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Homework Help: Question: Solubility

  1. Aug 20, 2006 #1
    As I was reading text “ The solubility of most compounds increase with increasing temperature, but a substance that is retrograde soluble is one whose solubility decreases with increasing temperature.”

    Why dose this happen? How dose “retrograde soluble” substance work and why? Is there an example?
  2. jcsd
  3. Aug 21, 2006 #2
    Cerium sulfate, Ce2(SO4)3, is the only example I know of. According to my table, the nonahydrate has a solubility of 17.35g/100mL at 0C, but only 3.73g/100mL at 60C. As for the "why?", I would like to know as much as you. I never could get a good answer from my teacher(s).
  4. Aug 21, 2006 #3
    destroing crystal lattice needs energy from outside (E1), and during aquation energy liberation occures(E2). if E2 is greater than E1, solubility decreases with temperature (sorry about bad English)
  5. Aug 21, 2006 #4
    what does "aquation energy liberation occures" mean?:bugeye:
  6. Aug 21, 2006 #5
    Here is a site with more examples. I found two sites that gave explanations that were clearly wrong and two others that said "there is no easy way to tell".


    According to the scientist on that site, most rare-earth sulfates show this property. Interesting...
  7. Aug 22, 2006 #6
    when ions interact with water molecules, energy liberates.
  8. Aug 22, 2006 #7
    Disolving NaOH in water, for example, gives off heat and is an exothermic reaction. Hydrating NH4NO3, absorbs heat and is an endothermic reaction. Both the solubility of NaOH and NH4NO3 increase with increasing temperature, however. So I don't really understand what you mean geo_alchemist.

    vincikai, in some cases it appears that the temperature starts to decrease with increasing temperature due to hydrate formation. See here http://en.wikipedia.org/wiki/Sodium_sulfate under the solubility of sodium sulfate. A similiar argument may help explain cerium sulfate's solubility graph as it forms several waters of hydrations.

    According to my solubility tables, however, lanthanum sulfate, La2(SO4)3, like Ce2(SO4)3 decreases in solubility with increasing temperatures and it forms no hydrates.
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