Can H2CO3 and NaHCO3 be in the same solution at the same time?

In my opinion they can't. Because the product of reaction will have $\ce{H2CO3}$, and end up with $\ce{H2O}$ and $\ce{CO2(g)}$. And gas can not be part of a solution.


$$\ce{H2CO3 +2NaHCO3 -> Na2CO3 + 2H2O + 2CO2(g)}$$




There are ongoing four linked equilibrium reactions with their respective equilibrium constants:


$$\ce{CO3^2-(aq) <=>[+H+][-H+]HCO3-(aq)<=>[+H+][-H+]H2CO3(aq)<=>[-H2O][+H2O]CO2(aq)<=>CO2(g)}$$


For drinking water with natural bicarbonate $\mathrm{pH}$ buffer, the concentrations of $\ce{HCO3-(aq)}$ and $\ce{CO2(aq)}$ are comparable, with minor concentration of $\ce{H2CO3(aq)}$. See also [carbonic acid](https://en.wikipedia.org/wiki/Carbonic_acid) on Wikipedia.


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