Oxidation and reduction are two halves of the same story: whenever one substance loses electrons, another gains them. These paired changes — together called redox reactions — drive the burning of fuels, the spoiling of food, and the slow rusting of iron. These notes move from the meaning of oxidation and reduction, through oxidising and reducing agents, into corrosion and the rusting of iron, its prevention, the corrosion of other metals, and finally everyday oxidation such as combustion and rancidity.
Oxidation can be recognised in three equivalent ways — by what happens to oxygen, to hydrogen, or to electrons.
In terms of electrons, oxidation is the:
During oxidation, the oxidation number:
Oxidation is the gain (addition) of by a substance.
Oxidation is the loss (removal) of from a substance.
Reduction is the exact opposite of oxidation, and can likewise be spotted by oxygen, hydrogen, or electrons.
In terms of electrons, reduction is the:
During reduction, the oxidation number:
Reduction is the loss (removal) of from a substance.
Reduction is the gain (addition) of by a substance.
The simplest way to never confuse the two is the classic mnemonic based on electrons.
In the mnemonic OIL RIG, 'OIL' stands for:
If a species gains electrons, it is:
In OIL RIG, 'RIG' stands for Reduction Is (of electrons).
The other mnemonic is LEO the lion says : Loss of Electrons = Oxidation, Gain of Electrons = Reduction.
The cleanest way to keep the two straight is to set them side by side.
| Feature | Oxidation | Reduction |
|---|---|---|
| Electrons | loss of electrons | gain of electrons |
| Oxygen | gain (addition) of oxygen | loss (removal) of oxygen |
| Hydrogen | loss (removal) of hydrogen | gain (addition) of hydrogen |
| Oxidation number | increases | decreases |
| Mnemonic | OIL — Oxidation Is Loss | RIG — Reduction Is Gain |
With respect to hydrogen, oxidation is the loss of hydrogen while reduction is the:
Which change to the oxidation number indicates oxidation?
With respect to electrons, oxidation is the loss of electrons and reduction is the of electrons.
With respect to oxygen, reduction is the loss (removal) of .
Oxidation and reduction never happen alone — they always occur together in the same reaction.
A reaction in which oxidation and reduction take place at the same time is called a:
In a redox reaction, the electrons lost in oxidation are:
Combustion, respiration, photosynthesis, rusting and are all redox reactions.
In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, zinc is because it loses electrons.
In every redox reaction one substance drives the oxidation and another drives the reduction.
| Agent | What it does | What happens to it |
|---|---|---|
| Oxidising agent | oxidises the other substance | is itself reduced (gains electrons) |
| Reducing agent | reduces the other substance | is itself oxidised (loses electrons) |
An oxidising agent brings about oxidation of another substance and is itself:
Which of these is a common reducing agent?
A reducing agent brings about reduction of another substance and is itself .
An oxidising agent brings about oxidation by accepting from another substance.
When metals are left exposed to their surroundings, they are slowly eaten away — this is corrosion.
Corrosion is best described as the:
The most common and most damaging example of corrosion is:
Corrosion is generally a slow of the metal at its surface.
Corrosion happens because metals react with oxygen, water and in the surrounding air.
The corrosion of iron has a special name — rusting — and it needs two things present together.
| Condition present | Does iron rust? |
|---|---|
| Air (oxygen) only — no water | No |
| Water only — no air (oxygen) | No |
| Both air and water together | Yes — rust forms |
| Both, plus salt / acid | Yes — rusts even faster |
Rusting of iron needs:
The reddish-brown rust formed on iron is:
If only air is present with no moisture, iron does rust.
Salt and acids in the air rusting, which is why rusting is faster near the sea.
Since rusting needs both air and water, most prevention works by keeping at least one of them away from the iron surface.
| Method | How it prevents rusting |
|---|---|
| Painting | forms a barrier of paint over the surface |
| Oiling / greasing | forms an oily film that keeps out air and water |
| Galvanising | coats iron with a protective layer of zinc |
| Electroplating | deposits a protective metal layer by electrolysis |
| Alloying (stainless steel) | mixing with chromium & nickel makes it rust-resistant |
Galvanising protects iron by coating it with a layer of:
Mixing iron with chromium and nickel makes:
A coat of keeps air and moisture off the iron surface to prevent rust.
Electroplating deposits a thin layer of another metal by .
Iron is not the only metal that corrodes — other metals corrode too, but each forms a different coating with a different colour.
| Metal | Coating formed | Colour |
|---|---|---|
| Iron | hydrated iron(III) oxide (rust) | reddish-brown |
| Copper | basic copper carbonate | green |
| Silver | silver sulphide | black |
| Aluminium | aluminium oxide (protective) | dull grey, protective layer |
Copper corrodes to form a coating of which colour?
Silver tarnishes to form a black coating of:
Aluminium forms a thin, tough layer that sticks to the surface and protects the metal underneath.
The green coating that forms on copper is basic copper (verdigris).
The most familiar oxidation of all is burning, in which a substance reacts rapidly with oxygen.
Combustion is a rapid oxidation reaction in which a substance reacts with oxygen and releases:
Respiration is best described as a:
A substance that burns is combustible, also called a .
In the reaction C + O₂ → CO₂, carbon is oxidised to carbon .
When oily and fatty foods are left in air, oxidation spoils them — this is rancidity.
Rancidity is the spoiling of oily and fatty foods when they are:
Chips packets are filled with which gas to slow rancidity?
Rancidity can be slowed by adding to the food.
Rancidity gives the food a bad (unpleasant) smell and .
Take 5 questions at a time — tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.