What is described by an oxidation-reduction (redox) reaction?

An oxidation-reduction (redox) reaction is fundamentally characterized by the transfer of electrons between substances. In these reactions, one species undergoes oxidation, which is the loss of electrons, while another species undergoes reduction, which is the gain of electrons. This simultaneous process of oxidation and reduction ensures that the total charge is conserved and drives the chemical change observed in the reaction.

In this context, oxidation is often associated with an increase in oxidation state, while reduction corresponds to a decrease in oxidation state. This interplay is crucial in various chemical processes, including combustion, respiration, and many types of electrochemical cells, highlighting the significance of electron transfer in transforming reactants into products.

The other options fail to capture the essence of redox reactions, as they either describe processes where no electron transfer occurs or focus on the transfer of protons. Since redox reactions specifically hinge upon the movement of electrons and the corresponding changes in oxidation states, the description provided by the second choice accurately reflects these important features.