What characterizes a reduction reaction?

A reduction reaction is characterized by the gain of electrons. In the context of redox (reduction-oxidation) reactions, reduction refers specifically to the process where an atom or molecule acquires electrons and thus decreases its oxidation state.

When a substance gains electrons, it becomes more negative in charge or more reduced, which implies that it has undergone a reduction reaction. This is fundamental in various chemical processes, including those involved in electrochemistry and metabolism.

The other aspects mentioned in the incorrect choices do not accurately describe a reduction reaction. For instance, the loss of electrons represents oxidation, not reduction. The production of hydrogen ions and the generation of heat may occur in certain reactions but are not defining characteristics of reduction itself. Thus, the core defining feature of a reduction reaction is indeed the gain of electrons.