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?

Question

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?

In: 5

Answer 1

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.