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Reflux still

  1. Mar 23, 2008 #1
    I've read info on it's operation but some questions are still unanswered. This is concerning reflux stills that create the reflux by condensing rising vapor at the top of the still.
    How does the still work on a molecular level?
  2. jcsd
  3. Mar 24, 2008 #2

    Andy Resnick

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    Are you asking for a description of a phase change at the molecular level?
  4. Mar 24, 2008 #3
    Yes, I organized my thoughts better in the following paragraph.
    A theoretical mixture is composed of liquid A and liquid B. There is more of liquid A in the mixture than liquid B and liquid B has a lower boiling point. They are fully miscible. The mixture is distilled to try to seperate the two liquids. It is my understanding that when the mixture is heated both liquids will vaporize but liquid B will vaporize more. Why does liquid B vaporize more readily and what is happening on a molecular level/how are the molecules of liquid A and B interacting? In a reflux still the vapor is condensed at the top and flows back down interacting with rising vapor. Why does liquid A condense more readily than liquid B at the top of the still?
  5. Mar 24, 2008 #4
    Their boiling points are different. Temperature is the average kinetic energy of the molecules. In your example, a B molecule requires less energy to escape the liquid, so the higher the temperature, the more B molecules will be above their minimum threshhold for evaporation, than A molecules above their (higher) minimum threshhold. Thus B evaporates at a faster rate.

    At the top of the still, A has a higher boiling point and thus will condense at a higher temperature than B.