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How does bicarb soda dissolve gypsum in plaster form?

  1. Nov 4, 2015 #1
    Ehecatl posted very helpful content on this ,.. Just wondering if anyone can describe the actual reaction that takes place?
     
  2. jcsd
  3. Nov 5, 2015 #2

    Borek

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    Staff: Mentor

  4. Nov 5, 2015 #3
    Yes, that is the thread, I have, And it was brilliant advice, -I tried the bicarb solution and it worked well to dissolve the plaster/ soften enough for easy scraping.
    I am hoping to be able to explain it a little to primary school kids - they will be playing archeologist with buried dinosaurs, though I now have to also give them bicarb soda to access the toys inside the plaster as I made the plaster too hard. I can talk to them about it being a base, but I don't really understand why it is working to eat away the plaster,....
    There is a reaction that happens first when the water and gypsum combine, then another when the bicarb works on the plaster,. Any laymen's terms would be appreciated,
    Thanks,
     
  5. Nov 5, 2015 #4

    Borek

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    Every insoluble salt is in fact a bit soluble. CaSO4 has solubility around 100 times higher than CaCO3. When you add bicarbonate plaster (calcium sulfate) slowly dissolves and calcium carbonate precipitates:

    CaSO4(s) + NaHCO3(aq) → CaCO3(s) + NaHSO4(aq)

    It happens the precipitating CaCO3 is much easier to remove.

    Disclaimer: this is just my guess, what I wrote is thermodynamically correct, but it is not necessarily the correct explanation.
     
    Last edited: Nov 5, 2015
  6. Nov 6, 2015 #5
    Cannot Ca++ stay in solution as Ca(HCO3)2(aq)?
    This is the normal species of calcium hardness in well water.
     
  7. Nov 6, 2015 #6

    Borek

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    Yes, but there is still a limit to how much of them can be present.

    Besides, in NaHCO3 solution concentration of CO32- is quite high, which means the maximum concentration of Ca2+ quite low. In 0.1 M NaHCO3 [CO32-] = 1.1×10-3 M, which puts a limit on the [Ca2+] at 2.5×10-6 M. Compare that to the concentration of Ca2+ in the saturated CaSO4, which is around 0.005 M - almost 2000 times higher than what is required required for the CaCO3 to start precipitate (assuming above concentration of CO32-).
     
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