The starting substance for a chemical reaction is called a reactant. Reactants are the initial substances that undergo a transformation during the reaction, leading to the formation of products.
In a chemical equation, reactants are typically written on the left side, while products are on the right side. For example, in the reaction A + B → C + D, A and B are the reactants, and C and D are the products.
Understanding the role of reactants is crucial in studying chemical reactions, as they are essential for driving the changes that occur and determining the outcome of the reaction.