Question

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

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

Answer 1

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

Explanation:

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.

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