Eli5: “Above the critical point of water, there is no difference between liquid and gas.” I don’t understand why it’s not just gas. What does this fluid look like or what does it mean to be supercritical on the molecular level?
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A gas’ behavior is based on the free movement of particles. A liquid’s is caused by the requirement that the particles always maintain the same distance from one another.
When supercritical, there is no longer a distinction. The “requirement” of a consistent distance is overcome. The fluid can expand or contract but it no longer enters a state where the particles can bounce around freely.
There is also no phase transition. Unlike boiling, where a liquid undergoes a clear transition (with a corresponding heat change), supercritical fluids form “smoothly” from a liquid or a gas.
If you take a glass tube half-full of a liquid and heat the tube to the critical point, you’ll see the gas and liquid start to blur and mix together as if they are the same.
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