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While reading about the Clausius Clapeyron equation from the Feynman lectures on Physics, I couldn't understand a few things from its derivation:

Although the argument was pretty clear, when the system consists both gas(vapour) and liquid why would it have constant pressure on heating and increasing volume??

Second and more fundamental, how can we assign a single pressure value to this composite system in spite of it having two states of matter; wouldnt the pressure in the gas part (on top part of container) be different from that at the bottom or side in the liquid part? On top of this how can this be the vapour pressure (which I think is the pressure of the vapours on the surface of liquid)?

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If the pressure of gas and liquid differ, then one will expand while the other will contract, implying non-equilibrium. –  Siyuan Ren Jan 24 '13 at 4:37

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The system has constant pressure by definition. Even in a system with changing pressure, though, for some small time, dt, there would be constant pressure. In that moment, and with those conditions, the relationship holds. Adjust the pressure slightly, and a new, similar relationship is set up.

The second question is related to this being a thought experiment (an ideal situation). If gravity isn't a factor, the pressure remains constant throughout the container. The International Space Station may be the best environment for testing this out, and I suspect someone may get around to trying it, if they haven't already.

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Then, in abscence of gravitational effects, would not liquid be dispersed rather than be settled at bottom? then how can two states of matter, gas and liquid, be equally dispersed within a container when liquid is just gas with hightened intermolecular forces of attraction? –  Satwik Pasani Feb 5 '13 at 12:50

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