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Density of liquid

  1. Jun 21, 2014 #1
    Please, tell me really simply from the viewpoint of molecular physics... Why is liquid having a smaller density on liquid having a bigger density? Thanks
     
    Last edited by a moderator: Jun 21, 2014
  2. jcsd
  3. Jun 21, 2014 #2

    berkeman

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    Staff: Mentor

    Your question did not translate very well -- are you asking why a less dense liquid "floats" on top of a more dense liquid?
     
  4. Jun 22, 2014 #3
    Sorry. Yes. This is my question,
     
  5. Jun 22, 2014 #4

    Lok

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    For this question (expertly found by Berkeman) there is no need for Molecular physics just Archimedes.
    If you place 2 different density liquids with a vertical separator and then remove the separator liquids will search the lowest potential energy position. So the heavy liquids bottom position will be lower in potential energy than the vice-versa. it is not as the lighter liquid does not seek it's lower potential energy position, it is just that it's potential energy is lower.

    For some reason I think you are trying to ask why two liquids separate into a boundary. As a fine mixture of oil and water (can be done and it will settle into oil above and water below). It is just because the Water molecules attract each other much stronger that Water molecules attract oil molecules. So the water molecules seek their respective most stable configuration (again with the potential energy - although Molecular Physics) and gather into a big clump leaving the oil to clump with it self. Then you get the above situation where the heavy and light clumps go into the configuration with the lowest potential energy.
     
  6. Jun 22, 2014 #5
    Ok, thank you very much. However, if some liquid has bigger density and some liquid had lower density, they can be mixed, if is the force between molecule of liquid with lower density and molecule of liquid od bigger density stronger than force of same molecules. It is right? Must occur chemical reaction?
     
  7. Jun 22, 2014 #6

    Lok

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    A chemical reaction is not needed to occur.

    For two liquids to dissolve, it is necessary for the intermolecular forces to be close irrespective of their density.
     
  8. Jun 22, 2014 #7
    So, the intermolecular forces must not be bigger than forces between same moleculas of liguid? Only must be the two different molecules close?
     
  9. Jun 22, 2014 #8

    Lok

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    Too put it simply.

    The intermolecular forces (liquid 1 with liquid 2) must be close in value to the intermolecular force of liquid 1 with liquid 1.
    Example:
    Liquid1-Liquid1 - strong force ...... Liquid2 -Liquid2 weak force
    If Liquid1-Liquid2 is close in strength to Liquid1-Liquid1 they will dissolve nicely. In this case Liquid2 -Liquid2 force does not contribute to their solvation.

    For a more detailed picture you can look up solvation in a Chemistry Introductory Course or Book.
     
  10. Jun 22, 2014 #9
    Ok, thank you very much :-)
     
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