In a chemical reaction, substances (elements or compounds) called reactants are changed into other substances (compounds or elements) called products Hence, reactants are those which start the chemical reaction For example: 1 In the burning of natural gas, methane (CH4) and oxygen (O2) are the reactants in the chemical reaction are the substances formed by a chemical reaction In the burning of natural gas, carbon dioxide (CO2) and water (H2O) are the products formed by the reaction 2 H2+O2 ----> H2O Here H2 & O2 are called Reactants and H2O is called Product