Limestone is calcium carbonate Ca(Co3) it's dissociation equation is as follows:
Ca(CO3) <----> Ca+2 + CO3-2
but the solubility is very small
in the presence of acid by LeChatlier's Principle you can shift to the right by adding acid
H+ + CO3-2 <-----> H+CO3- (the bicarbonate ion which is really soluble)
The system will pull more and more limestone out into solution as acid is added and neutralize the acid coming from the rain.
