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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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.

pretty sure nilered made a video showing water going supercritical, might help you understand

Here’s [Applied Science](https://www.youtube.com/watch?v=-gCTKteN5Y4) with a video showing carbon dioxide going supercritical. Check it out!

As a liquid and its vapor reach the critical point, the density and molecular interactions between the two phases becomes more and more similar, until at the critical point there’s no difference in physical properties between them. This can be seen with the eye, as the visible boundary between the gas and liquid fades away and disappears.

Is this fluid a gas? Is it a liquid? Who cares! It’s definitely a fluid, it moves and flows easily and conforms to the shape of its container. Its volume is neither constant with increasing pressure (as it would be for an ideal liquid) nor inversely proportional to pressure (as it would be for an ideal gas).

The three (or four) phases of matter we teach kids in grade school science classes are useful, but they’re only a starting point for the many weird forms matter can take.