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Volume Contraction (Water & Methanol Mixture)

  1. Aug 17, 2011 #1
    Hey there. I've been searching the internet on volume contraction with water and methaol mixture. Although I have found a little bit of infomation I don't know why it happens.

    I found it quite intriguing that mixing 50ml of water and 50ml of Methanol =/= 100ml of liquid. Infact there is a 4% difference.

    I know it has something to do with the strength of the hydrogen bond, but is there anything else that affects this. Size of the hydrocarbon? Temperature? Percentage of Mixture? if so, I cannot find much on this on the internet.

    I'd like to know more about this :smile: it does seem very interesting.

    Does anyone else know anything about Volume Contraction in Methanol & Water mixtures?
  2. jcsd
  3. Aug 17, 2011 #2
    I think some factors could be applied. Size of the molecule might be a factor, because the larger the alcohol molecule is, the less significant the hydrogen bond appears. Because non-polarised parts prefer join with other hydrophobic parts, which means attraction between alcohol and water molecules becomes less significant.

    Also, temperature, I guess, has significant impact on strength of hydrogen bonds. As the more violent the movement of molecules is, the more possible that the bonds could break. I don't quite know many details involved, and you probably require more authoritative answers.
  4. Aug 20, 2011 #3
    So what you are suggesting is that when you mix equivalent volumes of CH3OH and H20 you do not get 100mL of solution in your vessel? My thinking is that the -OH group on the methyl group is a electron withdrawing functional group and will shift the electron density toward the -OH region of the molecule, creating a small dipole moment. This can interact with the polar water molecules so what you have is some limited miscibility. I think increasing the temperature (increasing the kinetic energy of the molecules in solution) of the system would facilitate the solution process and make both substances more "agreeable" with each other. The -OH group on alcohols makes them useful as solvents for both polar and non-polar substances so, however I would not expect the methanol and water to be perfectly miscible with each other.
  5. Aug 21, 2011 #4


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    You don't need any fancy interactions for volume contraction, add sand to marbles and final volume is lower than the sum of volumes.
  6. Aug 21, 2011 #5
    Interesting, I've always thought that intermolecular forces were at play here. But I haven't looked at it that way...thanks mate

    Yeah, that would work. But it wouldn't show how different temperatures affects volume contraction :smile:
  7. Aug 21, 2011 #6


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    Not sure about methanol but ethanol/water mixtures density are defined in tables/graphis in handbook of chemistry and physics.

    If you have access to that book, have a look.

  8. Aug 23, 2011 #7


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    It works with water due to the unique structure of water in the liquid state. Liquid water is a fairly open network of hydrogen bonds. Methanol (and ethanol) get in there and mess up the latticework a bit. The more open pure water hydrogen bond network is altered by that methyl group and the whole house of cards comes down.

    Be careful where you go looking for answers to this question. I've seen answers on Yahoo Answers that claim that ethanol molecules are smaller than water molecules and they can slip between those big 'ol water molecules! Were that true, the ethyl group would need to be substantially smaller than a hydrogen atom!

    Oh yeah, methanol and water are completely miscible in all proportions as is ethanol and water.
  9. Aug 24, 2011 #8
    Sorry, I was meant to say Ethanol... sorry

    Could someone please explain how the abundence of electrons on the end of the hydroxyl group affect volume contraction?

    I can't find anything on the internet or the books.
  10. Feb 6, 2013 #9
    I have been reading about this in the pchem book as well. I thought it was interesting indeed. Of course, I cound not find any good explanation online and eventually ended up here!
    Thermodynamics have been an active research in the 1900s but I doubt it still is right now.
    Here is a paper that one might be interested in reading.
    Its paper about mixing methanol and water. Methanol has much lower molar volume than water. But upon addition of methanol, molar volume do not change for a while. It is different from ethanol one where volume just decrees and then goes up to ethanol's molar volume(not sure if ethanol has higher molar volume)
    by A. J. Easteal and L. A. Woolf. Journal of CHem. Thermodyn.
  11. Feb 7, 2013 #10


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    Staff: Mentor

    Is this an exercise in misreferencing?
  12. Feb 7, 2013 #11
    Was in a hurry I forgot to add the rest. A.J. Easteal and L.A. Woolf, J. Chem. Thermodyn., 17, 49 (1985)
  13. Feb 8, 2013 #12


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    To get further insight, you could also study e.g. a two component van der Waals equation.
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