Final answer:
The materials at the beginning of a chemical reaction are known as reactants, which transform into products during the reaction.
Step-by-step explanation:
The materials present at the beginning of a chemical reaction are called the reactants. These are the substances that undergo a chemical change and are usually found on the left side of a chemical equation. During a chemical reaction, reactants interact and transform into new substances known as products, which are the outcomes of the reaction and are placed on the right side of the equation. For instance, in the chemical reaction where sodium and chloride ions combine, they are the reactants that produce table salt as the product.