1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the significance of Mg(OH)2's solubility?-Chatlier's

  1. Mar 2, 2016 #1
    We did a lab on Le Chatlier's principle. A post-lab question asks wether or not Mg(OH)2 fits the pattern of solids being soluble in warm water.

    NaOH was added to MgCl2 to provide the OH. The phenolphthalein indicator within the solution changed color with NaOH's addition--magenta. When heated it turned pink. When cold it turned violet.


    Is the answer to my question that MgOH doesn't fit the pattern, because the indicator said so? because the forward dissociative reaction is exothermic?
     
  2. jcsd
  3. Mar 2, 2016 #2
    The equilibrium reaction:
    Mg(OH)2(s) <==> Mg++ + 2 OH-
    According to Le Chatlier's principle for an exothermic reaction, if you lower the temp., the equilibrium reaction will shift right (the dissociation gives off heat and if there is less heat in the outer environment (lower temp.), then the reaction wishes to produce more heat (via more products forming)).
    Does the indicator turn more pink (ex. turning from pink to violet) when you raise or lowered the temperature? What does this say about the amount of reactants/products that formed?
    So putting this all together, you will clearly see if Mg(OH)2 would be more or less soluble in warmer water.
     
  4. Mar 3, 2016 #3

    Borek

    User Avatar

    Staff: Mentor

    Indicator and its color change doesn't matter much here. You have not checked how it behaves when it is heated alone, without a presence of Mg(OH)2, so you can't be sure its color changes are related to Mg(OH)2, or not.

    Did the Mg(OH)2 dissolve on heating?
     
  5. Mar 3, 2016 #4
    The indicator goes from low (pink) to high (violet) depending on the presence of OH-s. Violet means more of the latter, so I think it means that cold water caused the production OH ions.

    MgOH<--> OH + Mg
     
  6. Mar 3, 2016 #5
    So no, it didn't o_O
     
  7. Mar 3, 2016 #6

    Borek

    User Avatar

    Staff: Mentor

    Not exactly. Yes, how deep is the color depends on the pH, but around pH 9 (which is 10-5 M strong base) the color gets saturated. If you added strong base in typical for a student lab amounts pH was not lower than 11, so even ±1 pH unit changes didn't matter at all.
     
  8. Mar 3, 2016 #7
    What do you mean by saturated?

    If the indicator isn't telling me anything useful, then how am I suppose to answer the question? It's the only way of telling wether or not MgOH is dissociating that I can think of.
     
  9. Mar 3, 2016 #8
    Violet, to my understanding means more OH ions. It turned violet when the temperature lowered. The presence of ions means that MgOH dissociated, which means that it's soluble in colder water.

    Unless I'm interpreting my lab results incorrectly.
     
  10. Mar 3, 2016 #9

    Borek

    User Avatar

    Staff: Mentor

    the color won't change any further when the pH goes up.

    Assuming there is no indicator in the test tube containing some salt, how would you test if it dissolves in a warm water?
     
  11. Mar 3, 2016 #10
    If you no longer see it's color/shape in the solution?
     
  12. Mar 3, 2016 #11

    Borek

    User Avatar

    Staff: Mentor

    Have you seen Mg(OH)2 after heating?
     
  13. Mar 3, 2016 #12
    no
     
  14. Mar 3, 2016 #13
    That's the way I thought of it, but I'm just learning Le Chatlier's principle right now in my own class, so I'm far from an expert on this.
     
  15. Mar 4, 2016 #14

    Borek

    User Avatar

    Staff: Mentor

    So does it dissolve on heating, or not?
     
  16. Mar 4, 2016 #15
    If you're saying that the indicator is unreliable, then I don't know how I would tell within these circumstances. The experiment was driven by recording the changes in color depending on the "stresses".
     
  17. Mar 4, 2016 #16

    Borek

    User Avatar

    Staff: Mentor

    I am not saying it is "unreliable" - I am saying its indications are not directly related to the solubility changes vs temperature. Could be it was there to help you control the amount of added NaOH - but you have never explained what was the experimental procedure.

    No matter what - you don't use a pH indicator to check whether solubility of a substance changes with a temperature. To do so you take a sample of the substance and you test if it dissolves in a cold and hot water.
     
  18. Mar 4, 2016 #17
    The indicator was there to tell us when a reaction has occurred. Since its color measures the Ph, I would think....
     
  19. Mar 4, 2016 #18

    Borek

    User Avatar

    Staff: Mentor

    No, it doesn't tell you whether the reaction occurred, it tells if pH is above 8.2. It happens that if the pH is high enough Mg(OH)2 will precipitate, so there is a correlation, but in general these two things are separate.
     
  20. Mar 4, 2016 #19
    It sounds like I'm back where I started.

    This was the college level version of the wells mixtures experiment.
     
  21. Mar 5, 2016 #20

    Borek

    User Avatar

    Staff: Mentor

    No idea what it is. Please describe the experimental procedure you have followed.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: What is the significance of Mg(OH)2's solubility?-Chatlier's
  1. Zn(OH)2 and Al(OH)3 (Replies: 2)

Loading...