Why do CaCO3 + CO2 + H2O form Ca(HCO3)2?

Question

Is it because CO2 + H2O create the carbonic acid H2CO3 and it somehow reacts with CaCO3?

Mateo Aguilar · Accepted Answer

Considering poor solubility of #CaCO_3(s)# a probable explanation may be given as follows

Calcium carbonate is poorly soluble in pure water (47 mg/L) Ksp = 3.7×10−9 to 8.7×10−9 at #25^oC# there exist an ionic equilibrium with the ions formed due to ionization of soluble calcium carbonate. #CaCO_3(s) ⇌CO_3(soln) ⇌ Ca^(2+)(aq) + CO_3^(2−)(aq)# Now in presence of #CO_2# in water #H_2CO_3# is formed and as a result medium becomes acidic. In this acidic medium there exists #HCO_3^-# ion and #H^+#.The #CO_3^(2−)(aq)#ions get protonated by this#H^+# ions and #HCO_3^-# ions are formed .So decrease in concentration of #CO_3^(2−)(aq)#ions in solution shifts the above equilibrium towards right and dissolution of more #CaCO_3(s)# in solution takes place. And the reaction proceeds as stated in question. Please provide feedback.

Cameron Andrews · Answer

undefined CaCO3 + CO2 + H2O form Ca(HCO3)2 because when carbon dioxide dissolves in water, it forms carbonic acid (H2CO3), which then reacts with the calcium carbonate (CaCO3) to produce calcium bicarbonate (Ca(HCO3)2). This reaction occurs in natural settings, such as in the dissolution of limestone by carbonic acid in rainwater.