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  GR9277 #47
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Isotherms and coexistence curves are shown in the pV diagram above for a liquid-gas system. The dashed lines are the boundaries of the labeled regions.

In which region are the liquid and the vapor in equilibrium with each other?

  1. A
  2. B
  3. C
  4. D
  5. E

Thermodynamics}Liquid-Vapor Equilibrium

The liquid-vapor region is where the substance can coexist as both a liquid and vapor. (A gas is just a vapor at normal temperatures.)

In this region, the liquid and vapor are in equilibrium, hence their coexistence. Equilibrium occurs when P_v=P_l and \mu_v=\mu_l, i.e., when the pressure and chemical potential of the liquid and vapor are equal to each other.

Since region B shows a constant pressure behavior, despite the volume-decrease, it is the region of liquid-vapor equilibrium.

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Alternate Solutions
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2009-09-28 14:15:56
If you have no idea what this question is talking about, region B is the only one with different behavior from the others. The ETS has also conveniently shaded the correct answer for you.
2010-09-25 07:34:32
how very nice of them,, playing mind games with us
2010-11-09 02:34:21
That is not entirely true. In region A is is definitely a liquid. In region C it is definitely vapor.

Follow the path from one isotherm through the region (lets go left to right with 4). Due to high pressure, it's condensed into a liquid. As pressure decreases, and temperature increases, it starts to evaporate into a liquid. Pressure stays the same for a phase transition, so the volume increase as more and more liquid transitions into a gas. Eventually, when the isotherm exits the shaded region, it is a gas.

Now following the critical isotherm, 2, we see it skips this whole process.

The region E is most likely a liquid, the region A is a liquid, the region C is a gas, and the region B is transition region where Vapor and Liquid are in equilibrium. Thus, the answer is B.

You might ask "What about D?" Well, the distinction between gas and liquid falls apart up here. You'll get something that can only be classified as both. This is called a supercritical fluid and an excellent video you should watch is here:
2010-11-12 22:32:50
Thank you gravity

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