Rusting Iron and Rancid Oil Are the Same Chemistry
Sort reactions into combination, decomposition, displacement and double displacement, tell the three kinds of decomposition apart by their energy source, label what is oxidised and reduced, and explain corrosion and rancidity.
What do a rusting gate and a packet of stale snacks have in common?
An iron gate left through a monsoon grows a flaky brown coat. A packet of fried snacks left open for a week tastes and smells wrong. Those look like unrelated problems — one is metal, the other is oil.
They are the same reaction type. In both cases oxygen from the air has combined with the substance, and in both cases the product is something nobody wants. Rusting is called corrosion; the spoiling of fats and oils is called rancidity; and both are oxidation.
That is why this chapter sorts reactions into types. Once you can say this is an oxidation, you already know what will slow it down: keep the oxygen away. Paint and grease do it for the gate; an airtight packet, a refrigerator or an added antioxidant does it for the snacks. One classification, two very different-looking solutions from the same principle.
The chapter recognises four main types by counting what goes in and what comes out, plus one cross-cutting idea — oxidation and reduction — that can apply to reactions of any type.
This page covers the second part of the CBSE Class 10 Science chapter on chemical reactions and equations: the four reaction types, the three kinds of decomposition, redox reactions with their agents, and corrosion and rancidity.
They are the same reaction type. In both cases oxygen from the air has combined with the substance, and in both cases the product is something nobody wants. Rusting is called corrosion; the spoiling of fats and oils is called rancidity; and both are oxidation.
That is why this chapter sorts reactions into types. Once you can say this is an oxidation, you already know what will slow it down: keep the oxygen away. Paint and grease do it for the gate; an airtight packet, a refrigerator or an added antioxidant does it for the snacks. One classification, two very different-looking solutions from the same principle.
The chapter recognises four main types by counting what goes in and what comes out, plus one cross-cutting idea — oxidation and reduction — that can apply to reactions of any type.
This page covers the second part of the CBSE Class 10 Science chapter on chemical reactions and equations: the four reaction types, the three kinds of decomposition, redox reactions with their agents, and corrosion and rancidity.
How do you classify a reaction as combination, decomposition or displacement?
Count the reactants and products, and look at whether elements are joining, splitting or swapping.
Combination — two or more substances make one.
The first of these is the slaking of lime, and it releases so much heat that the container becomes hot — an exothermic combination. Burning is always exothermic too.
Decomposition — one substance breaks into two or more.
Decomposition is the opposite of combination, and it always needs energy supplied — it is endothermic. That is a useful generalisation: if a reaction needs continuous heating, light or electricity to keep going, it is almost certainly a decomposition.
Displacement — a more reactive element pushes a less reactive one out of its compound.
The direction is fixed by reactivity. Iron displaces copper because iron is the more reactive of the two; copper cannot displace iron, and putting a copper wire into iron sulphate solution produces no change at all. A displacement question is really a reactivity question.
Double displacement — two compounds exchange partners.
When one of the products is insoluble, it drops out as a precipitate, and such reactions are also called precipitation reactions. Neutralisation is a double displacement too, with water as one of the products.
The distinction that is tested most often. In a displacement an uncombined element takes the place of another element. In a double displacement both reactants are compounds and the ions simply change partners — no element is free on either side. Look for a lone element among the reactants: if there is one, it is a displacement; if there is none, it is a double displacement.
Combination — two or more substances make one.
The first of these is the slaking of lime, and it releases so much heat that the container becomes hot — an exothermic combination. Burning is always exothermic too.
Decomposition — one substance breaks into two or more.
Decomposition is the opposite of combination, and it always needs energy supplied — it is endothermic. That is a useful generalisation: if a reaction needs continuous heating, light or electricity to keep going, it is almost certainly a decomposition.
Displacement — a more reactive element pushes a less reactive one out of its compound.
The direction is fixed by reactivity. Iron displaces copper because iron is the more reactive of the two; copper cannot displace iron, and putting a copper wire into iron sulphate solution produces no change at all. A displacement question is really a reactivity question.
Double displacement — two compounds exchange partners.
When one of the products is insoluble, it drops out as a precipitate, and such reactions are also called precipitation reactions. Neutralisation is a double displacement too, with water as one of the products.
The distinction that is tested most often. In a displacement an uncombined element takes the place of another element. In a double displacement both reactants are compounds and the ions simply change partners — no element is free on either side. Look for a lone element among the reactants: if there is one, it is a displacement; if there is none, it is a double displacement.
What is the difference between thermal, electrolytic and photolytic decomposition?
All three split one substance into several; they differ only in the form of energy supplied.
Thermal decomposition — energy supplied as heat.
The second is the reaction behind the brown fumes seen when lead nitrate is heated in a test tube. Count the gas produced: formula units of solid give molecules of gas, which is why the fumes appear so suddenly.
Electrolytic decomposition — energy supplied as electricity.
In the electrolysis of water, hydrogen collects at one electrode and oxygen at the other, **in a volume ratio of — which the coefficients predict. Seeing twice as much gas in one tube as in the other is the visible proof that the equation is right.
Photolytic decomposition — energy supplied as light.**
White silver chloride turns grey in sunlight because metallic silver is left behind. The word photolytic says exactly that: photo for light, lysis for splitting.
The common thread, and the exam point. Every one of these needs energy put in, so all three are endothermic. The energy source is named above the arrow and is the only thing that distinguishes the three names. **A question asking what type of decomposition is this is asking which form of energy is being supplied — nothing more.
One boundary case worth knowing. Some decompositions need no external supply at all once started, and a few even release heat. The syllabus examples, though, all require a continuous supply, and that is the useful generalisation: stop the heating, the current or the light, and the reaction stops.** Compare that with the burning of magnesium, which continues on its own once lit — the mark of an exothermic reaction.
Thermal decomposition — energy supplied as heat.
The second is the reaction behind the brown fumes seen when lead nitrate is heated in a test tube. Count the gas produced: formula units of solid give molecules of gas, which is why the fumes appear so suddenly.
Electrolytic decomposition — energy supplied as electricity.
In the electrolysis of water, hydrogen collects at one electrode and oxygen at the other, **in a volume ratio of — which the coefficients predict. Seeing twice as much gas in one tube as in the other is the visible proof that the equation is right.
Photolytic decomposition — energy supplied as light.**
White silver chloride turns grey in sunlight because metallic silver is left behind. The word photolytic says exactly that: photo for light, lysis for splitting.
The common thread, and the exam point. Every one of these needs energy put in, so all three are endothermic. The energy source is named above the arrow and is the only thing that distinguishes the three names. **A question asking what type of decomposition is this is asking which form of energy is being supplied — nothing more.
One boundary case worth knowing. Some decompositions need no external supply at all once started, and a few even release heat. The syllabus examples, though, all require a continuous supply, and that is the useful generalisation: stop the heating, the current or the light, and the reaction stops.** Compare that with the burning of magnesium, which continues on its own once lit — the mark of an exothermic reaction.
How do you identify what is oxidised and what is reduced?
Oxidation is the gain of oxygen or the loss of hydrogen; reduction is the loss of oxygen or the gain of hydrogen. Both always happen together, which is why such reactions are called redox reactions.
Worked example 1. Identify the changes in
- has lost oxygen, so copper oxide is reduced
- has gained oxygen, so hydrogen is oxidised
Now name the agents. The substance that causes oxidation is the oxidising agent, and it is the one that gets reduced:
- Oxidising agent: , because it supplied the oxygen
- Reducing agent: , because it removed the oxygen from the copper oxide
That cross-over is the whole difficulty of this section. The oxidising agent is reduced and the reducing agent is oxidised. Read it as a job description: an oxidising agent does oxidation to something else, and pays for it by being reduced itself.
Worked example 2. In the extraction of zinc,
Zinc oxide loses oxygen, so it is reduced; carbon gains oxygen, so it is oxidised. Carbon is the reducing agent — which is why coke is used in a furnace.
Worked example 3. In
manganese dioxide loses oxygen and is reduced, while hydrogen chloride loses hydrogen and is oxidised to chlorine. ** is the oxidising agent and the reducing agent.
Worked example 4 — a reaction that is two types at once.**
Aluminium is the more reactive metal, so this is a displacement reaction. It is also a redox reaction: iron oxide is reduced and aluminium is oxidised. The classifications are not exclusive — a reaction can be a displacement and a redox at the same time, and a question may ask you to name both.
This one releases so much heat that the iron is produced molten, which is how railway tracks are joined on site.
The electron version, which you meet fully in Class 11. Oxidation is the loss of electrons and reduction is the gain:
That definition covers every case, including reactions with no oxygen and no hydrogen in sight — such as , where sodium is oxidised by losing an electron and chlorine reduced by gaining one. The oxygen definition is a special case of the electron one, and that is why the subject moves to electrons as soon as it can.
Worked example 1. Identify the changes in
- has lost oxygen, so copper oxide is reduced
- has gained oxygen, so hydrogen is oxidised
Now name the agents. The substance that causes oxidation is the oxidising agent, and it is the one that gets reduced:
- Oxidising agent: , because it supplied the oxygen
- Reducing agent: , because it removed the oxygen from the copper oxide
That cross-over is the whole difficulty of this section. The oxidising agent is reduced and the reducing agent is oxidised. Read it as a job description: an oxidising agent does oxidation to something else, and pays for it by being reduced itself.
Worked example 2. In the extraction of zinc,
Zinc oxide loses oxygen, so it is reduced; carbon gains oxygen, so it is oxidised. Carbon is the reducing agent — which is why coke is used in a furnace.
Worked example 3. In
manganese dioxide loses oxygen and is reduced, while hydrogen chloride loses hydrogen and is oxidised to chlorine. ** is the oxidising agent and the reducing agent.
Worked example 4 — a reaction that is two types at once.**
Aluminium is the more reactive metal, so this is a displacement reaction. It is also a redox reaction: iron oxide is reduced and aluminium is oxidised. The classifications are not exclusive — a reaction can be a displacement and a redox at the same time, and a question may ask you to name both.
This one releases so much heat that the iron is produced molten, which is how railway tracks are joined on site.
The electron version, which you meet fully in Class 11. Oxidation is the loss of electrons and reduction is the gain:
That definition covers every case, including reactions with no oxygen and no hydrogen in sight — such as , where sodium is oxidised by losing an electron and chlorine reduced by gaining one. The oxygen definition is a special case of the electron one, and that is why the subject moves to electrons as soon as it can.
Why do iron rust and cooking oil spoil, and how can each be prevented?
Both are oxidation by atmospheric oxygen, and both are prevented by keeping the oxygen out.
Corrosion is the slow attack on a metal surface by air, moisture and other substances around it.
- Iron forms a flaky reddish-brown hydrated oxide — rust — which can be written as
- Silver develops a black coating of silver sulphide from sulphur compounds in the air
- Copper develops a green coating of basic copper carbonate
Rusting needs both air and water. An iron nail sealed in dry air does not rust, and one kept under boiled water free of dissolved air does not rust either. Only the nail exposed to both does — which is why humid coastal air is so much harder on ironwork than dry inland air, and why a cycle chain rusts fastest during the rains.
How rusting is prevented, all by separating the iron from air and moisture:
- Painting, oiling or greasing — a physical barrier, cheap but easily scratched
- Galvanising — coating with a layer of zinc, used for roofing sheets, buckets and pipes
- Chrome plating or tin plating — a bright, hard, non-reactive surface
- Alloying — mixing iron with chromium and nickel to make stainless steel, which does not rust at all
Galvanising is the interesting one. Zinc is more reactive than iron, so it is attacked first and protects the iron underneath even where the coating has been scratched. A tin coating cannot do that — tin is less reactive than iron, so a scratch in a tin-plated surface lets the iron beneath rust faster than it would have unprotected.
Rancidity is the oxidation of the fats and oils in food, which produces the unpleasant smell and taste of stale fried snacks, old biscuits or reused cooking oil.
How rancidity is delayed:
- Antioxidants added to packaged food, which are oxidised in preference to the fat
- Airtight packaging, so there is little oxygen inside to begin with
- Flushing the packet with nitrogen, which is why a bag of fried snacks is puffed up — that gas is nitrogen, not air, and it has no oxygen for the oil to react with
- Refrigeration, since cooling slows every reaction down
The unifying sentence. Corrosion and rancidity are both oxidation, so every prevention method is a way of removing the oxygen, blocking the oxygen, or slowing the reaction down. Naming which of those three a given method uses is the way to answer a suggest a method question with a reason rather than a list.
Corrosion is the slow attack on a metal surface by air, moisture and other substances around it.
- Iron forms a flaky reddish-brown hydrated oxide — rust — which can be written as
- Silver develops a black coating of silver sulphide from sulphur compounds in the air
- Copper develops a green coating of basic copper carbonate
Rusting needs both air and water. An iron nail sealed in dry air does not rust, and one kept under boiled water free of dissolved air does not rust either. Only the nail exposed to both does — which is why humid coastal air is so much harder on ironwork than dry inland air, and why a cycle chain rusts fastest during the rains.
How rusting is prevented, all by separating the iron from air and moisture:
- Painting, oiling or greasing — a physical barrier, cheap but easily scratched
- Galvanising — coating with a layer of zinc, used for roofing sheets, buckets and pipes
- Chrome plating or tin plating — a bright, hard, non-reactive surface
- Alloying — mixing iron with chromium and nickel to make stainless steel, which does not rust at all
Galvanising is the interesting one. Zinc is more reactive than iron, so it is attacked first and protects the iron underneath even where the coating has been scratched. A tin coating cannot do that — tin is less reactive than iron, so a scratch in a tin-plated surface lets the iron beneath rust faster than it would have unprotected.
Rancidity is the oxidation of the fats and oils in food, which produces the unpleasant smell and taste of stale fried snacks, old biscuits or reused cooking oil.
How rancidity is delayed:
- Antioxidants added to packaged food, which are oxidised in preference to the fat
- Airtight packaging, so there is little oxygen inside to begin with
- Flushing the packet with nitrogen, which is why a bag of fried snacks is puffed up — that gas is nitrogen, not air, and it has no oxygen for the oil to react with
- Refrigeration, since cooling slows every reaction down
The unifying sentence. Corrosion and rancidity are both oxidation, so every prevention method is a way of removing the oxygen, blocking the oxygen, or slowing the reaction down. Naming which of those three a given method uses is the way to answer a suggest a method question with a reason rather than a list.
Exam tip
What layout keeps a reaction-types answer complete?
Name the type, give the balanced equation with states, and then justify the naming in one clause. All three are separately creditable, and the justification is what most answers leave out.
- Write the type with its reason: displacement, because the more reactive zinc replaces copper. The reason is the mark
- Look for a free element among the reactants to separate displacement from double displacement
- Name the energy source above the arrow for a decomposition, and use it to say which of the three kinds it is
- For redox, write two lines: X is oxidised because it gains oxygen; Y is reduced because it loses oxygen. Then name the agents on a third line
- Remember the cross-over: the oxidising agent is the substance reduced, and the reducing agent is the substance oxidised
- Say that a reaction can belong to two classes when it does — displacement and redox together, for instance
- For prevention questions, give the method and the mechanism: galvanising, because the zinc coating is more reactive and is attacked first
- **Use or for a precipitate and state its colour when the question asks what you would see
The distinction to state carefully. Rusting needs both** air and moisture, so an answer that says only air or only water is incomplete. And galvanising protects even a scratched surface while tin plating does not — the reactivity order is the whole reason, and quoting it turns a recalled fact into an explained one.
- Write the type with its reason: displacement, because the more reactive zinc replaces copper. The reason is the mark
- Look for a free element among the reactants to separate displacement from double displacement
- Name the energy source above the arrow for a decomposition, and use it to say which of the three kinds it is
- For redox, write two lines: X is oxidised because it gains oxygen; Y is reduced because it loses oxygen. Then name the agents on a third line
- Remember the cross-over: the oxidising agent is the substance reduced, and the reducing agent is the substance oxidised
- Say that a reaction can belong to two classes when it does — displacement and redox together, for instance
- For prevention questions, give the method and the mechanism: galvanising, because the zinc coating is more reactive and is attacked first
- **Use or for a precipitate and state its colour when the question asks what you would see
The distinction to state carefully. Rusting needs both** air and moisture, so an answer that says only air or only water is incomplete. And galvanising protects even a scratched surface while tin plating does not — the reactivity order is the whole reason, and quoting it turns a recalled fact into an explained one.
Did you know
Why is a packet of fried snacks filled with gas?
Open a bag of fried snacks and the first thing you notice is that most of the bag was air. Except that it was not air.
The packet is filled with nitrogen. Nitrogen makes up most of the atmosphere already and is almost unreactive, so it takes up the space inside the bag without doing anything to the food. What has been deliberately excluded is the oxygen, because oxygen is what turns the frying oil rancid.
The reasoning is exactly the reasoning behind painting a gate. Rancidity is oxidation, oxidation needs oxygen, so remove the oxygen and the reaction cannot proceed. Flushing with nitrogen removes the reactant rather than blocking it, which is a cleaner solution than any coating.
And the inflated bag has a second job. It cushions brittle food against crushing in transport. One design choice solves a chemical problem and a mechanical one — which is why almost all packaged fried food is sold this way.
Compare the three strategies against the same reaction:
- Remove the reactant — nitrogen flushing, vacuum packing
- Block the reactant — an airtight seal, or paint and grease on iron
- Slow the reaction — refrigeration, which works on rancidity but is useless against rust at ordinary temperatures
A fourth strategy is more subtle: give the oxygen something else to attack. That is what an antioxidant does — it is oxidised in preference to the fat, so the fat survives while the additive is used up. Galvanising works the same way for iron, with the zinc playing the part of the antioxidant: more reactive, attacked first, sacrificed on purpose.
So a roofing sheet and a biscuit packet are protected by the same idea, arrived at independently in metallurgy and in food science. Recognising that is the real value of classifying reactions — the classification carries the solution with it.
The packet is filled with nitrogen. Nitrogen makes up most of the atmosphere already and is almost unreactive, so it takes up the space inside the bag without doing anything to the food. What has been deliberately excluded is the oxygen, because oxygen is what turns the frying oil rancid.
The reasoning is exactly the reasoning behind painting a gate. Rancidity is oxidation, oxidation needs oxygen, so remove the oxygen and the reaction cannot proceed. Flushing with nitrogen removes the reactant rather than blocking it, which is a cleaner solution than any coating.
And the inflated bag has a second job. It cushions brittle food against crushing in transport. One design choice solves a chemical problem and a mechanical one — which is why almost all packaged fried food is sold this way.
Compare the three strategies against the same reaction:
- Remove the reactant — nitrogen flushing, vacuum packing
- Block the reactant — an airtight seal, or paint and grease on iron
- Slow the reaction — refrigeration, which works on rancidity but is useless against rust at ordinary temperatures
A fourth strategy is more subtle: give the oxygen something else to attack. That is what an antioxidant does — it is oxidised in preference to the fat, so the fat survives while the additive is used up. Galvanising works the same way for iron, with the zinc playing the part of the antioxidant: more reactive, attacked first, sacrificed on purpose.
So a roofing sheet and a biscuit packet are protected by the same idea, arrived at independently in metallurgy and in food science. Recognising that is the real value of classifying reactions — the classification carries the solution with it.
Exam relevance
How do reaction types feed into JEE and NEET Chemistry?
This is foundation work whose redox half becomes one of the largest and most examined topics in Class 11 and 12 Chemistry.
Where it leads. The Class 11 chapter Redox Reactions replaces the oxygen-and-hydrogen definitions with oxidation numbers and the electron-transfer definition, and teaches systematic balancing by the half-reaction method. From there it runs into Class 12 Electrochemistry, where a redox reaction is split between two electrodes and its voltage calculated, and into the p-block and d-block chapters, where oxidising and reducing behaviour is the main property being compared. JEE Main, JEE Advanced and NEET all examine redox heavily.
Where the reactivity idea leads. Displacement reactions are ordered by the reactivity series, which becomes the electrochemical series in Class 12 — the same ordering, now quantified by electrode potentials. The rule a more reactive metal displaces a less reactive one becomes a numerical comparison, and questions ask which of two given reactions is feasible. The galvanising explanation above is the Class 12 idea of sacrificial protection, arriving three years early.
Where corrosion and rancidity go. Corrosion reappears in Electrochemistry as an electrochemical process with anodic and cathodic regions on the same piece of metal. Rancidity and antioxidants appear in Biomolecules and in NEET Biology when lipids and free radicals are discussed.
Question types to expect. At this level: classify, balance, name the agents, suggest a prevention with a reason. In competitive papers: assign oxidation numbers, balance a redox equation in acidic or basic medium, and identify the oxidising agent among four options. Assertion-reason items love the agent cross-over.
The single trap that costs marks. Saying that the oxidising agent is oxidised. It is the most frequently set distractor in this whole area, and it survives into Class 12 where a wrong agent means the wrong electrode. Write the job description out: the oxidising agent oxidises something else and is itself reduced.
A second trap. Treating the four types as mutually exclusive. Many reactions are both a displacement and a redox, and a combination can be a redox too — is both. **A question asking for the type may expect two names.
Board versus competitive emphasis. The CBSE paper marks the equation, the state symbols and the named type with its reason; a competitive paper marks an oxidation number or a balanced half-reaction. The transferable habit is tracking what happened to each species** — gained oxygen, lost hydrogen, lost electrons — rather than memorising which reaction is which.
Where it leads. The Class 11 chapter Redox Reactions replaces the oxygen-and-hydrogen definitions with oxidation numbers and the electron-transfer definition, and teaches systematic balancing by the half-reaction method. From there it runs into Class 12 Electrochemistry, where a redox reaction is split between two electrodes and its voltage calculated, and into the p-block and d-block chapters, where oxidising and reducing behaviour is the main property being compared. JEE Main, JEE Advanced and NEET all examine redox heavily.
Where the reactivity idea leads. Displacement reactions are ordered by the reactivity series, which becomes the electrochemical series in Class 12 — the same ordering, now quantified by electrode potentials. The rule a more reactive metal displaces a less reactive one becomes a numerical comparison, and questions ask which of two given reactions is feasible. The galvanising explanation above is the Class 12 idea of sacrificial protection, arriving three years early.
Where corrosion and rancidity go. Corrosion reappears in Electrochemistry as an electrochemical process with anodic and cathodic regions on the same piece of metal. Rancidity and antioxidants appear in Biomolecules and in NEET Biology when lipids and free radicals are discussed.
Question types to expect. At this level: classify, balance, name the agents, suggest a prevention with a reason. In competitive papers: assign oxidation numbers, balance a redox equation in acidic or basic medium, and identify the oxidising agent among four options. Assertion-reason items love the agent cross-over.
The single trap that costs marks. Saying that the oxidising agent is oxidised. It is the most frequently set distractor in this whole area, and it survives into Class 12 where a wrong agent means the wrong electrode. Write the job description out: the oxidising agent oxidises something else and is itself reduced.
A second trap. Treating the four types as mutually exclusive. Many reactions are both a displacement and a redox, and a combination can be a redox too — is both. **A question asking for the type may expect two names.
Board versus competitive emphasis. The CBSE paper marks the equation, the state symbols and the named type with its reason; a competitive paper marks an oxidation number or a balanced half-reaction. The transferable habit is tracking what happened to each species** — gained oxygen, lost hydrogen, lost electrons — rather than memorising which reaction is which.
Key takeaways
What should you be able to classify before the next chapter?
Four types by counting, three decompositions by energy source, and one cross-cutting idea.
- Combination: two or more substances give one, usually exothermic
- Decomposition: one gives several, always endothermic
- Displacement: a free, more reactive element replaces another from its compound — look for a lone element among the reactants
- Double displacement: two compounds exchange ions, often giving a precipitate
- Thermal, electrolytic and photolytic decomposition differ only in whether the energy is heat, electricity or light
- **Electrolysis of water gives hydrogen and oxygen in a volume ratio, as the coefficients predict
- Oxidation is gain of oxygen, loss of hydrogen, or loss of electrons; reduction is the reverse, and the two always occur together
- The oxidising agent is the substance reduced, and the reducing agent is the substance oxidised
- Corrosion and rancidity are both oxidation by atmospheric oxygen
- Rusting needs air and moisture; prevention works by removing, blocking or slowing the oxygen
- Galvanising protects a scratched surface because zinc is more reactive than iron**; tin plating does not
The sharpest self-test is the thermite equation. Write , then name both of its types and label all four roles — oxidised, reduced, oxidising agent and reducing agent.
- Combination: two or more substances give one, usually exothermic
- Decomposition: one gives several, always endothermic
- Displacement: a free, more reactive element replaces another from its compound — look for a lone element among the reactants
- Double displacement: two compounds exchange ions, often giving a precipitate
- Thermal, electrolytic and photolytic decomposition differ only in whether the energy is heat, electricity or light
- **Electrolysis of water gives hydrogen and oxygen in a volume ratio, as the coefficients predict
- Oxidation is gain of oxygen, loss of hydrogen, or loss of electrons; reduction is the reverse, and the two always occur together
- The oxidising agent is the substance reduced, and the reducing agent is the substance oxidised
- Corrosion and rancidity are both oxidation by atmospheric oxygen
- Rusting needs air and moisture; prevention works by removing, blocking or slowing the oxygen
- Galvanising protects a scratched surface because zinc is more reactive than iron**; tin plating does not
The sharpest self-test is the thermite equation. Write , then name both of its types and label all four roles — oxidised, reduced, oxidising agent and reducing agent.