In a redox reaction, the substance that causes another substance to get oxidized, but itself gets reduced, is known as an oxidizing agent, or oxidant. In the same manner, the substance that causes another substance to get reduced, but itself oxidized, is referred to as a reducing agent or reductant.

Oxidizing agents are substances that:

• are reduced (gain electrons)
• contain elements whose oxidation number decreases

Reducing agents are substances that:

• are oxidized (lose electrons)
• contain elements whose oxidation number increases

For example,

(i) Permanganate ion (MnO4–) in acidic solution changes color from purple to colorless.

MnO₄^– → Mn²⁺

(ii) Dichromate in acidic solution changes color from orange to green.

Cr₂O₇^2– → Cr³⁺

Other common oxidizing agents are chlorine, potassium chromate, sodium chlorate and manganese(IV) oxide.

Similarly, certain reducing agents undergo a visible color change with a substance which is easily reduced.

For example,

(i) A moist starch solution changes potassium iodide paper to blue-black to show that iodine is formed, 2I^– → I₂. That is potassium iodide is a reducing agent.

(ii) Hydrogen sulphide bubbled through a solution of an oxidizing agent forms a yellow precipitate, S^2– → S. The oxidizing or reducing ability of substances depend on many factors. Some of these are:

• Electronegativity: Elements with high electronegativity such as F2, O2, N2 and Cl2 are good oxidizing agents. Elements with low electronegativity for example, metallic elements like Na, K, Mg and Al are good reducing agents.
• Oxidation states: In a compound or ion, if one of its elements is in a higher oxidation state, then it is an oxidizing agent. Similarly, if an element of a compound or ion is in a lower oxidation state, then it is a reducing agent.