Every pure substance has a critical point — a unique temperature and pressure (Tc, Pc) where the liquid and gas phases become indistinguishable. Below it, a sealed vessel shows a sharp meniscus separating dense liquid from sparse vapour. As you heat the vessel toward Tc along the liquid-vapour boundary, the density difference between the phases shrinks, the meniscus thins and flickers with critical opalescence — then vanishes entirely. Above Tc and Pc the substance becomes a single supercritical fluid: it fills its container like a gas but dissolves things like a liquid, which is exactly why supercritical CO₂ is used to decaffeinate coffee and extract plant oils without leaving solvent residue.
(1 − T/Tc)^0.5 and reaches zero exactly at the critical point.Supercritical CO₂ extraction runs near 31°C and 74 bar — mild enough to process delicate botanicals — while supercritical water at 374°C and 221 bar is used to oxidise hazardous organic waste completely, since almost everything becomes miscible in a supercritical fluid.
A sealed vessel of CO₂, water or ethane sits on a lab stand; heat it toward its critical temperature along the liquid-vapour coexistence curve and watch the meniscus thin, shimmer with critical opalescence, and vanish into a single supercritical phase.
Liquid and vapour densities converge toward a common critical density as temperature rises, following a density-gap that shrinks like (1 − T/Tc)^0.5 and hits zero exactly at the critical point — the same physics behind supercritical CO₂ extraction.
Pick a substance, drag the temperature slider (or hit Auto-heat), and adjust the fill amount to see the classic three-tube behaviour: only a critically-filled vessel keeps its meniscus alive all the way to Tc.
Supercritical CO₂ runs at a mild 31°C and 74 bar, making it the solvent of choice for decaffeinating coffee and extracting essential oils without leaving any residue behind.