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General Chemistry

  1. Aug 25, 2009 #1
    1. The problem statement, all variables and given/known data
    a) In general, can we determine the identity of a pure substance based only on its density?
    b) During the summer the temperature of water is warmer at the surface of a lake than in depth. Disregarding the fact that the lake is heated by the sun light, explain this phenomenon.
    2. Relevant equations

    3. The attempt at a solution
    a) Yes because all pure substances have their own density. Density for all pure substances is different.
    b) I believe that this phenomenon is called Thermocline.

    I'm not sure if these answers are correct, if anyone can shine some light, it would be great. It's my first day in college, and I'm having trouble. Thanks :D.
  2. jcsd
  3. Aug 26, 2009 #2
    For the density, it seems reasonable to argue both ways, since one could always bring up the argument of pure water freezing to ice, which floats on pure liquid water since the density is less. I personally found both arguments when I used a search engine on this problem, since I was interested.
  4. Aug 27, 2009 #3


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    Staff: Mentor

    Not necesarilly, optical isomers have crystals of idetical densities, yet they are different substances. Even assuming densities are different, question is - are we able to measure density precisely enough? Assuming you have measured density with +/- 1mg/mL accuracy (high one!) and assuming all substances we know have densities in the range 0-22.61, that gives place for 22610 substances. Not much.

    No, thermocline is a surface at which temperature of the water (or air) changes abruptly.

    Think in terms of densities. What will happen to warm water immersed in the lake of cold water? To cold water in the lake of warm one? Remember they won't mix fast.
    Last edited: Aug 27, 2009
  5. Aug 27, 2009 #4
    1) Nope.

    2) Consider the fact that the bottom of the ocean is dark while the top of the ocean is light.
  6. Aug 27, 2009 #5
    Oh. Disregarding the fact that the lake is heated by the sunlight.

    If this is a general chemistry class, quite frankly, a chemistry class is not going to explain thermocline; it's more complicated than a chemistry class can explain.
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