Answer: [tex]CaCO_3[/tex] is a reactant; it is present before the reaction occurs.
Explanation:
In a chemical reaction the chemical formulas written before the arrow are described as reactants as they react together to form products which are written after the arrow.
[tex]CaCO_3 + 2HCl \rightarrow CaCl_2 + CO_2 + H_2O[/tex]
Thus [tex]CaCO_3[/tex] and HCl are reactants here whereas [tex]CO_2[/tex], [tex]CaCl_2[/tex] and [tex]H_2O[/tex] are products.