A supercritical fluid is a substance at a temperature and pressure above its
critical temperature and pressure. The critical point represents the highest
temperature and pressure at which the substance can exist as a vapour and liquid
in equilibrium. It has the unique ability to diffuse through solids like a gas,
and dissolve materials like a liquid. Additionally, it can readily change in
density upon minor changes in temperature or pressure. These properties make it
suitable as a substitute for organic solvents.
Carbon dioxide usually behaves as a gas in air or as a solid called dry ice
when frozen. Above its critical temperature and pressure, it behaves like a
supercritical fluid and can adopt properties midway between a gas and a liquid.
Supercritical carbon dioxide is for instance used by coffee manufacturers as a
decaffeinating solvent, because of its low toxicity and environmental impact..
