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Homework Help: Question about phase transitions and PV, VT diagrams

  1. Apr 21, 2014 #1

    A) Explain what happens when a gas is kept in a cylinder under constant pressure while being cooled. Assume that constant pressure is less than critical pressure. Draw PT and VT diagrams for this?

    B) Describe and explain what is seen when a strong transparent container of constant volume, rather greater than critical volume of the gas it contains, is cooled. Assume that initial temperature is above the critical temperature. What would be the difference if the container volume were exactly critical?

    My attempt:

    A) For question A I know that if the pressure is kept constant and the temperature is decreasing then the volume would also decrease with the temperature. So the gas would condense slowly into a liquid as the temperature decreases and if the temperature continued to decrease it would condense further into a solid

    My problem was drawing the graphs, the way I thought it was initially down was on the PT graph pressure would be constant so it would just be a horizontal line and in the VT graph it would be a straight line but this has no information about the phase transitions involved?

    B) I haven't really got much on this question so far! All I know (think I know) is that if volume is held constant pressure would decrease as temperature decreases??

    Any help would be great for this

    Thanks in advanced :)
  2. jcsd
  3. Apr 21, 2014 #2
    In part A, on the VT diagram, if you have a pure substance, the volume change from a vapor to a liquid would all occur at constant temperature; this constant temperature would be the saturation temperature corresponding to system pressure.

  4. Apr 21, 2014 #3
    Ok, thank you! Is everything else I said correct for part A?
  5. Apr 21, 2014 #4
    You can see by eye what the answers to all these questions are by Googleing "enthalpy diagram" or "Mollier diagram." A typical pressure-enthalpy diagram shows lines of constant temperature, lines of constant specific volume, and even lines of constant entropy. They also show the two phase region, and the critical point. These diagrams are of great practical value and are also very educational. Check it out.

  6. Apr 22, 2014 #5
    Ok I will do! Thanks for all your help
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