SPI Chemistry · Chemical Reactions

Oxidation, Corrosion and Rancidity

Oxidation is gain of oxygen or loss of hydrogen, reduction is the reverse, and slow oxidation by air rusts iron and spoils oily food.

All Chemical Reactions notes

Why this matters

Two questions in the bank, both EASY, and one is from the 2022 paper: what makes oily food go rancid and how to stop it, and what rust is chemically. Both are the same idea at work, oxidation by the air.

Concept 1 of 3: Oxidation and reduction

Follow the oxygen. The substance that gains oxygen is oxidised; the one that loses it is reduced. The two always happen together in one reaction, so such reactions are called redox reactions.

Definition

Oxidation: gain of oxygen or loss of hydrogen.
Reduction: loss of oxygen or gain of hydrogen.
The oxidising agent gives oxygen (or takes hydrogen); it is itself reduced.
The reducing agent takes oxygen (or gives hydrogen); it is itself oxidised.

Redox in one line

\text{oxidation} = +\,\mathrm{O} \text{ or } -\,\mathrm{H} \qquad \text{reduction} = -\,\mathrm{O} \text{ or } +\,\mathrm{H}

Worked example

\mathrm{CuO + H_2 \rightarrow Cu + H_2O}

, which substance is oxidised and which is reduced?

Practice this conceptself-check · 4 quick reps

Looking for oxidation on its own

Oxidation and reduction always happen together. If one substance gains oxygen, another in the same reaction has lost it.

Saying the oxidising agent is oxidised

The oxidising agent gives away oxygen, so it is reduced. The substance it oxidises is the one that gains oxygen.

Concept 2 of 3: Corrosion and rusting

Metals left in air slowly react with oxygen, water and other gases and are eaten away. For iron this is rusting, and it needs both air and water at the same time.

Definition

Corrosion is the slow eating away of a metal by air, moisture or chemicals around it.
Rust is hydrated iron(III) oxide, $\mathrm{Fe_2O_3 \cdot xH_2O}$ , a reddish brown flaky solid.
Iron rusts only when both air (oxygen) and water are present.
Rusting is prevented by painting, oiling or greasing, galvanising (a coat of zinc), chrome plating and making alloys such as stainless steel.

Metal	Coating that forms	Colour
Iron	Hydrated iron(III) oxide (rust)	Reddish brown Ferric means iron(III); rust carries water, so plain ferric oxide is not rust.
Silver	Silver sulphide	Black
Copper	Basic copper carbonate	Green

Galvanising coats iron with zinc; the zinc corrodes first and protects the iron.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionEasy

Example 2 · Chemical Reactions · Oxidation, Corrosion and Rancidity

Chemically rust is:

Calling rust plain ferric oxide

Rust is hydrated ferric oxide,

\mathrm{Fe_2O_3 \cdot xH_2O}

. Leaving out the water of hydration gives a different compound.

Thinking water alone rusts iron

Iron needs air and water together. In dry air, or in water with no dissolved air, it does not rust.

Concept 3 of 3: Rancidity and how to prevent it

Fried food left open for days smells and tastes bad. The fats and oils in it have been oxidised by the air. Anything that keeps oxygen away, or slows the reaction, keeps the food fresh longer.

Definition

Rancidity: fats and oils in food are oxidised by air, so their smell and taste turn unpleasant.
Antioxidants added to food slow down this oxidation.
Packets of chips are filled with nitrogen, an unreactive gas, so no oxygen is left inside.
Airtight containers, refrigeration and keeping food away from light also slow it down.

Method	How it helps
Adding antioxidants	They slow the oxidation of fats and oils
Filling packets with nitrogen	Nitrogen pushes out the air, so no oxygen reaches the oil Nitrogen, not oxygen: it is chosen because it does not react.
Airtight containers	Less air reaches the food
Refrigeration	Low temperature slows the reaction

Every method either keeps oxygen out or slows the oxidation down.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

SPI · 2022 · SPI Entrance Examination 2022 · Q95Easy

Example 3 · Chemical Reactions · Oxidation, Corrosion and Rancidity

Rancidity of food cooked in oil or ghee is due to____reaction. It can be prevented by using____.

Blaming bacteria for rancidity

Rancidity is the oxidation of fats and oils by air, not the work of bacteria. Antioxidants stop it; antibiotics would not.

Thinking any gas will do in a chips packet

The gas must be unreactive. Nitrogen is used because it does not react with the oil and keeps oxygen out.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Oxidation and reduction
Redox in one line
$\text{oxidation} = +\,\mathrm{O} \text{ or } -\,\mathrm{H} \qquad \text{reduction} = -\,\mathrm{O} \text{ or } +\,\mathrm{H}$

Reference tables (2)

Corrosion and rusting3 rows

Metal	Coating that forms	Colour
Iron	Hydrated iron(III) oxide (rust)	Reddish brown Ferric means iron(III); rust carries water, so plain ferric oxide is not rust.
Silver	Silver sulphide	Black
Copper	Basic copper carbonate	Green

Galvanising coats iron with zinc; the zinc corrodes first and protects the iron.

Rancidity and how to prevent it4 rows

Method	How it helps
Adding antioxidants	They slow the oxidation of fats and oils
Filling packets with nitrogen	Nitrogen pushes out the air, so no oxygen reaches the oil Nitrogen, not oxygen: it is chosen because it does not react.
Airtight containers	Less air reaches the food
Refrigeration	Low temperature slows the reaction

Every method either keeps oxygen out or slows the oxidation down.

Watch out for (6)

Looking for oxidation on its own
→ Oxidation and reduction
Saying the oxidising agent is oxidised
→ Oxidation and reduction
Calling rust plain ferric oxide
→ Corrosion and rusting
Thinking water alone rusts iron
→ Corrosion and rusting
Blaming bacteria for rancidity
→ Rancidity and how to prevent it
Thinking any gas will do in a chips packet
→ Rancidity and how to prevent it

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