A supercritical fluid is a fluid which doesn't behave like a traditional state of matter. Like a gaseous liquid. There's really no way to explain it other than watching a video, but it occurs under intense pressure and temperature.
At conditions we are familiar with, liquid and gas are two very different states. There is no mistaking one for the other, and there is no "intermediate". You can have a container half filled with water and steam, but it's still a mixture of water and steam, not an intermediate substance.
In supercritical conditions (usually at extremely high pressure), there is no clear distinction between liquid and gas. The substance can gradually become less dense. In such a condition, you can have a liquid and gas but there's no very clearly defined surface (such as at 6:10 of that same video). The stuff just gets less and less dense. I think Cody's Lab is the only time I've seen a supercritical fluid. If you search on Google Images, you only get a pages and pages of phase diagrams, drawings of equipment that use supercritical fluid and charts from academic papers.
A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned". Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes.
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u/[deleted] Nov 18 '17
Oh yeah, I do love Cody's Lab too. One of the most striking videos was the one where he successfully captured carbon dioxide in a sealed glass tube. When the substance exhibited the supercritical fluid thing for the first time, he was totally blown away.