
Ethylene - C2H4 283.06 50.50 124 0.270
Helium - He 5.19 2.26 58 0.308
Hydrogen - H2 33.24 12.797 65 0.304
Methane - CH4 190.7 45.8 99 0.290
Nitrogen - N2 126.2 33.54 90 0.291
Oxygen - O2 154.78 50.14 74 0.292
Propane - C3H8 370.01 42.1 200 0.277
Sulfur Dioxide - SO2 430.7 77.8 122 0.269
Water - H2O 647.27 218.167 56 0.230

Note! - the critical point is not the same as the triple point.

  • 1 cm3/gmol = 10-3 liter/mol

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