How does bicarb soda dissolve gypsum in plaster form?

[Gypsy Moonlight]

Ehecatl posted very helpful content on this ,.. Just wondering if anyone can describe the actual reaction that takes place?

 

[Borek]

I guess you refer to this thread: https://www.physicsforums.com/threads/acid-for-plaster-of-paris.139990/

The only chemistry I can think of is that he converted sparingly soluble CaSO₄ into even less soluble CaCO₃, changing the structure of the solid in the process.

 

[Gypsy Moonlight]

I guess you refer to this thread: https://www.physicsforums.com/threads/acid-for-plaster-of-paris.139990/

The only chemistry I can think of is that he converted sparingly soluble CaSO₄ into even less soluble CaCO₃, changing the structure of the solid in the process.

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,

 

[Borek]

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.

 

[insightful]

Cannot Ca⁺⁺ stay in solution as Ca(HCO₃)₂(aq)?
This is the normal species of calcium hardness in well water.

 

[Borek]

Cannot Ca⁺⁺ stay in solution as Ca(HCO₃)₂(aq)?
This is the normal species of calcium hardness in well water.

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

Besides, in NaHCO₃ solution concentration of CO₃^2- is quite high, which means the maximum concentration of Ca²⁺ quite low. In 0.1 M NaHCO₃ [CO₃^2-] = 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 CO₃^2-).