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$\mathbf{Q:}$ While teaching "Real Gases", my professor remarked last day that "Liquid phase is a highly compressed gaseous phase." But he did not explain the reason behind it and left it as food for our thought.

$\mathbf{A:}$ What I could make out of this was that he was referring to the isotherms of real gases. So I got this graph.

enter image description here

Now I can see from the graph that a certain finite amount of pressure needs to be applied in order that we can change the gaseous state from vapor to liquid. Ideal gases have considerable or high compressibility while ideal liquids are almost incompressible. But still can I call this "highly compressed"? So how do I prove the statement made by my professor?

Any help in the form of hints or answers is welcome.

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    $\begingroup$ The correct thing to Google is 'critical point' - yes, at high enough pressures indeed the liquid and gas phases become a single phase. This is used, for example, in CO2 drying where you can flood the part with liquid CO2 (displacing any water), and then run around the critical point by varying temperature and pressure, and Poof! the liquid disappears without leaving a meniscus (i.e. no surface tension). $\endgroup$
    – Jon Custer
    Dec 16, 2015 at 19:25

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In a gas, each molecule always moves freely. In a liquid, groups of two or more molecules (typically many) weakly "bond" for some amount of time (typically electrostatically). That is, a liquid has 'super' molecules coming in and out of existence.

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