What characterizes a redox reaction?

A redox reaction is characterized primarily by the transfer of electrons between species. In these reactions, one substance gets oxidized (loses electrons) while another gets reduced (gains electrons). This transfer of electrons is what facilitates the changes in oxidation states of the reacting species, effectively causing one to become more positive and the other more negative.

For instance, in a simple redox reaction like the reaction between zinc and copper ions, zinc loses electrons and is oxidized, while copper ions gain electrons and are reduced. This electron transfer is fundamental to the reaction mechanism and to the identification of a process as a redox reaction.

The other options describe phenomena that may occur in chemistry but do not specifically characterize redox reactions. The synthesis of new compounds can happen in a variety of reaction types, including but not limited to redox reactions. Similarly, changes in temperature and phase changes of substances are often coincidental or secondary to the primary electron transfer process that defines redox reactions. Thus, the essence of redox reactions is tightly linked to the underlying electron transfer mechanism.