In the chemical equation \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \), \( \text{CaCO}_3 \) represents:
a chemical formula representing a reactant.
It is the substance that undergoes a chemical change to produce the products \( \text{CaO} \) (calcium oxide) and \( \text{CO}_2 \) (carbon dioxide).