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Melting point and impurities

Discussion in 'MCAT Study Question Q&A' started by DJtiesto, Sep 2, 2009.

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  1. DJtiesto

    DJtiesto 7+ Year Member

    Apr 5, 2009
    I know that impurities in a solution will cause it to decrease its vapor pressure, have a higher boiling point, and a lower freezing point, but what is the effect on melting point? I can't seem to find this in any of my books. Thanks
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  3. Omni

    Omni 5+ Year Member

    Sep 17, 2007
    I have a question to add to this as well:
    I've always heard that impurities affect the hydrogen bonds in water, which lower the BP of water. What impurities are they talking about? Why don't solutes like NaCl salt lower the BP instead? Is it because salts are charged?
  4. Omni

    Omni 5+ Year Member

    Sep 17, 2007
    This is weird.
    This whole time the thing has been telling me "unable to post" and whenever I clicked refresh, none of these messages showed up so I kept trying and then now I see a bunch of these.
  5. inaccensa

    inaccensa 7+ Year Member

    Sep 5, 2008
    It will decrease the melting pt,since it will affect the crystallization. Imagine pure water forming crystals and now you add some solute like salt. It interferes with the process, hence decreasing the mp.
  6. jung1

    jung1 2+ Year Member

    Aug 14, 2009
    I imagine the MP and BP are nearly the same. I mean, if it transitions from a solid to liquid, then it will transition from a liquid to solid at the same temp in a reverse reaction right? I guess at equilibrium, if there is such a thing, same amounts would be melting and freezing.

    Dissolving a non-volatile substance decreases vapor pressure of the solution and thus increases boiling point because there is simply less volatile substance in a given mass of solution. It's a dilution effect. That means adding a volatile solute would decrease the boiling point of that solution.
  7. inaccensa

    inaccensa 7+ Year Member

    Sep 5, 2008

    That is an incorrect statment. MP - Substances go from Solid-Liquid
    BP - Substances go from Liquid -gases
  8. PiBond

    PiBond Call me Bond...PiBond 7+ Year Member

    Aug 12, 2009
    It will definitely decrease melting point (or equivalently, decrease the freezing point).

    The presence of solute particles interferes with the efficient arrangement of solvent molecules into a solid lattice. Therefore, it will take colder temperatures (freezing point will decrease) for the solution to effectively freeze. Melting point decreases because, due to the effect the solute particles interfering with efficient arrangement, the intermolecular forces between solvent molecules are not as strong to prevent disruption when heated. Therefore, it will melt at a lower temperature (or be more easily melted).

    Hope that helps.
  9. Hah

    Hah 5+ Year Member

    Sep 4, 2009
    I actually did this experiment in orgo lab a year ago. MP DECREASES with impurities like water or other substances. In addition, the MP RANGE INCREASES. For example, pure substance X has a mp of 142 C - 143C. An impure sample of substance X would have a mp of 90 C -120 C.

    Hope this helps!

    EDIT: As the person who posted before me said BP AND MP are not the same but MP and Freezing point IS (since freezing is liquid to solid and melting is solid to liquid). So you basically answered your own question when you said in your original post that Freezing point decreases with impurities. Since MP=FP, logic dictates MP will decrease too.
    Last edited: Sep 28, 2009
  10. boaz

    boaz shanah alef 7+ Year Member

    Dec 31, 2007
    bachelor pad
    Remember, melting pt. = freezing pt.

    e.g., what are the melting and freezing points of water?

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