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Chemistry

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  • $\begingroup$ HCO3- + H2O = H2CO3 + OH-; H2CO3 = H2O + CO2, Standard hydrolysis equation. H2O is the acid in highest concentration in pure water. Atmospheric CO2 is also involved [or not]. $\endgroup$
    – jimchmst
    Commented Feb 5 at 23:12
  • 2
    $\begingroup$ ... and as a kitchen experiment, adding a little vinegar to the water donates acidic ions, raising the concentration of $\ce{CO_2}$ enough to cause bubbling. $\endgroup$
    – Eric Towers
    Commented Feb 6 at 6:33
  • $\begingroup$ As a side-note, improperly stored samples of sodium hydroxide will react with moisture and carbon dioxide from the air to form the carbonate. $\endgroup$
    – Jonathan S.
    Commented Feb 6 at 13:39
  • $\begingroup$ I was wondering how likely the direct decomposition $\ce{HCO3- <=>CO2 +OH-}$ in neutral water solution would be and ended up going down a quantum chemistry rabbit hole. It seems to be a topic of ongoing research, but AFAICT (from e.g. this recent paper) it may be a relevant route (maybe more than $\ce{HCO3- +H3O+<=>CO2 +2H2O}$ depending on pH), so it doesn't seem like water autodissociation is a necessary initial step. Of course, higher pH still drives the equilibrium left either way, so all this is really just mostly pointless quibbling. $\endgroup$
    – Ilmari Karonen
    Commented Feb 6 at 16:32
  • $\begingroup$ (Also, AFAICT, apparently modern consensus is that $\ce{CO2 + H2O <=> H2CO3}$ doesn't happen in a single step in water, but goes through $\ce{HCO3-}$ first.) $\endgroup$
    – Ilmari Karonen
    Commented Feb 6 at 16:34