Definition · Plain-language
Oxidation and reduction
Oxidation is the loss of electrons by a substance; reduction is the gain of electrons. The two always happen together in a redox reaction.
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Loss and gain of electrons
In modern chemistry, oxidation and reduction are defined by the movement of electrons. Oxidation is the loss of electrons by a substance; reduction is the gain of electrons. The widely used memory aid OIL RIG captures this: "Oxidation Is Loss, Reduction Is Gain". Because electrons cannot simply disappear, the electrons one substance loses are exactly the electrons another substance gains, so oxidation and reduction are two halves of the same event. This is why they are treated together as a redox reaction.
Oxidising and reducing agents
In a redox reaction, the substance that gains the electrons is being reduced, but in doing so it causes the other substance to be oxidised — so it is called the oxidising agent. Conversely, the substance that loses electrons is oxidised and acts as the reducing agent, because it brings about reduction in its partner. It can feel counter-intuitive that the oxidising agent is itself reduced, but it follows directly from the definitions: an agent makes something happen to the other substance, not to itself.
Redox in everyday life
Redox reactions are everywhere. The rusting of iron is the metal being oxidised by oxygen and water over time. Combustion — burning fuel — is a rapid redox reaction that releases energy as the fuel is oxidised. Respiration in living cells oxidises glucose to release energy, and photosynthesis runs redox chemistry in reverse to store it. Batteries generate electricity by separating the oxidation and reduction steps and forcing the electrons to travel through a circuit. The older meaning of oxidation as "reaction with oxygen" survives as a special case of electron loss.
Key facts
At a glance
- Oxidation: loss of electrons by a substance
- Reduction: gain of electrons by a substance
- Together: a redox (reduction–oxidation) reaction
- Memory aid: OIL RIG — Oxidation Is Loss, Reduction Is Gain
- Oxidising agent: gains electrons (is reduced) and oxidises the other substance
- Examples: rusting, combustion, respiration, batteries
Common misconceptions
What people often get wrong
Often heard: Oxidation only means a substance reacting with oxygen.
Actually: Reaction with oxygen is just one example. The general definition of oxidation is the loss of electrons, which can happen with no oxygen involved at all.
Often heard: Oxidation can happen without reduction.
Actually: They always occur together. Electrons lost in oxidation must be gained by something else in reduction, so the two are inseparable halves of a redox reaction.
Often heard: The oxidising agent is the substance that gets oxidised.
Actually: The opposite is true. The oxidising agent gains electrons and is itself reduced; it causes the other substance to be oxidised, which is why it earns that name.
