When a substance is oxidized, it:
A. loses protons.
B. gains protons.
C. acts as a reducing agent.
D. acts as an oxidizing agent.
E. None of the above.
Eliminate choice A and B because electrons not protons are lost or gained in oxidation-reduction reactions. Oxidation is a loss of electrons and these electrons are gained by a second substance (reduction). The oxidized substance causes reduction to occur. In other words, it is the reducing agent.
Hint: Reducing agents cause oxidation (loss of e-); oxidizing agents cause reduction. Reducing diets cause weight to be lost.