In which type of reactions do both reactants and products exist around the same concentrations and are in constant reaction with one another?

In reversible reactions, both the reactants and products are present at similar concentrations and continually react with each other to form a dynamic equilibrium. This means that the forward and reverse reactions occur at the same rate, allowing the concentrations of reactants and products to remain relatively unchanged over time.

This characteristic distinguishes reversible reactions from other types. For example, exothermic and endothermic reactions refer to the heat exchange during the reactions but do not inherently imply the concentration dynamics described. Combustion reactions are generally irreversible, leading to products that do not revert to the original reactants under normal conditions. Thus, the nature of reversible reactions allows for a balance where both reactants and products can coexist, continuously interacting in a state of equilibrium.