# Pressure-Volume Diagram

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.

• Can you at least accept that, in the superheated region, at constant temperature, if volume increases, pressure decreases? (Think ideal gas law) Commented Aug 20, 2017 at 12:30
• Is it like a universal law? I just thought that there might be an important reason behind the relationship. Commented Aug 20, 2017 at 16:30
• Are you asking about the applicability of the ideal gas law , or are you asking about its derivation? Commented Aug 20, 2017 at 16:36
• "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? Commented Sep 18, 2017 at 20:20