What is a Supercritical Fluid?

You’ve probably heard of it on crime scene TV shows and maybe even in sci-fi movies. Whichever character says “supercritical fluid” always says it nonchalantly. I mean, who doesn’t know what it is, right? False. Not that many people know what a supercritical fluid really is or how difficult or easy one is to create. Even after a few years of chemistry lectures I never really knew what a supercritical fluid was. I would also just give the ever-so-mature middle school response of, “what, you don’t know what it is? Everyone knows.”

A supercritical fluid is a fluid (note – fluid means either liquid or gas) that is neither a liquid nor a gas. That’s right, it has properties of both. The supercritical point occurs when the density of both gas and liquid are identical. This phenomena occurs at specific temperature-pressure combinations.

The cool thing about a supercritical fluid is that it can “seep” into solids like a gas would and also dissolve substances just as a liquid would. A supercritical fluid is one of the coolest phenomena in science.

What are some applications of supercritical fluids? Do you ever enjoy decaffeinated coffee in the evening? Supercritical CO2 is used to remove caffeine from raw coffee beans. Biodiesel can be produced using supercritical methanol and some dry cleaners employ supercritical CO2.

Here’s a video showing some supercritical CO2:

[Photo Credit] Mark Chadwick

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