A supercritical fluid is a compound with properties between those of a gas and a liquid. It can move (diffuse) through solids like a gas, and dissolve materials like a liquid.

Supercritical fluids can be made to behave more like a gas or a liquid by changing temperature and pressure. This alters their density, viscosity and diffusivity, and allows fine control over the way that they dissolve other materials.

Carbon dioxide and water are both supercritical fluids, and are widely used in industry as a safer alternative to organic solvents, for example for decaffeination and extracting pharmaceutical products from plant material.

Supercritical liquids are also used in power generation, biofuel production, refrigeration, the dying industry and many other applications, including carbon dioxide capture and storage and enhanced oil recovery.