The concept of oxidation and reduction are the basics of elementary chemistry classes in schools and colleges. When students are introduced to chemical reactions, they are made to understand the concept of oxidation and reduction that help them to interpret the change in chemical equations and the formation of end products or compounds. The word 'redox' is the acronym for reduction-oxidation that occurs in a certain chemical reaction. Put more simply, these reactions are those sets of chemical equations in which the oxidation number of the atoms involved in the chemical reaction changes when the reaction occurs. This definition introduces us to another fundamental concept in chemistry that is of crucial importance, the oxidation number. Let us define it. Oxidation number is exactly similar to the valency of an atom but carries a sign with it. This sign expresses the nature of charge on the corresponding species when it is formed from a neutral atom. Let us understand this through an example. The oxidation number of chlorine in hydrochloric acid (HCl) is -1, in Chloric acid (HClO₃) is +5, and in perchloric acid (HClO₄) it is +7. How do we calculate oxidation number? Well, for calculating oxidation number, one has to consider various oxidation states of all atoms in a molecule and then equate the sum of all of them to the net charge on the molecule. There are various rules for finding oxidation number of an atom in a molecule however, here we will just see how to find oxidation number of Cl in HClO₃. Oxidation number of H = +1 Oxidation number of Cl = Z (let us suppose) Oxidation number of O = -2 Therefore, +1 + Z + 3 (-2) = 0 (since net charge on HClO3 = 0), Hence, +1 + Z + (-6) = 0 This implies, Z = +5 So on a general note, oxidation generally involves any of the following changes: Similarly, reduction is said to occur when any of the following changes occur: Here is one of the simplest examples of these reactions, that will help you to get a better idea of this concept. Reaction: H₂ + F₂ = 2HF Substance oxidized: Hydrogen Substance reduced: Fluorine Oxidation Reaction: H₂ → 2H⁺ + 2e^- Reduction Reaction: F₂ + 2e^- → 2F^- In the first reaction, hydrogen oxidizes by increasing its oxidation number from 0 to +1 while in the second reaction, fluorine is reduced by decreasing its oxidation number from 0 to -1. Eventually, net charge on the molecules formed is zero as number of electrons gained during oxidation are consumed during reduction process. Finally, the equation turns out to be, H₂ → 2H⁺ + 2e_- + F₂ + 2e^- → 2F^- = H² + F² = 2H+ + 2F^- H₂ + F₂ → 2H+ + 2F^- → 2HF In the examples given below, only the complete reactions have been mentioned instead of the half cell reactions. Fe + CuSO₄ + → FeSO₄ + Cu (Fe oxidized, Cu reduced) Oxidation of Fe(II) to Fe(III) by hydrogen peroxide and an acid: 2 Fe₂⁺ + H₂O₂ + 2 H⁺ → 2 Fe³+ + 2 H₂O Glucose oxidation to carbon dioxide: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O Reaction between Potassium Permanganate and Sodium Sulfite: 2 KmnO₄ + 3 Na₂SO₃ + H₂O → 2 MnO₂ + 3 Na₂SO₄ + 2 KOH Reaction of an acid and a base: 3 CuS + 8 HNO₃ → 3 CuSO₄ + 8 NO (g) + 4 H₂O In chemistry and biology, there are innumerable examples in which the process of oxidation and reduction occur. Redox reactions, in fact, play a crucial role in biochemical reactions, industrial processes, and other chemical works. Transfer of cells and glucose oxidation in the body are also classic examples of these type of reactions. Reactions in chemical factories, electrochemical reactions, obtaining metals from their ores, and preparation of certain fertilizers are all examples of redox reactions.