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Although some of the earliest views of oxidation related to a process where oxygen is added, it is more widely defined as when an atom, ion or molecule loses an electron. Conversely, reduction is when an atom, ion or molecule gains an electron. The acronym "OIL RIG" is used to help remember this.:
"Oxidation Is Loss, Reduction Is Gain"
You may first have heard oxidation in relation to say, rusting of iron where we say Iron has been oxidised:
Although the formation of an oxide helps make an intuitive link to iron being oxidised, we can still describe this using "OILRIG"
In the above, iron will have undergone the following reaction:
Hence it has lost electrons - i.e. it has been oxidised. However, oxygen has gained electrons so it has been reduced:
Oxygen does not have to be involved though. In the following:
Magnesium is oxidised and Chlorine is reduced:
Both of the above are referred to a redox reactions (there has been reduction and oxidation)
There are some particular rules to follow when determining if something has been oxidised or not relating to its oxidation state (and explains why acid base reactions are not redox reactions). The following web site helps if you need to go into this detail