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Questions from a lab on pH and water

  1. Feb 23, 2006 #1

    Math Is Hard

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    ok, sorry. Couple of dumb questions..:redface:

    We had a lab last night, and one part involved testing the pH of various substances (we used the little strips of litmus paper and measured the color against a chart). My group tested pure water and we obtained a pH measurement of 7. Another group tested sugar water and obtained a pH of 6. One of the questions was if the OH- concentation of pure water and sugar water differed by a magnitude of 10 times or greater, and while it did, I can't figure out why. Why would the pure water be more basic? What is it about sugar that would make water more acidic? Should I trust the test result from the other group?

    We also did an experiment related to capillary action, where we placed plastic straws and glass tubes of different sizes in beakers half full of water or half full of alcohol. In all cases the water climbed in the tube or straw. The alcohol also climbed, but not quite as high. One of the lab questions was "for capillary action to occur is it necessary for 1) only the fluid or 2) both the fluid and the tube to be polar?" My thought is that only the fluid would need to be polar. I think the plastic material is polar, but the glass is not. But I am not sure about this. Is it strictly the evaporating molecules on top that pull the water column upwards?

    Thanks!
     
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  3. Feb 24, 2006 #2

    Hootenanny

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    As far as my knowlage goes, I believe that it is only the fluid that needs to be polar. I think it is refered to as the cohesion / tension theory. Beyond that I don't know much else. Sorry.
     
  4. Feb 24, 2006 #3
    I'm not sure about the water,
    bat I wouldn't trust the relevance of the
    other groups testing. I recommend you
    repeat the experiment yourself.
    :)
     
  5. Feb 24, 2006 #4

    honestrosewater

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    Sorry, this isn't a serious answer, but I thought it might give you a little chuckle. How old was the sugar water? Maybe there were some little critters in there that fermented the sugar and converted the ethanol to acetic acid. That could happen, right?
     
  6. Feb 25, 2006 #5

    Math Is Hard

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    Thanks for looking at that, y'all. I think I feel more confident about my answer to the second question. I still don't know what to make of the sugar water result. I wish there were some way to retest it, but I don't have any testing strips laying around. HRW, thanks for the suggestion, but I don't think that dog will hunt. :) I am wondering, though, if that sugar water sample was contaminated. There were stir sticks in each of the solutions and maybe some got swapped around and the sample was contaminated? My group was very careful, but I don't know how cautious the group who tested the sugar water was. The color differences on the matching chart were quite subtle as well.
     
  7. Feb 25, 2006 #6

    siddharth

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    Math Is Hard, what sugar did the group use? If you take a simple sugar like sucrose, on hydrolysis it forms a mixture of glucose and fructose.
    Now both glucose and fructose have hydroxyl groups (-OH) which can disassociate and release H+ into the solution and as a result, lower the pH. Have you tried using a digital pH meter to measure the pH?


    As for that question, the capillary action happens because the adhesive forces between the solid and the liquid is more than the cohesive force between the molecules of the liquid. Both these forces are essentially due to intermolecular attraction. When it's between like molecules, it's a cohesive force and when it's between unlike molecules, it's an adhesive force.

    Now both the liquid and solid molecules are neutral as a whole, so how do these attractive forces actually happen? If you take a polar molecule, the charge distribution in the molecule is not uniform. So even though the molecule is neutral as a whole, there will be dipole moment, which is a tiny separation of the positive and negative charge centers. This causes attraction between polar molecules.

    What about non-polar molecules? Even they can experience such forces. This is because, non-polar molecules will have instantaneous dipole moments which change with time, and the result of this is a net attraction.
     
    Last edited: Feb 25, 2006
  8. Feb 25, 2006 #7

    Math Is Hard

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    Hi siddharth! Sorry to pull you away from the cryptic crosswords. Thanks for looking at this.
    No, we didn't have a digital pH meter in the lab, unfortunately. Only the little test strips. I am almost positive the sugar water solution was made by dissolving table sugar into a cup of water. (I could see a little bit of the sugar in the bottom of the cup).
    I am thinking about what you said, and I am looking at a glucose molecule.
    I can see how the -OH groups could possibly dissociate, but even in that case, it seems that either

    1) the dissociation of the hydroxyl groups increases the OH- concentration. So it would become more basic.
    2) the dissociated OH- ions would bond with any free H+ ions in the solution and cause it to become more basic by taking up the H+ ions.
    so I can't get my head around how the dissociation of the hydroxyl groups would release H+ into the solution.

    Thanks for the explanation. I was googling trying to find out if the surface of glass is polar or non-polar. I am not sure, but I think it is non-polar. I am also not clear about alcohol - I believe it has a non-polar end and a polar end (the -OH group). I am trying to make some deductions about the materials used in the experiment, the plastic tubes vs. the glass tubes, and the substances we tried, alcohol and water.
     
  9. Feb 25, 2006 #8

    siddharth

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    What I meant was that the -OH group does not dissociate as a whole. It ionizes into the alkoxide ion and H+.

    That is,
    R-OH ---------> H+ + R-O-

    In fact, because if this, most alcohols can act as weak acids.
     
  10. Feb 25, 2006 #9

    Math Is Hard

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    That's very interesting. I did not know that. Thanks!
     
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