A reactant is a substance that undergoes a chemical change during a reaction. In a chemical equation, reactants are typically found on the left side, indicating the starting materials that will transform into products. For example, in the reaction of hydrogen and oxygen to form water, both hydrogen and oxygen are considered reactants. During a chemical reaction, reactants interact with each other, breaking and forming bonds to create new substances. The amount and concentration of reactants can influence the rate of the reaction, which is a key concept in chemistry. Understanding reactants is essential for studying various processes, including those in biochemistry and industrial chemistry.