TITLE: Specific Volume vs Temperature Graph
QUESTION [0 upvotes]: When looking at a graph of comparing temperature and specific volume, we see that for increasing pressures, the boiling point increases and that the length of the saturated liquid-vapor line decreases.
So, there's really two questions here, but the following first question will help with answering my main question.
I understand this as because of an increase in pressure, molecules are harder to push apart (as essentially pressure is an external force keeping the molecules together). Is this correct?
However, the main question is why does the saturated liquid-vapor line decrease in length as pressure increases?
The way I understand it is that because we needed a higher boiling point at high pressures, we've already inputted a lot of energy to get to this point, and so the specific volume of the liquid is already quite high for higher pressures, as inputting energy essentially tries to push apart these molecules. Hence, to reach the saturated vapor point, we don't need as much energy to keep forcing particles apart (so the increase in specific volume is not as much).
Refer to graph 2-13 on http://engr.bd.psu.edu/davej/classes/thermo/chapter2.html

REPLY [0 votes]: First, I will give you my interpretation of the graph, because I had to stare at it for a bit to make sense out of it.  Here is what I imagine each line represents.
You fill a vessel completely with cold liquid and seal it.  At the very top (where vapor would be if there were any) there is a pressure relief valve set to allow fluid to escape if the pressure reaches a desired threshold. Each red line corresponds to a different threshold, increasing in pressure toward the top of the graph. Define "specific volume" as the volume of the container divided by the total mass of the stuff inside, whether it's liquid, vapor, or both. Then you start adding heat, which increases the temperature of the liquid and therefore the pressure. Pretty soon the threshold pressure is reached and the valve starts leaking. Things thereafter proceed in three stages, with the pressure always remaining at the constant determined by the valve setting: 1) the temperature of the liquid increases, and it expands (its specific volume increases) causing some liquid to be ejected through the valve; this is the straight section of each line on the left side of the graph. 2) As you continue adding heat the temperature reaches the boiling point and stops increasing; instead the added heat goes into boiling liquid into vapor, with vapor constantly being pushed through the valve to make room for more; this is the flat straight section of each line.  3) Once all the the liquid is boiled away, leaving only vapor in the container, the temperature begins to increase again, and as it does the vapor expands; this is the upward sloping part of each line on the right hand side. 
Now that that's out of the way, your "main question" is why does the middle, horizontal line decrease in length for higher pressures.  One answer is that the vapor is more compressible than the liquid: at higher pressures there's less difference in the densities of liquid vs. vapor because the vapor has shrunk more in response to being squeezed.  Less difference in densities means a smaller range in "average density" (or specific volume) in going from all liquid to all gas, which is what the length of the horizontal line shows. The horizontal line shrinks to zero when liquid and vapor densities are the same.
I know some of this answer is reiterating stuff already offered in the others. But even after reading them it took me a minute to make sense of the graph, so I figured I'd share my thoughts.
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