Substances above critical temperature and pressure with properties of both liquid and gas, such as CO2 used in decaffeination.
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A supercritical fluid is a substance held above both its critical temperature and its critical pressure, where it has properties intermediate between a liquid and a gas. Its density is close to that of a liquid, so it can dissolve many solutes, while its low viscosity and high diffusivity resemble those of a gas. Because the solvent power can be tuned by changing pressure and temperature, the fluid is a useful extraction medium. Supercritical carbon dioxide is non-toxic and leaves no residue, which makes it a green solvent. It is used to decaffeinate coffee and to extract essential oils.
Key ideas: Properties tune with density: diffusivity about 10 to 100 times that of liquids and viscosity near gases. Supercritical CO2 (above 31 C and 73 atm) dissolves nonpolar compounds, and adding a modifier such as ethanol helps dissolve polar ones. The process is simple: pressurize, extract, then depressurize so the CO2 evaporates, leaving pure product.
Common mistakes and connections: Students think supercritical fluids are just hot gases or compressed liquids; they have a single phase with no surface tension. Another is assuming the solvent power is constant, when it changes sharply with pressure. Supercritical water at above 374 C dissolves organics and oxidizes waste. Related to critical point and green chemistry.
Above its critical point (31 C, 73 atm for CO2) a supercritical fluid has gas-like diffusion with liquid-like solvent power, used for decaffeination. The interactive lesson shows this as a live animation or chart.
Decaffeinated coffee is often produced by extracting caffeine with supercritical carbon dioxide, which leaves no toxic residue.
Substances above critical temperature and pressure with properties of both liquid and gas, such as CO2 used in decaffeination.
Supercritical Fluids is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in States of Matter & Solutions first: Phase Diagram, Triple Point, Critical Point.
The next topics in States of Matter & Solutions are Phase Diagram of Water and Crystalline Solids.
Yes. The interactive lesson includes a self-check question on Supercritical Fluids, and the States of Matter & Solutions quiz draws on every topic in this area.
Supercritical Fluids is topic 53 of 109 in States of Matter & Solutions.