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I'm confused with pressure and condensation of a gas inside a sealed container. Say I boil a liquid to it's gaseous state inside a sealed container and exert more pressure on that container then should the gas not turn back into a liquid?

For instance on Mount Everest the atmospheric pressure is 5psi and water boils at 71c, at ground level the pressure is 14psi and water boils at 100c. My thinking is that If I raise the pressure even more the temperature at which gas condensation occurs should be reached and the gas would suddenly turn back to a liquid?

If that's correct, what pressure would I need to exert on that gas to turn it back to a liquid state instantly?

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2 Answers 2

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Water vapor is not an ideal gas. Due to this, when you compress it instantly, you do work on it, and the temperature of the water vapor rises. Instant compression is adiabatic compression, and without heat transfer to the environment, you will not be able to transfer the heat necessary for condensation to occur.

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  • $\begingroup$ Any suggestions on low boiling point liquids above 0c that act more like ideal gases? Or is your answer a flat out no for instant gas to liquid phase change? $\endgroup$
    – G Gr
    Commented Aug 10, 2019 at 2:28
  • $\begingroup$ I have a chemical engineering background with 21 years of industrial experience. I have never seen the case where a vapor could be turned back into a liquid under adiabatic conditions. $\endgroup$
    – David White
    Commented Aug 10, 2019 at 2:30
  • $\begingroup$ Thanks. Just find it strange how pressure increases the boiling point of a liquid but increases the temperature of a gas. I thought if a liquid becomes a gas at a certain temperature then the same would apply. For instance liquid becomes a gas at 100c.. increase pressure... now the boiling point is 150c and gas returns to being a liquid. $\endgroup$
    – G Gr
    Commented Aug 10, 2019 at 2:44
  • $\begingroup$ Think of it this way: the work of compression turns into heat, which raises the temperature of the vapor. $\endgroup$
    – David White
    Commented Aug 10, 2019 at 3:53
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Assume that only vapour (and no air) is inside your container and that the temperature is kept fixed. When you use a piston to decrease the volume, and so increase the pressure until it reaches the vapour pressure, then a small amount of the liquid will condense and lower the pressure until it is again at the vapour pressure (Vapour pressure = 5psi at T=71C acording to your data) . Reduce the volume a bit more and a bit more fluid will condense again lowering the pressure back to the vapour pressure. At T=71C this will continue, the pressure remaining constant at 5psi, until you have turned all the vapour back into liquid and the piston is in contact with the liquid surface. If you try to reduce the volume even more you will be compressing the liquid and will have to exert a lot more pressure.

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