Occurs when the oxidation number of an element decreases in chemical reaction
When the oxidation number of an element decreases in a chemical reaction, this process is known as reduction. Reduction is one half of a redox (reduction-oxidation) reaction and occurs simultaneously with oxidation, where another element's oxidation number increases.
In the context of redox reactions, loss of oxygen, gain of hydrogen, or gain of electrons constitutes reduction for an element. An easy way to remember this is through the acronym OIL RIG, which stands for "Oxidation Is Loss, Reduction Is Gain" – referring to the gain or loss of electrons.
An example of a reduction reaction is when iron(III) oxide reacts with hydrogen gas to produce iron and water:
Fe2O3 + 3H2 → 2Fe + 3H2O
In this reaction, the oxidation number of iron (Fe) decreases from +3 in Fe2O3 to 0 in Fe, indicating that iron is reduced. Simultaneously, the oxidation number of hydrogen increases from 0 in H2 to +1 in H2O, indicating that hydrogen is oxidized. This is a classic redox reaction where reduction and oxidation processes happen simultaneously.