This is the chemical reaction for the decomposition of calcium carbonate $CaCO3$ into calcium oxide $CaO$, carbon dioxide $CO2$, and water $Ca(OH)2$ when heated.

CaCO3 $s$ -> CaO $s$ + CO2 $g$ CaCO3 $s$ -> CaO $s$ + CO2 $g$ + H2O $l$