What defines an oxidation-reduction (redox) reaction?

An oxidation-reduction (redox) reaction is defined by the transfer of electrons between reactants. In these reactions, one substance loses electrons (oxidation), while another gains electrons (reduction). This transfer of electrons is fundamental to the chemical changes that occur during the reaction, leading to alterations in the oxidation states of the elements involved.

In a redox reaction, the ability to lose or gain electrons is crucial, as it facilitates the energy changes that typically accompany chemical transformations. For example, in the case of combustion, while energy is released, the defining characteristic of redox processes is the actual movement of electrons, not merely the energy changes or the production of heat and light.

The other choices do not encapsulate the essence of a redox reaction. For instance, splitting and forming atoms or separating mixtures are related to different kinds of reactions and processes, but they do not specifically address the transfer of electrons, which is central to redox chemistry.