Oxidation and Reduction (Redox Reaction).

First, Oxidation is opposite of Reduction.

Second, there are three different ways of defining each: In terms of OXYGEN, HYDROGEN & ELECTRONS.
The best, and fullest definition is in terms of electrons.

Third, since one substance can only gain electrons at the expense of a second substance, Oxidation and Reduction always occur together! For this reason such reactions are better described as REDOX reactions rather than either oxidation or reduction.

So!

Reduction is when a substance loses oxygen during a reaction. Oxidation is when a substance gains oxygen during a reaction. The reduction and oxidation take place together in a reaction (always).  This reaction is called a redox reaction.

Here is a word equation for an example:

Copper (ii) oxide + Hydrogen    =    Copper + Water. 

In this reaction the copper  is losing oxygen and so it is being reduced, whereas the hydrogen is gaining oxygen, and so the hydrogen is being oxidised. In this particular experiment the hydrogen is the reducing agent (bringing about reduction by removing oxygen), and the copper is the oxidising agent.

Oxidation also occurs when Hydrogen is removed from a substance. In this reaction: -

C₂H₆   =   C₂H₄  + H₂

_{The substance on the left is being oxidised because hydrogen is being removed. Whereas, in this reaction: -}

H₂  +  Cl₂   =    2HCl

The chlorine is being reduced because it is gaining hydrogen.

 There is, however, another definition for oxidation and reduction, which is: -

A substance is oxidised if it loses electrons during a reaction, and, a substance is reduced if it gains electrons during a reaction.

 A memory aid for this is OIL  RIG! Oxidation is loss. Reduction is gain.

An example of an experiment in which this definition of oxidation and reduction is seen,  is when magnesium burns in oxygen, and magnesium oxide is formed. The equation for this is:

2Mg (s) + O₂ (g)    =   2Mg O(s)

This is a redox reaction as well as an example of combustion. Seeing the magnesium being oxidised is easy but seeing the oxygen being reduced is not so easy.

In this reaction a magnesium atom loses two electrons and an oxygen atom gains them. Therefore the magnesium is oxidised. In giving  electrons to another substance (oxygen), it is acting as the reducing agent. The oxygen is reduced in the process. In taking electron it is acting as the oxidising agent.

Lots of different substances can take electrons and behave as oxidising agents.

Equally, lots of different substances can give away electrons and behave as reducing agents.

(Thanks to Clare Mc Aleese, Loreto College.)