The atoms, molecules, or compounds present at the start of a chemical reaction that participate in the reaction. which part of a chemical reaction does this define? a. products b. reactants c. subscripts d. coefficients which is a chemical reaction? a. solid chocolate melting into liquid chocolate b. a peanut shell cracking open into two halves c. water vapor condensing into liquid water on a window d. hydrogen gas and oxygen gas synthesizing into liquid water