Reef Chemistry Question of the Day #245 Osmosis

Discussion in 'Reef Chemistry by Randy Holmes-Farley' started by Randy Holmes-Farley, Nov 6, 2017.

  1. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    Reef Chemistry Question of the Day #245

    Suppose that I have a special balloon that is readily permeable to water but not to ions (like a cell membrane, but stronger).

    I fill it with fresh water and drop it in a reef aquarium.

    What happens?

    A. The balloon gets bigger
    B. The balloon gets smaller
    C. There is no change in the size of the balloon

    Good luck!

























    .
     
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  2. happyschneiders

    happyschneiders New Member

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    The balloon gets smaller :)
     
  3. luis carreiro

    luis carreiro Well-Known Member

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    balloon gets smaller due to the ionic gradient and permeability of the balloon
     
  4. Grey Guy

    Grey Guy Well-Known Member

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    The balloon gets smaller because the pure water coming through the membrane will seek equilibrium, or the balloon will stay the same with no exchange, or the osmotic pressure wil make the balloon get larger. Not sure, I guess.
     
    Last edited: Nov 6, 2017
  5. Grey Guy

    Grey Guy Well-Known Member

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    OK I think that as pure water seeps out and increases concentration of ions, inside, then pure water will seep back inside in order to maintain equilibrium.
     
  6. Reef Monkie

    Reef Monkie Well-Known Member

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    B.

    But, if you put a balloon filled with 19999.999999 gallons of water in a 20000 gallon reef tank with 0.000001 gallons of salt water in it I don't think you are going to notice that the balloon gets smaller with the naked eye. :p
     
  7. Grey Guy

    Grey Guy Well-Known Member

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    I think it is logical that the balloon will stay the same size, if the balloon is not trying to return to a smaller size. If there is more pressure inside then out, then it must get smaller. But Randy says it’s a special balloon, which indicates to me, that there is no air or water pressure trapped inside. Therefore, the special balloon will stay the same size. The right answer is probably that it gets bigger, because that’s the one that makes the least sense. LOL
     
  8. Grey Guy

    Grey Guy Well-Known Member

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    O.K. You’ve really got me going on this one. Here is my final pick. The balloon will get larger because the ion density of salt water will be higher than the fresh water inside. This will force water from outside to inside, so that when the ion count is equal on both sides, the balloon will have increased in volume.
     
  9. KMench

    KMench Well-Known Member

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    Balloon gets smaller. Water molecules migrate out of the balloon in an attempt to reach equilibrium. Since the ions can't move across the membrane the water does.
     
  10. luis carreiro

    luis carreiro Well-Known Member

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    I believe that fresh water has a lower osmotic concentration then salt water thus since the balloon is impermeable to ions. Water will flow from a lower osmotic concentration to one of higher osmotic concentration. Thus the balloon will get smaller
     
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  11. redfishbluefish

    redfishbluefish Stay Positive, Stay Productive R2R Supporter R2R Excellence Award Reef Squad Build Thread Contributor NJRC Member Article Contributor

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    I believe the OP unwittingly added additional parameters to this problem. As an example, if I was not aware of osmosis, but realized this overfilled balloon is exerting pressure, I would come to the correct answer of B, because the pressure exerted by the extended rubber balloon forces the water out.

    However, if the OP stated no pressure differences on either sides of this semipermeable balloon, than the ionic gradient would cause for water to flow from least to greatest ionic strength, thereby shrinking said balloon.....answer B.
     
    Last edited: Nov 6, 2017
  12. dansreef

    dansreef Well-Known Member

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    Smaller..... but because that monster emerald crab I put in my sump a year ago would mistaken it for the mother of all bubble algae.... and would try to munch it....

    In all seriousness.... I would think that the fresh water would migrate out of the balloon.
     
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  13. SDReefer

    SDReefer Well-Known Member Build Thread Contributor

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    The balloon gets smaller since the concentration of water outside the balloon is less than the concentration of water inside the balloon. Therefore, water will leave the balloon until the concentrations are equal. The reef tank is considered a hypoosmotic solution due to the high concentration of salt in the water.
     
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  14. rkpetersen

    rkpetersen Well-Known Member Partner Member 2018

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    B. Much like an animal cell dropped into hypertonic saline. Except in this case and unlike with a cell, eventually there will be virtually no fluid in the balloon at all, since it can never come to equilibrium.
     
    Last edited: Nov 6, 2017
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  15. MnFish1

    MnFish1 Well-Known Member R2R Supporter Build Thread Contributor Partner Member 2018

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    B. It gets smaller. The opposite of a freshwater dip(which causes cells to explode ich)
     
  16. Alfrareef

    Alfrareef Well-Known Member

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    B. Fresh water shall leave the ballon.
     
  17. MnFish1

    MnFish1 Well-Known Member R2R Supporter Build Thread Contributor Partner Member 2018

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    So both ways you stated the answer was b. What was the complicating factor? Btw. If there was pressure inside the balloon and the membrane is permeable to water water would start leaking before it even hit the reef tank
     
  18. beaslbob

    beaslbob Well-Known Member

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    smaller

    hey only 1 chance in 3. LOL
     
  19. SDchris

    SDchris Member

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    A or C
    Devil's advocate. Pressure exerted by the salt water (depth) is greater or equal to the osmotic pressure of the two different solutes.


    Chris
     
  20. FunkEngine

    FunkEngine Active Member

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    Considering the topic title, I'm going to assume this balloon has negligible forces acting on it from pressure and elastic contraction. Water will move out of the balloon, and it will shrivel.
     
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