In this example, I will describe a pressure-volume diagram, and just the basics of the
diagram. So what I plot in the y axis is pressure. This is in mega pascals, and the x-axis I
have plotted the log of the volume, because large volume change between liquid, and vapor.
Let's look at the various aspects of this line that separates liquid and vapor
in a two phase region. So this region here is liquid vapor. So here is the critical point,
and the line along here is saturated liquid, and by saturated liquid, we mean liquid that
is at its boiling point. It is a pure component diagram. It's liquid, that if we put any energy
in, some of it will convert to vapor. So saturated liquid is on the left side, and then the line
along here is saturated vapor. So this is all vapor. We remove any energy at constant
temperature. We will see some liquid form, and so the region in between indicates the
two phase region. Where if we pick a point. What that point corresponds to is some fraction that
is liquid here, and some fraction that is vapor over here. If we pick this point. Then
we are going have majority liquid, and if I pick a point over here. The majority vapor,
and so, we can divide this into regions. The region above the saturated liquid and
saturated vapor line. So the region above the saturated liquid line is a liquid. Not
a saturated liquid, but a liquid at higher pressure. The region to the right. Above the saturated vapor
line is a vapor. Super-heated vapor, and the region near and above the critical point
is a supercritical fluid. It is important to note there is no dividing lines between
these regions. There is nothing that says that when we cross this condition we go from
vapor to supercritical fluid, or we go from supercritical fluid to a liquid. So in some
ways it's an arbitrary distinction. So what we like to look at now is the behavior along
an isotherm. So I am going to draw in isotherm, and then discuss the behavior. So I have indicated
a constant temperature line, an isotherm. In the liquid region we can see this region.
Where we have liquid. As we lower the pressure there is almost no change in volume. Or another
way of course in looking at it. Is raising the pressure, does not decrease the volume,
significantly, and this is because liquids are close to being in-compressible. This is
almost a vertical line, but not quite. In this region we have conversion as we try to lower
the pressure. We will get to a point where an attempt to lower the pressure results in
vaporization, and we go from saturated liquid over to saturated vapor, and of course we
have to have energy added to the system for this to take place, and as we continue to
lower the pressure the isotherm out here then is a gas, and at the lower pressure of course
it's going to behave like an ideal gas. So pressure, volume, where this is volume per mole. Pressure
times volume is a constant, at a constant temperature. I have drawn a second isotherm.
So this green line notice this is now a lower temperature isotherm, because the
saturation pressure is lowered. The temperature have, vapor-liquid equilibrium, but it shows the
same general behavior of in-compressible liquid, constant pressure, and temperature phase change,
and as we get to the lower pressures, ideal gas behavior. And finally, I have
indicated here, a higher temperature isotherm. Where now we don't exhibit any phase change
because we are above the critical point. So the idea is that this diagram can be useful for
solving problems for 1 component system, where there is phase changes making it easier to
visualize the behavior. For example if we have a constant volume process.