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This is the plot of the Van der Waals equations:

enter image description here

As you can see, the Van der Waals equation predicts a "stretched liquid" area (MNS). Is this physically feasible and what exactly happens there? Are there any intuitive explanations for this?

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  • $\begingroup$ There is no M-N-S, much like there is no F-G-H, but just the horizontal dotted line F-H. $\endgroup$ Commented Oct 21, 2019 at 15:57
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    $\begingroup$ Be aware the equation has limited applicability and is not applicable in the area below the triple point, when the formula extrapolation leads to not existing physical states. Sea also en.wikipedia.org/wiki/Real_gas#Models $\endgroup$
    – Poutnik
    Commented Oct 21, 2019 at 15:59
  • $\begingroup$ Although you can reach the superheated liquids and supercooled vapours, these are not at equilibrium and tend not to last long. $\endgroup$
    – Jan
    Commented Oct 21, 2019 at 16:53
  • $\begingroup$ I am aware that this is only a metastable state and that there is no M-N-S area in the chemical sense. What I'm inquiring about is the concept of negative pressure in a real gas context. There are papers out there that do say that there is negative pressure for liquids (ilm-perso.univ-lyon1.fr/~fcaupin/fichiersPDF/…). I still do not understand what's happening with a gas/liquid under negative pressure conditions. It's not called "stretched liquid" for no reason in the graphic above... $\endgroup$ Commented Oct 21, 2019 at 17:33
  • $\begingroup$ Well, if you consider compressing a positive pressure, then stretching is negative, as simple as that. If you have viscous liquid then some measure of stretching is possible. $\endgroup$
    – Mithoron
    Commented Oct 21, 2019 at 21:55

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You can have a stretched liquid to some degree. The same way as pulling a solid body in all directions creates stresses that can have negative trace, you can stretch a liquid, which then holds together by cohesion forces.

The state is metastable and subject to cavitation, but not completely unstable. In thin capillary tubes (or rather very thin pores in some porous media) you can achieve negative pressures and hence be able to lift the water column above 10 m, as trees are able to do in their vascular tissue (Quenching Even the Tallest Thirst: How trees suck up water from the ground and avoid embolisms) and as can be reproduced in experiments (The transpiration of water at negative pressures in a synthetic tree).

A lot more, with some thermodynamic diagrams: Negative pressure of stretched liquid water. Geochemistry of soil capillaries

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