What is the formula for Calcium Carbonate?

$C a C {O}_{3}$
$\text{Calcium carbonate}$ is reasonably insoluble. When you bubble carbon dioxide gas thru lime water ($C a {\left(O H\right)}_{2} \left(a q\right)$) you see a white precipitate of the carbonate:
$C a {\left(O H\right)}_{2} \left(a q\right) + C {O}_{2} \left(g\right) \rightarrow C a C {O}_{3} \left(s\right) \downarrow + {H}_{2} O \left(a q\right)$