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Caffeine sublimation and melting point

  1. Sep 13, 2009 #1
    Why caffeine has 178 oC as point of sublimation but 237 oC as melting point?

    Is it about the pressure? If so, then what is the preesure that makes 237 as melting point instead of sublimation at 178 oC? Cuz the science websites always just said 237 is the melting point, but didn't mention the pressure or any reason why.
     
  2. jcsd
  3. Sep 13, 2009 #2

    chemisttree

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    If you keep the caffeine at 178C long enough it will sublime. If you quickly raise the temperature through 178 to 237C you will observe the solid/liquidus transition even though it is slowly subliming at temperatures above 178C. When you finish doing the melting point test and remove your capillary, you will observe the sublimed and recrystallized material some distance above the melted sample.
     
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