Question

In an oxidation-reduction reaction, what substance is reduced always?

Answer 1

The OXIDANT is always reduced in a redox reaction.

Explanation:

Formally, a species that gains electrons is reduced, and a species that loses electrons is oxidized. When oxygen (typically) is reacted with a hydrocarbon fuel, the oxygen is formally reduced to `CO_2`, in which the oxygen number of `O` is `-II`, REDUCED from the zerovalent elemental state. On the other hand, the `C` has been oxidized from `-II` or so to `+IV`.