Reactants are the starting substances in a chemical reaction. They undergo a transformation during the reaction to form products. In a chemical equation, reactants are typically written on the left side, while products are written on the right side, separated by an arrow which indicates the direction of the reaction. For example, in the reaction:
\[ \text{A} + \text{B} \rightarrow \text{C} + \text{D} \]
A and B are the reactants, while C and D are the products. Reactants can be elements, compounds, or mixtures, and their chemical properties determine the nature of the products formed during the reaction.