What is Oxidation? — Definition, Process & Examples

Written by

Malcolm McKinsey

January 12, 2023

Edited by

Courtney Adamo

Fact-checked by

Paul Mazzola

Oxidation definition

Oxidation is the loss of electrons of an atom, ion, or atoms in molecules during a chemical reaction. Oxidation is an increase in the oxidation state of an atom. Oxidation and reduction form a redox reaction that is remembered by the acronym OIL RIG (Oxidation Is Loss, Reduction Is Gain).

Oxidation was discovered in 1697 by chemist Georg Ernst Stahl and has evolved in meaning over time. The first two of three following definitions of oxidation are outdated:

1. Oxidation as a gain of oxygen – An oxidizing agent gives oxygen atoms to another substance in a compound.

2. Oxidation as a loss of hydrogen – An oxidizing agent removes hydrogen atoms from another substance in a compound.

3. Oxidation as a loss of electrons – The creation of ions when atoms lose electrons—not just hydrogen and oxygen—and has a mnemonic device to remember, OIL RIG--Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons)

Oxidation vs. reduction

In chemistry, oxidation and reduction reactions are opposites but occur simultaneously in many chemical reactions. When oxidation and reduction occur simultaneously, it is called a redox reaction (reduction/oxidation). During an oxidation-reduction reaction, the oxidation number of atoms changes, and there is a transfer of electrons between two substances.

The following is what happens to oxygen, hydrogen, and electrons during oxidation versus reduction:

• Oxygen transfer – oxidation is a gain of oxygen; reduction is loss of oxygen.

• Hydrogen transfer – oxidation is loss of hydrogen; reduction is a gain of hydrogen.

• Electron transfer – oxidation is loss of electrons; reduction is a gain of electrons (OIL RIG)

Oxidation and reduction reactions

Removing iron from iron ore is a classic example of a redox reaction. At the same time, iron oxide $\left(F{e}_{2}{O}_{3}\right)$ converts to iron $\left(Fe\right)$, carbon monoxide $\left(CO\right)$ converts to carbon dioxide $\left(C{O}_{2}\right)$:

It can be easier to think of what happens in terms of two half-reactions.

Consider this oxidation reaction. The reactant of three carbon monoxide molecules $\left(3CO\right)$ gains oxygen to yield the product of three carbon dioxide molecules $\left(3C{O}_{2}\right)$.

The reactant of one molecule of iron oxide $\left(F{e}_{2}{O}_{3}\right)$ loses oxygen to yield the product of two atoms of iron $\left(2Fe\right)$\left(2Fe\right), a reduction reaction. You probably recognize iron oxide as rust.

An oxidizing agent gives atoms to another substance. A reducing agent removes atoms from another substance. In this example, iron is the oxidizing agent, and carbon monoxide is the reducing agent.

The oxidation process

Oxidation can occur in many substances in many different chain reactions. In general, the three steps of oxidation are:

1. Initiation – a net increase in free radicals (reactive oxygen species or other elemental species) triggered by heat or light

2. Propagation – no gain or loss of free radicals

3. Termination – a net decrease in free radicals

You see and benefit from the oxidation process in everyday and commercial activities:

• Rusting metal that changes iron $\left(Fe\right)$ into iron oxide $\left(F{e}_{2}{O}_{3}\right)$

• Burning fuels

• Digesting food

• Water forming from hydrogen and oxygen gases.

• Cellular functions (catalase and dehydrogenase reactions)

• Industrial oxidation of alcohols to aldehydes or ketones in the production of magnetic nanocatalysts

• Epoxidation (used in organic chemistry)

• Fenton’s peroxidation (used to treat wastewater)

Oxidation examples

Rusting iron is a clear example of an oxidation reaction, but so are thousands of industrial and commercial manufacturing processes and countless biological processes. Most of these are redox reactions, in which oxidation and reduction are unbreakably linked.

Uses of oxidation in daily life

• Bleach is a chemical that uses several redox reactions to brighten and whiten clothes. Removing color from fabric occurs because electrons change energy levels, and the process forms ions.

• Traditional film photography depends on redox reactions. Silver ions in activated silver bromide are reduced to silver atoms using hydroquinone or pyrogallol.

• Extraction of aluminum from bauxite requires electrolysis to force the reduction of $A{l}^{3+}$ ions to neutral ions.

• Oxidizing laundry cleaners lift the stains out of your clothes.

• Hydrogen peroxide disinfects wounds and bleaches household products.

Biological oxidation

You have seen biological oxidation at work if you have ever seen fruit turning brown. Fruit ripening and browning are only one of many biological oxidation examples:

• Cellular respiration – carbon dioxide is reduced; water is oxidized to yield oxygen.

• Photosynthesis – as with cellular respiration, water is oxidized, and carbon dioxide is reduced.

• Decomposition of organic matter – a prolonged, slow-motion redox reaction

Oxidation quiz

Got a grip on oxidation? Test yourself with these questions:

1. In your own words, what are three definitions of oxidation, and which is the most current definition?

2. What is a redox reaction?

3. Give one human-made example of a redox reaction and one biological example.

Review the material or do more research on your own if you struggled with these questions. Check your answers below:

1. You may have said the three definitions of oxidation are a gain of oxygen, a loss of hydrogen, or the loss of electrons, with the loss of electrons being the modern, universal meaning of oxidation.

2. A redox reaction is a combination of oxidation and reduction reactions occurring simultaneously, such as converting iron oxide to pure iron.

3. You might have mentioned that a human-made redox reaction exists in breathalyzer tests used by law enforcement. A biological example of oxidation is the browning of exposed fruit flesh or photosynthesis.