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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.