Question

Why do${\mathrm{CaCO}}_3+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$ form $\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2$?

Answer 1

Dissociation of Calcium Carbonate

Calcium Carbonate is in the equilibrium with the Carbonate ion and the calcium ion as shown below.

• ${\mathrm{CaCO}}_3\left(\mathrm{s}\right)\rightleftharpoons {\mathrm{Ca}}^{2+}\left(\mathrm{aq}\right)+{\mathrm{CO}}_3^{-2}\left(\mathrm{aq}\right)$
• Here, formation of the carbonate takes place.

In the presence of Water $\left({\mathrm{H}}_2\mathrm{O}\right)$ and Carbon dioxide $\left({\mathrm{CO}}_2\right)$:

• Formation of hydrogen carbonate ( ${\mathrm{H}}_2{\mathrm{CO}}_3$) takes place.
• Medium of the solution becomes acidic, as a result the carbonate ions $\left({\mathrm{CO}}_3^{-2}\right)$ get protonated and bicarbonate ions $\left({{\mathrm{HCO}}_3}^{-}\right)$ start forming.
• Further, it causes shifting of equilibrium and more and more formation of carbonate ions $\left({\mathrm{CO}}_3^{-2}\right)$ and hence after protonation, formation of bicarbonate ions $\left({{\mathrm{HCO}}_3}^{-}\right)$ takes place which then forms Calcium bicarbonate $\left(\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\right)$.