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Q: What’s a supercritical fluid?

A: You might have learned in science class that substances are found in three states: gas, liquid, and solid, and that each has different characteristics. For example, a gas expands to fill its container. A liquid flows and cannot be compressed.

But there’s another physical state – one that’s often shortchanged in textbooks. At both high temperatures and high pressures, substances can be found in a dense phase, also called a supercritical fluid. In this dense phase, a material has properties of both a liquid and a gas: it can expand to fill a container like a gas, but it’s dense like a liquid. And this density has a benefit. In the same amount of storage space, you can store over 300 times more carbon dioxide as a supercritical fluid compared to a gas.

Each person in the U.S. produces about 376,000 cubic feet of carbon dioxide each year. That much carbon dioxide gas fills over 4 Olympic-sized swimming pools. But in its denser supercritical phase, the same amount of carbon dioxide only takes up 1,165 cubic feet. That’s about 1/8th of one swimming lane.


Dr. Susan Hovorka of the Gulf Coast Carbon Center approved this FAQ.



Dr. Steve Bryant from The University of Calgary approved this FAQ.



Click on the link at the left for a lab demo from the University of Nottingham showing the formation of a supercritical fluid from liquids and gases, and the reverse process too.