A bicycle chain left in the rain rusting red and a log blazing in a fire are the same reaction at different speeds: both are oxidation, a substance combining with oxygen.
The opposite, losing oxygen, as when copper is recovered from copper oxide, is reduction; and since the oxygen one substance loses must be taken up by another, oxidation and reduction always happen together.
The three modes of the diagram above show, in turn, oxidation by gaining oxygen, oxidation by losing electrons, and magnesium and copper oxide being oxidized and reduced at the same moment.
The iron nail below shows rust spreading as oxidation goes on, and every way of preventing rust comes down to keeping water and oxygen away from the iron.
What Is Oxidation?
🔥 Gaining Oxygen Is Oxidation, Losing It Is Reduction
①Rusting is slow oxidation, iron combining gradually with oxygen
②Burning wood is fast oxidation (combustion), carbon and hydrogen combining quickly with oxygen
③A reaction that loses oxygen, like copper oxide turning into copper, is reduction
④In terms of electrons, losing electrons is oxidation and gaining them is reduction
Picturing Oxidation and Reduction
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Oxidation by combining with oxygen, oxidation by losing electrons, and a reaction where oxidation and reduction happen together
Whenever one substance is oxidized, another is reduced
🎯 Common Test Points
①Oxidation is gaining oxygen or losing electrons
②Reduction is losing oxygen or gaining electrons
③Oxidation and reduction always occur together
④Rust (slow oxidation) and burning (fast oxidation) are the same kind of reaction at different speeds
⑤Smelting metals from ores uses reduction
Examples and Unit Review
Example 1
Is iron rusting into iron oxide oxidation or reduction? What about removing oxygen from copper oxide to get copper?
1
A reaction that gains oxygen is oxidation; one that loses oxygen is reduction.
2
Iron gains oxygen to become iron oxide, so it is oxidized; copper oxide loses oxygen to become copper, so it is reduced.
▸ Rusting iron is oxidation; copper oxide turning into copper is reduction
Whatever takes the oxygen from copper oxide (carbon, for example) is oxidized at that very moment.
Example 2
Heating copper oxide with carbon produces copper and carbon dioxide. Do oxidation and reduction happen separately or together here?
1
The oxygen lost by copper oxide does not vanish; it combines with carbon.
2
Copper oxide loses oxygen and is reduced, while carbon gains oxygen and is oxidized to carbon dioxide. Both processes take place in the same reaction.
▸ Together: copper oxide is reduced and carbon is oxidized
As an equation it is 2CuO + C → 2Cu + CO₂, with the same numbers of copper, oxygen and carbon atoms on each side.
Unit-test style
Which statement about oxidation and reduction is correct?
①Losing oxygen is called oxidation
②Oxidation and reduction always happen together
③Gaining electrons is called oxidation
④Painting iron speeds up its oxidation
⑤Combustion is a reduction reaction that loses oxygen
▸ ② Oxidation and reduction always happen together
1
When one side loses oxygen (reduction), the other gains oxygen (oxidation), so oxidation and reduction always happen together.
2
Losing oxygen is reduction (①), gaining electrons is reduction (③), painting blocks oxygen and prevents oxidation (④), and combustion is an oxidation reaction that combines with oxygen (⑤).