This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
Supercritical fluids have some very unusual properties. When a compound is
subjected to conditions around the critical point ( which is defined as
the temperature at which the gas will not revert to a liquid regardless how
much pressure is applied ), the properties of the supercritical fluid become
very different to the liquid or the gas phases. In particular, the solubility
behaviour changes. The behaviour is neither that of the liquid or that of the
gas. The transition between liquid and gas can be completely smooth.
The pressure-dependant densities and corresponding Hildebrand solubility
parameters show no break on continuity as the supercritical boundary is
crossed. Physical properties fall between those of a liquid and a gas.
Diffusivities are approximately an order of magnitude higher than the
corresponding liquid, while viscosities are an order of magnitude lower.
These properties ( along with low surface tension ) allow SCFs to have
liquid-like solvating power with the mass transport characteristics of
a gas.
Potential Supercritical Fluids Compound Critical Critical Density Temperature Pressure ( C ) ( bar ) (g cm^-3) Ammonia 132.4 112.8 0.235 Carbon dioxide 30.99 73.75 0.468 CFC-12 111.8 41.25 0.558 Dimethyl ether 126.9 52.7 0.271 Ethane 32.4 49.1 0.212 HCFC-22 96.15 49.90 0.524 HCFC-123 183.68 36.62 0.550 HFC-116 19.7 29.8 0.608 HFC-134a 101.03 40.57 0.508 Methanol 240.1 83.1 Nitrous oxide 36.4 72.54 0.453 Propane 96.8 42.66 0.225 Water 374.4 227.1 Xenon 16.6 58.38 1.105
 
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