[femto]

If you heat water under a high enough pressure, when you release the pressure it instantly becomes stream.

Normally, when you boil water the vapourisation happens piecemeal. That's why you see bubbles rising to the surface. For supercritically heated water, the vapourisation is a runaway chain reaction, triggered by a reduction in pressure, so the whole body of water flashes into stream.

--- Edit: added note about reduction in pressure being the trigger.

[neltnerb]

Apologies, I don't want to be pedantic, but this has nothing to do with being supercritical.

https://en.wikipedia.org/wiki/Supercritical_fluid#mediaviewe...

You can note in this diagram that what you are describing is true whether or not the system is supercritical, and can be seen in how water will boil into steam when the pressure is decreased from atmospheric as well.

What you are describing is how higher pressures allow you to add energy to the water while it remains liquid, and how if you add enough energy it will overcome the enthalpy of vaporization and cause it to convert to steam as the pressure is reduced.

[femto]

Point taken! I'm not an expert.

As I understand it, a container of boiling water will have liquid in the bottom half and steam in the top half. As the pressure and temperature rise, the steam/water goes supercritial, meaning the water/steam boundary disappears and the whole container becomes a homogenous mush of supercritical fluid.

Am I right in thinking that this supercritical fluid can flash into steam faster than a combination of water and steam? My thinking is that for a water/steam combination to convert into steam, the water molecules have to take the time to break their bonds and separate into a gas. For a supercritical fluid it's faster because there are no bonds to be broken?

I'd be grateful if you can correct the above, as I can learn something here.

[neltnerb]

There aren't really formal bonds being broken transitioning from liquid to gas, but I suppose it is fair to say that the supercritical state will transition more quickly to steam than a subcritical liquid with enough energy to become steam at atmospheric pressure.

The reason for that would be that there is a nucleation process in forming gas from liquid, which does take some time. Or at least more than not needing to do so.

There is some terminology you're using that bothers me, like "flash into steam" isn't really a good way to describe it. At that point you'd be better off describing it as "super pressurized steam" converting to "normal pressure steam" or something. It's just expanding, but there is no flash (which to implies a sudden change). It's gradually and continuously decreasing in density.

I think part of this may be confusion over how we overload the word "water" to mean "liquid water" as well as "water the chemical". I am meaning "water the chemical" which can be a solid, liquid, or gas. Steam is water that is a gas.

With a supercritical system, you can take liquid water, stay in the liquid state until the water becomes supercritical (where the liquid and gas phases are indistinguishable), and then move across a boundary from "liquid like" to "gas like", go back into a sub-critical state as gas, and never formally boil/flash/etc.

[femto]

Thanks for the explanation.

[jjoonathan]

If you release pressure slowly you never see boiling/evaporation either, but at the end of the process you have steam.