Hi!
As far as I remember sodium bicarbonate + any acid, for example
CO3HNa + ClH = CO3H2 + ClNa, and the carbonic acid decomposes
CO3H2 = CO2 + H2O. Of course, Alka Seltzer does not contain
clorhidric acid, but some organic acid, I guess citric or tartaric.
Regards Emilio