Why might it not always be possible to determine the reactants and the products in a reaction

When atoms react they form a chemical bond which is defined as a force of attraction that holds atom together. A force of attraction is defined as a kind of force that draws two or more objects together regardless of distance. There are two major categories of forces of attraction, one is intramolecular and intermolecular. Intramolecular forces is the presence of forces in atoms internally. While intermolecular is the force by which the force that is existent in two or more elements.