# Question on Super Heated Water

1. Feb 9, 2016

### irobot2020

Hi all, thanks for adding me to the forum.

Simple question - If 705 degrees F is the highest temperature water can exist as a liquid, wouldn't air pressure have to be reduced to a vacuum (or near vacuum) to remain liquid at that temperature?

2. Feb 9, 2016

### SteamKing

Staff Emeritus
What does air pressure have to do with it?

What you are describing is the critical point of water, that condition where the liquid phase and the gaseous phase of water can coexist. This point occurs only when the temperature is 705 °F and the pressure is 3200 psia.

https://en.wikipedia.org/wiki/Critical_point_(thermodynamics)

3. Feb 10, 2016

### Staff: Mentor

Lemme reword that, since I just stepped out of a foggy shower...

Above the critical point, the liquid and gaseous phases of water are no longer separate: they are indistinguishable from each other.

4. Feb 10, 2016

### SteamKing

Staff Emeritus
The critical point is the upper end of the saturation line for the liquid and the gaseous phases, where liquid water and water vapor can coexist in equilibrium with one another; above this point, water is a supercritical fluid, and there is no longer any distinction between the liquid and gaseous phases.

5. Feb 10, 2016

### irobot2020

Thanks for the responses. I probably should have taken another look at the phase diagram on Wiki (yep, I was already there before posting!) . . .

My bad . . my thinking is backwards. I can clearly see now, (by all means, correct me if I'm wrong) that in order for water to remain a liquid at 705F, is must be kept at an extremely high pressure (the critical pressure), or just a very slight fraction above, while marinating 705F, to keep it from phasing into a supercritical fluid or gas. Right?

6. Feb 10, 2016

### SteamKing

Staff Emeritus
Right. Above 3200 psia and 705 °F, there is no more vapor phase or liquid phase, only the supercritical fluid. There is some difference of opinion as to how this supercritical fluid behaves, as hinted to in the Wiki article. Even just below the critical point, the differences between the liquid phase and the vapor phase become less distinct.