I'm a first timer here. In a normal liquid-vapor P-V biphasic curve at a temperature below critical temperature, we can compress a vapor at a constant temperature till it becomes saturated, or is just about to turn into a liquid. Any further compression would lead to the existence of the two phases together, as we know. Here we can see from the phase rule that we have just one degree of freedom. So just specifying the temperature will set the pressure and volume, just like what the Antoine equation gives.

A pre-question here is that once we come into the zone inside the bell curve of mutual phase existence, what does the volume in the X-axis specify? (Like volume of the gas or volume of the liquid)

My main question is once it enters this zone where the temperature and pressure are set at a saturated value, what is the driving force for the conversion of the vapor to liquid. By this I mean, how can we ensure that two distinct desired compositions can be achieved if we perform the experiment two different times. Surely this isn't to be a transient process where the end result is always full saturated water. I would really appreciate if someone could clear this for me. I have tried looking everywhere for an answer to this, and I did not find it here either. Pardon me if I didn't look enough here.

  • $\begingroup$ So are you wondering why there is a liquid-vapor phase transition? If not, then what? The phase transition and resulting relative amounts are driven by thermodynamics, so 'transient' behavior does not enter in to the end states of the process. $\endgroup$
    – Jon Custer
    May 24, 2017 at 12:47
  • $\begingroup$ Hi! So, I've tried comparing this with the transition of a liquid to vapor. Over there, we are providing heat energy to drive this transition, and at the vaporization temperature, the temperature and the pressure remain constant, while the heat energy is used to fulfill the latent heat requirements effecting the transition. Over here, I can't understand what such a driving force is for this conversion, while the pressure and temperature remain at the saturated values. $\endgroup$ May 24, 2017 at 13:10
  • $\begingroup$ Is it also some sort of energy loss to the surroundings? $\endgroup$ May 24, 2017 at 13:20
  • 1
    $\begingroup$ If we are talking about a pure substance, homogeneous nucleation. If impurities are present, then heterogeneous nucleation. $\endgroup$
    – valerio
    May 25, 2017 at 11:50

3 Answers 3


The volume on the x axis is a weighted average of the liquid and vapor specific volumes, weighted in terms of the fraction of liquid water and water vapor present at the particular quality of the mixture. The quality is defined as the fraction of the total mass present that is vapor.


My main question is once it enters this zone where the temperature and pressure are set at a saturated value, what is the driving force for the conversion of the vapor to liquid.

Some type of "seed", as in crystalization.

In crystallization Nucleation is the step where the solute molecules or atoms dispersed in the solvent start to gather into clusters, on the microscopic scale (elevating solute concentration in a small region), that become stable under the current operating conditions. These stable clusters constitute the nuclei. Therefore, the clusters need to reach a critical size in order to become stable nuclei .

So in this case what happens is that energy is given up by the molecules binding into clusters that are the start of crystallization.

The corresponding mechanism from gas to liquid again needs a nucleation process, for water for example

Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. Nucleation is often found to be very sensitive to impurities in the system. Because of this, it is often important to distinguish between heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation occurs at nucleation sites on surfaces in the system. Homogeneous nucleation occurs away from a surface.

Again, energy will be radiated away when molecules start to bind to the liquid form( that is why it gets warmer when it rains).

Cloud chambers works this way, by having supersaturated vapor and nucleation supplied by the ions generated by charge particles.

Repeatability then depends on how clean surfaces are , or how many nucleation site impurieties there are.


My main question is once it enters this zone where the temperature and pressure are set at a saturated value, what is the driving force for the conversion of the vapor to liquid?

A wide range of liquid and vapor volumes can exist at a given equilibrium temperature and pressure based on the Antoine equation, from a container that is practically full of liquid with a small vapor bubble in it, to a container that is practically full vapor with a small liquid droplet in it. Given this fact, the energy balance of this system must still be maintained.

It is standard practice in chemical engineering to select a standard temperature and pressure (e.g., ideal gas at 0 K), and assign a "heat content" of zero to this standard state. Substances that have a higher temperature and pressure than the standard state are said to have a heat content, or enthalpy (and yes, I'm well aware that physicists do not define heat this way). For the example of water, this concept leads to the fact that a given mass of water has less heat content that the same mass of steam, and that heat content obviously differs by the heat of vaporization. This means that for a given container with a mixture of water and steam, the mass of water multiplied by its heat content plus the mass of steam multiplied by its heat content must equal the total heat content of the system. Any heat added to the system shifts the equilibrium toward more steam and less water, and any heat removed from the system shifts the equilibrium toward more water and less steam. Thus, the driving force for the proportion of liquid vs. vapor is the total heat content of the system, and whether or not heat is being transferred into or out of the system.


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