# 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?

Feb 18, 2016

Considering poor solubility of $C a C {O}_{3} \left(s\right)$ 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 ${25}^{o} C$
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 $C {O}_{2}$ in water ${H}_{2} C {O}_{3}$ is formed and as a result medium becomes acidic.
In this acidic medium there exists
$H C {O}_{3}^{-}$ ion and ${H}^{+}$.The CO_3^(2−)(aq)ions get protonated by this${H}^{+}$ ions and $H C {O}_{3}^{-}$ 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 $C a C {O}_{3} \left(s\right)$ in solution takes place.
And the reaction proceeds as stated in question.

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