Just above the critical temperature of the substance, liquid-like densities are rapidly approached with modest increases in pressure in the range of 0.7 to 2 times the critical pressure. Higher pressures are required to attain liquid-like densities for temperatures further above critical. Solvent power of a supercritical fluid unlike a liquid increases with density at a given temperature and increases with temperature at a given density. The increase in density is not linear with pressure, but rather the rate of increase is much greater in the vicinity of the critical point. A supercritical fluid exhibits physico-chemical properties intermediate between those of liquids and gases.
Mass transfer is rapid with supercritical fluids.
Their dynamic viscosities are nearer to those found in normal gaseous states. As was the case for density, values and subsequent changes for viscosity and diffusivity are dependent on temperature and pressure. The viscosity and diffusivity of the supercritical fluid approach that of a liquid as pressure is increased. Changes in viscosity and diffusivity are more pronounced in the region of the critical point. To understand the principles by which a SF behaves, it is beneficial to first picture a gas as a solvent.