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Homework Help: Why does melting point decrease for impure solid?

  1. Sep 20, 2008 #1
    Okay, i am doing a lab involving crude acetaminophen and pure acetaminophen. The crude acetaminophen is supposed to have a lower melting point than the pure acetaminophen...

    how does that work and why? does it have to do with thermodynamics or what??
  2. jcsd
  3. Sep 20, 2008 #2
    The substance its mixed with may have a lower melting point; that's my guess anyway.
  4. Sep 20, 2008 #3


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    2017 Award

    Mixtures have higher entropy than pure substances. Because the transition from solid to liquid is mostly driven by the increase in entropy (ΔS = Sliquid - Ssolid), increasing Ssolid lowers the overall ΔS, lowering the driving force for melting. Since melting is less favorable, you need higher temperatures to accomplish it.
  5. Sep 21, 2008 #4
    ahhh.... i see

    thanks man
  6. Sep 21, 2008 #5


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    Staff: Mentor

    You may also think in terms of intermolecular forces. In crystal molecules are packed in such a way that their interactions are strongest. When you add some other compound it stretches the crystal lattice, molecules are not in optimal positions and their interactions are weaker, thus less energy is required to break them apart.
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