Why does pressure start off high in the Pressure-Volume Diagram starting from a compressed liquid to a super heated vapor?

Noting that the temperature is constant in the process,I can't seem to understand why it would start at a high pressure going to lower pressure.

Meanwhile, I can understand the Temperature-Volume Diagram why temperature started of at a low value then increases eventually.

It seems that I am confused at the idea of changing saturation temperature and pressure. On the other hand, I understand the idea that boiling point and melting point temperatures can change if we are given with different pressures.

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  • 1
    $\begingroup$ Can you at least accept that, in the superheated region, at constant temperature, if volume increases, pressure decreases? (Think ideal gas law) $\endgroup$ Aug 20, 2017 at 12:30
  • $\begingroup$ Is it like a universal law? I just thought that there might be an important reason behind the relationship. $\endgroup$
    – Czar Luc
    Aug 20, 2017 at 16:30
  • $\begingroup$ Are you asking about the applicability of the ideal gas law , or are you asking about its derivation? $\endgroup$ Aug 20, 2017 at 16:36
  • $\begingroup$ "I can't seem to understand why it would start at a high pressure going to lower pressure." I don't understand your question. The right graph clearly shows pressure starting low, then going high and then going low again. What do you mean when you say that pressure starts high? $\endgroup$
    – Steeven
    Sep 18, 2017 at 20:20

1 Answer 1


Because liquids are in general much less compressible than gases, meaning that a small reduction in volume requires a big increase in pressure.


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