What are the substances that enter a chemical reaction called?

 Reactants are the substances that enter a chemical reaction. They are the starting materials that undergo a chemical change to form new substances called products. Reactants are consumed during the reaction and are usually written on the left side of a chemical equation. In contrast, products are substances that are formed as a result of the reaction and are written on the right side of the equation. Catalysts are substances that speed up the reaction but are not consumed in the process.