supercritical fluids

A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. 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 in a process called Supercritical Fluid Extraction.  Carbon dioxide and water are the most commonly used supercritical fluids.
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Highly compressed gases that combine properties of gases and liquids, they can lead to reactions that are difficult or even impossible to achieve in conventional solvents.