oxidation and reduction
/ ok-sih-DAY-shun and rih-DUK-shun /
Picture two children trading marbles: one always gives, the other always takes. In chemistry the marbles are electrons. Oxidation is the act of losing electrons; reduction is the act of gaining them. They are the two complementary halves of every electron-transfer reaction, and one can never happen without the other.
The names have a tangled history. Oxidation once meant "combining with oxygen" (as in rusting), and reduction meant a metal ore being "reduced" to the lighter, pure metal. Chemists later realized the deeper common thread was electrons, and redefined both in terms of electron transfer — a far wider net that catches reactions with no oxygen at all. A modern shortcut: track the oxidation state of each atom; if it goes up, that atom is oxidized; if it goes down, it is reduced. The classic mnemonic is OIL RIG — Oxidation Is Loss, Reduction Is Gain.
These two ideas are the foundation of all electrochemistry. Separating a reaction into its oxidation half and its reduction half is what lets us route the electrons through a wire — the whole trick behind every battery. They also describe respiration, combustion, bleaching, and metal-making, making this one of the most far-reaching pairs of concepts in chemistry.
When magnesium burns in air, each magnesium atom loses two electrons (oxidation) and each oxygen atom gains two (reduction). The brilliant white flame is oxidation and reduction happening together.
Burning magnesium: metal oxidized, oxygen reduced — the two always come as a pair.
Watch the wording: a substance that is oxidized causes the other to be reduced, so it acts as the reducing agent. The agent and the change have opposite names — a frequent source of confusion.