In our day to day activity, we are familiar with the chemical processes like rusting of iron, burning of substances, breathing of air, digestion of food and so on. All such types of processes or reactions are known as oxidation and reduction or redox reactions.

Oxidation

The term oxidation was originally used to describe reactions in which an element combines with oxygen. For example, the reaction between magnesium and oxygen involves the oxidation of magnesium: 2Mg(s) + O₂(g) → 2MgO(s). However, nowadays it has a broader meaning that includes reactions not involving only oxygen. Now, oxidation is the loss of electrons and is identified by an increase in oxidation number. For example, in the reaction

2Na + Cl₂ → 2NaCl

Each sodium atom has lost one electron and has turned to a sodium ion. Hence, sodium is oxidized.

Na → 1e^– + Na⁺

Reduction

The term reduction was originally used to describe the production of a metal from an ore. The term has been modified through time to include other reactions. Now, reduction is the gain of electrons and is identified by a decrease in oxidation number.

For example, in the reaction between sodium and chlorine each chlorine atom has gained an electron and has changed to chloride ion. Thus, chlorine is reduced;

Cl₂ + 2e^– → 2 Cl^–

The processes of oxidation and reduction always occur simultaneously because if one substance loses electrons, the other substance must gain these electrons. Since the process of oxidation and reduction involves the transfer of electrons, it also results in the changes of oxidation number.