IGCSE Chemistry Topic 9: Metals | Tara's Study Guide

9.1 Properties of Metals 9.2 Uses of Metals 9.3 Reactivity & Extraction

Hey Tara! Welcome to Topic 9 — Metals. This is one of the most rewardable topics on the whole syllabus, because almost every question has a predictable shape: state a property, then explain it, then link it to a use. The chemistry underneath is small and beautiful. There are really only three big ideas. One: metals are a lattice of positive ions in a sea of delocalised electrons, and that single picture explains conduction, malleability, high melting points, lustre and alloys. Two: a metal's reactivity is simply how readily its atoms lose electrons to form positive ions — and that one sentence explains reactions with water, acid and oxygen, displacement, extraction method, and sacrificial protection. Three: where a metal sits relative to carbon decides how humans get it out of the ground. Master those three and the rest of Topic 9 is bookkeeping. Three subtopics, sixty practice questions, and a lot of very satisfying explanations. Let's go!

9.1 Properties of Metals ▼

The Big Idea: One Picture Explains Every Property

Roughly 80% of the elements in the Periodic Table are metals, and they all share a family resemblance: shiny, bendable, conducting, high-melting. That is not a coincidence, and it is not a list to be memorised separately. Every one of those properties falls out of a single structural model called metallic bonding.

In a piece of metal, the atoms are packed together in a regular lattice — neat layers stacked on top of one another. Each atom releases its outer-shell electrons into the structure. Those released electrons are no longer attached to any one atom; they are delocalised, free to move anywhere through the whole lump of metal. What is left behind is a regular array of positive metal ions.

Metallic bonding = a lattice of positive ions in a "sea" of delocalised electrons

Positive ions — each metal atom has lost its outer electron(s), so it becomes Na⁺, Mg²⁺, Al³⁺ and so on, sitting at a fixed lattice point. Delocalised electrons — the released outer electrons, free to move through the whole structure. Also called "free electrons" or "mobile electrons". The bond itself — the strong electrostatic attraction between the positive ions and the negatively charged sea of electrons. It acts in all directions.

The standard IGCSE picture of a metal. Notice the two separate things you must name in an answer: a lattice of positive ions and a sea of delocalised electrons. Leave either one out and you lose the mark.

Explaining the Physical Properties

Now watch how the properties drop out of that one diagram. Each explanation is a short causal chain, and the examiner wants the chain, not just the conclusion.

1. Good conductors of electricity

An electric current is a flow of charge. Metals contain delocalised electrons that are free to move through the whole structure, so when a voltage is applied the electrons drift towards the positive terminal and carry charge from one end of the wire to the other. Crucially, the metal does not have to melt first — the electrons move even though the ions stay put. That is exactly why a metal conducts as a solid while an ionic compound does not.

2. Good conductors of heat

Two mechanisms operate, and either earns the mark. The delocalised electrons at the hot end gain kinetic energy, move faster, travel through the metal and transfer that energy by colliding with cooler ions further along. In addition, the tightly packed ions vibrate more strongly and pass the vibration on to their neighbours. The electron mechanism is much the faster of the two, which is why metals conduct heat far better than any non-metal solid.

3. Malleable and ductile

Malleable means it can be hammered or rolled into a sheet without shattering. Ductile means it can be drawn out into a wire. Both come from the same fact: the ions are arranged in regular layers of identical ions, so when a force is applied the layers slide over one another. As they slide, the delocalised electron sea flows with them and the metallic bonding is not broken — the structure simply takes up a new shape. Compare an ionic solid, where sliding a layer brings like charges next to like charges and the crystal cracks.

4. High melting and boiling points

The electrostatic attraction between the positive ions and the sea of delocalised electrons is strong, and there is an enormous number of these attractions throughout a giant lattice. A large amount of energy is therefore needed to overcome them and separate the particles, so most metals melt well above 600 °C — iron at 1538 °C, tungsten at 3422 °C. (Group I metals are the famous exceptions: each atom releases only one electron, so the electron sea is thin and the attraction comparatively weak.)

5. Shiny (lustrous) and dense

The delocalised electrons at the surface reflect light, giving a freshly cut or polished metal its characteristic lustre. And because the ions are packed closely together in a giant lattice with no gaps, most metals have a high density.

Exam Tip

Whenever a question says "explain, in terms of structure and bonding", the words delocalised electrons must appear somewhere in your answer, and for malleability the words layers slide must appear. Cambridge mark schemes for Topic 9 are built around those two phrases. Saying "because it is a metal" or "because the bonds are strong" without naming the particles scores nothing.

Metals Compared with Non-Metals

You met this table in Topic 8; here it matters because it tells you which side of the Periodic Table an unknown element sits on when all you are given is a set of measurements.

Property	Metals	Non-metals
Appearance	Shiny / lustrous when freshly cut	Dull; many are gases
Electrical conductivity	Good, as a solid and as a liquid	Poor (graphite is the exception)
Thermal conductivity	Good	Poor — good insulators
Malleable / ductile?	Yes — layers of ions slide	Brittle when solid; shatter
Melting and boiling points	Usually high	Usually low
Density	Usually high	Usually low
What the atoms do in reactions	Lose electrons → positive ions	Gain or share electrons
Oxide formed	Basic (MgO, CaO) — reacts with acids	Acidic (SO₂, CO₂) — reacts with alkalis
Reaction with dilute acid	Many give a salt + hydrogen	No reaction

Memory Trick

"Metals are Positive people." Metal atoms lose electrons to form Positive ions; their oxides are basic (think b for base and metal). Non-metals gain electrons to form negative ions and their oxides are acidic. One word — "positive" — unlocks the ion charge, the oxide behaviour and the reactivity definition all at once.

Alloys — Mixtures That Beat the Pure Metal

A pure metal is often disappointingly soft. Pure iron bends; pure gold scratches; pure aluminium dents. The fix is almost always the same: make an alloy.

An alloy is a mixture of a metal with one or more other elements

The other element is usually another metal, but it may be a non-metal — steel is iron mixed with carbon. An alloy is a mixture, not a compound: there is no fixed formula, the composition can be varied, and the properties can be tuned by changing the recipe. Alloys are harder and stronger than the pure metals they are made from.

Why alloys are harder — the explanation that earns the marks

This is the single most misquoted explanation in Topic 9, so learn it as a chain of three statements.

(1) In a pure metal, all the ions are the same size, so they pack into perfectly regular layers. When a force is applied, those layers can slide over one another easily — which is why a pure metal is soft and easily bent.

(2) In an alloy, atoms of a different size are mixed in. They distort / disrupt the regular arrangement of the layers.

(3) The layers can therefore no longer slide over each other easily, so the alloy is harder and stronger than the pure metal.

Left: identical ions in tidy layers that slip past each other. Right: the larger orange atoms wedge the layers apart and lock them in place. Nothing about the strength of the metallic bond has changed — only the geometry.

Exam Tip

The two answers that score zero every single session: "an alloy is harder because the atoms are held together more strongly" and "because the added metal is stronger". Neither mentions layers. The examiner is looking for the word layers (or "rows") plus the idea that they cannot slide. Also remember an alloy is a mixture, so it does not have a chemical formula — never write "CuZn" for brass.

The Alloys You Must Know

Alloy	Made from	Key property	Typical use
Brass	Copper + zinc	Hard, gold-coloured, resists corrosion, machines cleanly	Musical instruments, taps, door handles, temple bells, electrical plug pins
Bronze	Copper + tin	Harder than copper, does not corrode in sea water	Statues, ship propellers, bearings, cymbals, coins
Mild steel	Iron + a little carbon (<0.25%)	Strong, still malleable, cheap	Car bodies, girders, nails, railway wagons
Stainless steel	Iron + chromium + nickel	Strong and resists rusting — chromium forms a protective oxide layer	Cutlery, surgical instruments, kitchen sinks, chemical plant, cooking pots
Solder	Tin + lead (or tin + silver in modern lead-free solder)	Low melting point, so it melts before the components do	Joining electrical components on a circuit board, plumbing joints
Duralumin	Aluminium + copper (+ magnesium)	Much stronger than pure aluminium but still low density	Aircraft bodies and frames

Notice that solder breaks the usual pattern. It is not chosen because it is hard — it is chosen because its melting point is lower than that of either pure metal in it. Alloying can change several properties at once, and the exam question will always tell you which one matters.

Supplement

Why stainless steel does not rust — a preview of 9.3

Chromium is more reactive than iron, so it reacts quickly with oxygen in the air to form a thin, tough, impermeable layer of chromium(III) oxide on the surface. That layer is only a few atoms thick, it is transparent, and it seals the metal underneath from water and oxygen — and if it is scratched it immediately re-forms. This is exactly the same trick that protects aluminium (Section 9.2). It is a barrier method that the alloy provides for itself.

Do not confuse this with sacrificial protection, which you will meet in 9.3. Here the chromium is not being eaten away to save the iron; it is forming a coat.

Choosing a Metal: Property → Use

Every "suggest why metal X is used for Y" question is answered with the same two-part sentence: "X is [property], which means it can [do the job]". One clause is not enough; the link must be explicit.

Property	Because…	Used for
Good electrical conductor	delocalised electrons are free to move and carry charge	Wiring, power cables, connectors
Good thermal conductor	mobile electrons transfer kinetic energy quickly	Saucepans, kadhai bases, radiators, heat sinks
Malleable	layers of ions slide over one another	Rolled sheet: car bodies, roofing, foil
Ductile	layers slide, so the metal draws out without snapping	Wires and cables
High melting point	strong attraction between ions and electron sea	Engine parts, furnace linings, lamp filaments (tungsten)
Hard and strong (alloy)	distorted layers cannot slide	Bridges, rails, tools, cutlery
Low density	light ions, so little mass per unit volume	Aircraft, overhead cables, drinks cans
Resists corrosion	protective oxide layer, or low reactivity	Cladding, cookware, plumbing, jewellery

Worked Example 1 Explain, in terms of structure and bonding, why copper is a good conductor of electricity and is also ductile enough to be drawn into a thin wire. [5]

Step 1: Describe the structure first

Copper consists of a giant lattice of positive copper ions arranged in regular layers, surrounded by a sea of delocalised electrons. Every explanation that follows refers back to this sentence, so write it once at the top.

Step 2: Conduction — what moves and what does not

The delocalised electrons are free to move through the whole structure. A current is a flow of charge, so when a potential difference is applied these mobile electrons drift through the wire and carry charge from one end to the other. The ions themselves stay in place.

Step 3: Ductility — what slides

Because the ions are all identical in size and sit in regular layers, a pulling force makes the layers slide over one another. The electron sea flows with them, so the metallic bonding is not broken and the metal stretches rather than snapping.

Step 4: Check the command word

"Explain" means give reasons, so no sentence should end at a statement. "Copper conducts electricity" is a fact; "copper conducts because delocalised electrons are free to move and carry charge" is an explanation.

Copper is a lattice of positive ions in a sea of delocalised electrons [1]. The electrons are free to move and carry charge through the metal, so it conducts electricity [2]. The ions are the same size and lie in regular layers which can slide over one another when a force is applied [1], and the metallic bonding is not broken as they slide, so the copper is drawn into a wire without breaking [1].

Worked Example 2 A cutlery factory in Sheffield adds 18% chromium and 8% nickel to iron. A student writes: "The alloy is harder than iron because chromium atoms form stronger bonds with iron atoms." Explain why this answer scores no marks, and write a correct explanation. [4]

Step 1: Identify the misconception

The student thinks hardness comes from bond strength. It does not. An alloy is a mixture; no new chemical bonds are formed between chromium and iron, and the metallic bonding is the same kind as before. Hardness in an alloy is a structural / geometric effect, not a bonding-strength effect.

Step 2: Start from the pure metal

In pure iron every ion is the same size, so the layers are perfectly regular and slide over each other easily when a force is applied. That is why pure iron is comparatively soft and bends.

Step 3: Add the different-sized atoms

Chromium and nickel atoms are a different size from iron atoms, so they distort the regular arrangement of the layers.

Step 4: State the consequence

The distorted layers cannot slide over one another easily, so more force is needed to deform the metal — the alloy is harder and stronger than pure iron.

The student's answer is wrong because an alloy is a mixture, not a compound, and no stronger bonds are formed [1]. Correct explanation: in pure iron the ions are all the same size and lie in regular layers that slide over one another easily [1]; chromium and nickel atoms are a different size and disrupt the regular layers [1]; so the layers can no longer slide easily and the stainless steel is harder and stronger [1].

Worked Example 3 Four solids W, X, Y and Z were tested. W: shiny, melts at 1085 °C, conducts as a solid, bends when hammered. X: dull yellow, melts at 115 °C, does not conduct, shatters when hammered. Y: melts at 801 °C, does not conduct as a solid but conducts when molten, dissolves in water. Z: shiny, melts at 1450 °C, conducts as a solid, much harder than W and contains two different elements. Identify the type of substance in each case and justify your answers. [8]

Step 1: W — the conductivity-as-a-solid test

Conducting as a solid while still being malleable is the signature of a metal: delocalised electrons carry charge, and layers of identical ions slide. The melting point 1085 °C fits copper.

Step 2: X — low melting point plus brittleness

A low melting point and no conduction point to a simple molecular non-metal with weak intermolecular forces; brittleness rules out a metal. This is sulfur (melting point 115 °C).

Step 3: Y — the classic ionic fingerprint

"Does not conduct as a solid but conducts when molten" is the definition of an ionic compound. Its ions are held in fixed positions in the solid but become free to move in the melt. Soluble in water — sodium chloride, melting point 801 °C.

Step 4: Z — metal properties plus two elements plus extra hardness

Still conducts as a solid and is shiny, so it is metallic; but it contains two elements and is harder than the pure metal, so it is an alloy — the different-sized atoms distort the layers so they cannot slide. Steel fits the data.

W = metal (conducts as a solid, malleable) [2]; X = simple molecular non-metal (low melting point, no conduction, brittle) [2]; Y = ionic compound (conducts only when molten, because ions become mobile) [2]; Z = alloy (metallic properties, two elements, harder because different-sized atoms stop the layers sliding) [2].

Exam Tips for 9.1

1. Always name both halves of the metallic bond. "A lattice of positive ions surrounded by a sea of delocalised electrons." Half a description gets half the marks; "a sea of electrons" without the positive ions is the commonest omission.

2. Conduction = the electrons move and carry charge. Never write "the electrons pass the electricity along" or "the ions carry the current". The ions are fixed in a solid metal.

3. Malleable and ductile both need the word "slide". Layers of ions slide over one another without breaking the metallic bonding. Add that last clause for the extra mark on a 3-mark question.

4. Alloy hardness is about layers, never about bond strength. Different-sized atoms → distorted layers → layers cannot slide → harder. Three arrows, three marks.

5. An alloy is a mixture. No fixed formula, no chemical reaction on mixing, composition can be varied. Do not call brass a compound and do not give it a formula.

6. Learn the compositions exactly. Brass = copper + zinc. Bronze = copper + tin. Stainless steel = iron + chromium + nickel. Solder = tin + lead. Steel = iron + carbon. Swapping zinc and tin is a guaranteed lost mark.

7. Solder is the exception. It is used for its low melting point, not its hardness. If a question mentions circuit boards or plumbing joints, the answer is about melting below the components.

8. Justify a use with the matching property. "Aluminium is used in aircraft because it is a metal" is worth nothing. "Because it has a low density, so the aircraft is lighter and uses less fuel" is worth two.

9. High melting point needs the reason. "Strong electrostatic attraction between the positive ions and the delocalised electrons, so a large amount of energy is needed to overcome it."

10. Watch the metal-versus-ionic trap. Metals conduct as solids; ionic compounds conduct only when molten or aqueous. This one distinction identifies a mystery substance in a single line.

🌎 Apply It: Real-World Chemistry

From a temple bell in Thrissur to a violin string in Vienna, somebody once had to choose between a pure metal and an alloy — and got it right by thinking about sliding layers.

The bell metal used for temple bells in Kerala is about 78% copper and 22% tin — a high-tin bronze. Bell founders discovered centuries ago that a bell cast from pure copper gives a dull thud, while the bronze bell rings for many seconds. Bronze is also much harder, and a bell must survive being struck thousands of times a year.

Explain, in terms of structure, why bronze is harder than pure copper — and why hardness matters for a bell.

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Pure Copper: Layers That Slide

In pure copper every ion is the same size, so the lattice forms perfectly regular layers that slide over one another under an impact. The energy of the strike goes into permanently deforming the metal instead of into vibration — so the bell dents and thuds.

Bronze: Layers That Are Locked

Tin atoms are noticeably larger than copper atoms, so they disrupt the regular arrangement of the layers. The layers can no longer slide easily, so the bronze is harder: it springs back rather than denting, and the strike energy stays as vibration — a long, clear ring.

Chemistry Connection

A whole technological age — the Bronze Age — is named after this one structural trick. Nobody in 3000 BCE knew about delocalised electrons, but they had discovered empirically that mixing a big atom into a lattice of small ones stops the layers slipping. The same idea protects a Chennai railway rail today.

An electronics technician in Bengaluru repairs a circuit board using solder, an alloy of tin and lead that melts at about 183 °C. Pure tin melts at 232 °C and pure lead at 327 °C. The technician deliberately chooses a solder with the lowest possible melting point.

Why is a low melting point the property that matters here, and why can an alloy melt below both of its ingredients?

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Why Low Melting Point Is the Requirement

The solder must become liquid and flow into the joint while the delicate components and the plastic board stay solid and undamaged. A metal melting at 1000 °C would destroy everything around it. Once cooled, the solidified solder holds the joint and, being a metal, conducts the current.

Why the Mixture Melts Lower

The different-sized tin and lead ions disrupt the regular lattice, so the structure is less ordered than either pure metal and the particles are less efficiently packed. Less energy is therefore needed to break the structure down, and the melting point falls. Alloying disturbs the lattice — that raises hardness but can lower the melting point.

Chemistry Connection

This is the counter-example that proves you understand the topic. "Alloys are always harder and higher-melting" is a half-truth: alloying always disrupts the lattice, and disruption blocks sliding (harder) but also spoils the neat packing (often lower-melting). Modern electronics uses lead-free tin-silver-copper solder for the same reason, because lead is toxic.

A hospital in Delhi buys surgical instruments made of stainless steel (iron with 18% chromium and 8% nickel) rather than the cheaper mild steel used for the trolley frames. The instruments are sterilised in superheated steam at 134 °C several times a day.

Justify the choice of stainless steel for the instruments and mild steel for the trolley.

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The Instruments: Corrosion Is the Deciding Property

Repeated exposure to hot water vapour and blood would rust mild steel within days, and a rusty scalpel is both blunt and a source of infection. The chromium in stainless steel reacts with oxygen to form a thin, tough, impermeable layer of chromium oxide that seals the surface and re-forms if scratched. The alloy is also hard enough to take and hold a sharp edge, because the different-sized chromium and nickel atoms stop the layers sliding.

The Trolley: Cost Is the Deciding Property

A trolley frame is painted, kept dry indoors and never cuts anything. It needs to be strong and cheap, and mild steel is both. Chromium and nickel are expensive, so paying for corrosion resistance the trolley does not need would be a waste. Every property must be justified against the actual job.

Chemistry Connection

Engineers do not ask "which metal is best?"; they ask "which properties does this job need, and what is the cheapest material that has them?". Cambridge questions are written the same way, which is why the mark scheme always pairs a property with a use.

A goldsmith in Jaipur will not sell jewellery in 24-carat (pure) gold for everyday wear. She sells 22-carat instead: gold mixed with a little copper and silver. Customers sometimes complain that they are being sold "less gold", and ask why pure gold is not simply better.

Give the goldsmith a scientific answer.

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Pure Gold Is Too Soft

Gold is one of the most malleable metals known — a gram can be beaten into a sheet a square metre in area. Its identical ions form perfectly regular layers that slide extremely easily, so a pure gold ring bends out of shape, scratches on contact with a wall and eventually loses its stones.

The Alloy Fixes It Without Changing the Chemistry

Copper and silver atoms are a different size from gold atoms, so they distort the layers and stop them sliding. The 22-carat alloy is much harder and more hard-wearing, while gold's chemical unreactivity — it is below silver in the reactivity series and does not tarnish — is unaffected, because alloying is a physical mixing, not a reaction.

Chemistry Connection

Carat is simply a composition scale: 24 carat = 24/24 gold, 22 carat = 22/24 = 91.7% gold. Because an alloy is a mixture, the proportions can be varied continuously — 18, 14 and 9 carat all exist. A compound could never do that; it would have a fixed formula.

A cookware company advertises a pan with a copper base, stainless steel body and a plastic handle. A rival sells an all-stainless-steel pan with a steel handle, which is cheaper. A physics-minded customer measures how long each pan takes to bring 1 litre of water to the boil, and how hot the handle gets.

Explain each material choice in terms of structure and bonding.

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Copper Base — Fast, Even Heating

Copper is an excellent thermal conductor: its delocalised electrons gain kinetic energy at the hot flame and travel rapidly through the metal, transferring energy to cooler regions by collision. The base heats quickly and evenly, so food does not burn in one spot.

Stainless Steel Body — Strength Plus Corrosion Resistance

Steel is an alloy, so its distorted layers cannot slide and the pan is hard and does not dent; the chromium provides an oxide layer that stops rusting when acidic foods such as tamarind or tomato are cooked. Its poorer thermal conductivity is why the copper is added to the base.

Plastic Handle — No Delocalised Electrons

Plastic is a simple molecular / polymeric non-metal with no free electrons, so it cannot transfer energy quickly — it is a thermal insulator. The steel-handled rival is cheaper because it is one material, but the handle conducts heat straight to your hand.

Chemistry Connection

One saucepan demonstrates the whole of 9.1: delocalised electrons for conduction, alloying for hardness, an oxide layer for corrosion resistance, and the absence of free electrons for insulation. If you can talk your way round a frying pan, you can answer any "suggest why this material was chosen" question.

Practice Questions: 9.1

20 multiple choice questions. Click an option to check your answer.

Your Score 0 / 20

Question 1

Which description of metallic bonding is correct?

A A lattice of negative ions in a sea of protons

B A lattice of positive ions in a sea of delocalised electrons

C Atoms sharing pairs of electrons in fixed directions

D Alternating positive and negative ions attracting each other

B is the definition. C describes covalent bonding and D describes ionic bonding. Note that the ions are positive because each metal atom has lost its outer electrons into the sea.

Question 2

Why can a solid metal conduct electricity?

A Its ions are free to move through the lattice

B It contains delocalised electrons that are free to move and carry charge

C Its atoms vibrate and pass the current along

D It has a giant covalent structure

In a solid metal the positive ions are fixed in the lattice — only the delocalised electrons move. A is the classic confusion with molten ionic compounds, where it is the ions that move.

Question 3

Which statement best explains why metals are malleable?

A The metallic bonds are weak, so they break easily

B Layers of identical ions can slide over one another without breaking the bonding

C The delocalised electrons repel each other

D The ions are far apart, leaving space to move into

The layers slide and the electron sea flows with them, so the metallic bonding is maintained throughout. Metallic bonding is strong, which is why melting points are high — A contradicts that.

Question 4

An alloy is best described as

A a compound of two metals with a fixed formula

B a mixture of a metal with one or more other elements

C a metal that has been heated and cooled quickly

D a metal coated with a thin layer of a second metal

An alloy is a mixture, so the composition can be varied and there is no formula. D describes electroplating or galvanising, not alloying.

Question 5

Why is brass harder than pure copper?

A Zinc forms stronger bonds with copper than copper does with itself

B Zinc atoms remove the delocalised electrons

C Different-sized zinc atoms distort the layers so they cannot slide easily

D Zinc has a higher melting point than copper

Hardness in an alloy is geometric. No new bonds form (it is a mixture), the electron sea is still there, and zinc actually melts lower than copper. Only C mentions layers.

Question 6

Brass is an alloy of copper and

A tin

B zinc

C carbon

D chromium

Brass = copper + zinc. Bronze = copper + tin. Remember "brass has zinc" versus "bronze has tin" — the letters are deliberately misleading, so learn the pair together.

Question 7

Stainless steel contains iron together with

A copper and zinc

B tin and lead

C chromium and nickel

D aluminium and magnesium

Chromium is the essential ingredient — it forms the protective oxide layer that stops rusting. Nickel adds strength and toughness.

Question 8

Solder is chosen for joining electrical components mainly because it has

A a very high melting point

B a low melting point and good electrical conductivity

C a very low density

D the greatest hardness of any alloy

Solder must melt at a temperature that will not destroy the components (about 183 °C), then solidify into a joint that still conducts. It is the standard exception to "alloys are hard".

Question 9

Which property is not typical of a metal?

A High density

B Good conductor of heat

C Forms an acidic oxide

D Malleable

Metal oxides are basic — they react with acids to give a salt and water. Acidic oxides such as SO₂ and CO₂ come from non-metals.

Question 10

Why do most metals have high melting points?

A The covalent bonds between the atoms are very strong

B There is strong electrostatic attraction between the positive ions and the delocalised electrons

C The atoms are very heavy

D The intermolecular forces between metal molecules are strong

Metals do not contain molecules or covalent bonds. The energy goes into overcoming the many strong ion–electron attractions in a giant lattice.

Question 11

A solid conducts electricity when solid, is shiny and bends when hammered. Its structure is

A giant ionic

B simple molecular

C giant metallic

D giant covalent

Conducting as a solid plus malleability is uniquely metallic. Giant ionic conducts only when molten or aqueous; giant covalent (except graphite) does not conduct and is brittle.

Question 12

Which pair of statements about a pure metal and its alloy is correct?

A The pure metal is harder; the alloy has a fixed formula

B The alloy is harder; its composition can be varied

C Both are compounds, but the alloy contains more elements

D The alloy always melts at a higher temperature than the pure metal

Alloys are mixtures, so composition is variable — that is exactly why 22-carat and 18-carat gold both exist. D is false: solder melts below both tin and lead.

Question 13

Why do metals conduct heat well?

A Their molecules move apart when heated

B Delocalised electrons gain kinetic energy and transfer it through the metal

C Positive ions travel from the hot end to the cold end

D Metals absorb infrared radiation particularly well

The mobile electrons are the fast route for energy transfer; the vibrating ions contribute too, but they stay at their lattice sites. C wrongly has the ions travelling.

Question 14

Bronze is used for ship propellers rather than pure copper because bronze is

A a better conductor of electricity

B harder and more resistant to corrosion in sea water

C less dense, so the ship floats better

D magnetic, so it can be steered

A propeller needs to survive constant mechanical stress and salt water. Alloying with tin blocks the sliding layers (hardness) and improves corrosion resistance.

Question 15

Steel is an alloy of iron with

A oxygen

B carbon

C silicon only

D hydrogen

Carbon is a non-metal, which proves that alloys need not be metal + metal. Mild steel has under 0.25% carbon; too much carbon makes the steel brittle.

Question 16

Which observation shows that a delocalised electron model is needed, rather than an ionic model, for sodium metal?

A It has a shiny surface when freshly cut

B It conducts electricity in the solid state

C It is less dense than most metals

D It reacts vigorously with water

Solid-state conduction requires charged particles that are mobile while the solid stays solid. In an ionic solid the ions are locked in place, so no conduction occurs — only free electrons can explain it.

Question 17

Aluminium foil can be rolled to a thickness of 0.01 mm without tearing. This is because

A aluminium has a low density

B aluminium is malleable — its layers of ions slide over one another

C aluminium has a protective oxide layer

D aluminium conducts heat well

All four statements about aluminium are true, but only B is the property that allows rolling into a sheet. Match the property to the job described, not to the metal in general.

Question 18

A student says: "An alloy is harder because the extra atoms fill the gaps, so the metal is more tightly packed." The main flaw is that

A metals have no gaps at all

B hardness comes from disrupting the layers so they cannot slide, not from tighter packing

C alloys are always less dense than pure metals

D the added atoms are always smaller

The added atoms are a different size — larger or smaller — and either way they distort the regular layers. Distortion, not density, is what stops the sliding.

Question 19

Which statement about the ions in a solid metal is correct?

A They are negatively charged and move towards the positive terminal

B They are positively charged and vibrate about fixed positions

C They are neutral atoms surrounded by their own electrons

D They are free to move, which is why metals are malleable

The ions are positive (electrons have been released) and are fixed at lattice points, vibrating more as temperature rises. Malleability comes from whole layers sliding, not from individual ions wandering.

Question 20

Duralumin (aluminium with copper and magnesium) is used for aircraft frames rather than pure aluminium because it

A conducts electricity better

B is denser, giving the aircraft stability

C is much stronger while remaining low in density

D is completely unreactive with oxygen

Pure aluminium is too soft for a structural frame. Alloying distorts the layers and gives strength, while the density stays low because the alloy is mostly aluminium — the whole point for an aircraft.

9.2 Uses of Metals ▼

The Rule of This Section: Every Use Must Be Justified

Section 9.2 is short on new facts and long on marks, because Cambridge asks the same style of question over and over: "Explain why aluminium is used for overhead power cables", "Suggest why copper rather than steel is used for water pipes". The answer is never the name of a property on its own. It is always a property joined to a consequence.

Property → because of structure → therefore suitable for the job

Weak answer (1 mark at best): "Aluminium is used for cables because it conducts electricity." Strong answer (full marks): "Aluminium has delocalised electrons that are free to move, so it is a good conductor; and it has a low density, so the cable is light enough for the pylons to support over a long span." If the question gives you a number of marks, count your property–consequence pairs. Two marks usually means two different properties, not one property explained twice.

Aluminium

Aluminium is the most abundant metal in the Earth's crust, and after iron it is the most widely used. Its selling point is an unusual combination: it is very light, yet it does not corrode away.

Use	Property that matters	Why that property does the job
Aircraft bodies and frames (as an alloy)	Low density; strong when alloyed	Less mass to lift, so less fuel is burned and a bigger payload can be carried. Alloying with copper and magnesium supplies the strength that pure aluminium lacks.
Overhead power cables	Low density + good electrical conductivity + ductile + corrosion resistant	Copper conducts better, but an aluminium cable of the same conductance is far lighter, so pylons can be further apart and cheaper. It is drawn into long wires and does not corrode in rain. (A steel core is added for tensile strength.)
Food containers, foil, drinks cans, cooking pots	Resists corrosion; malleable; non-toxic; good thermal conductor	The oxide layer keeps the metal out of the food and stops the container being eaten away. Malleability allows rolling into foil and pressing into cans; heat conduction cooks food evenly.
Window frames, cladding, ladders	Low density + corrosion resistance	Light enough to lift and fit, and never needs painting to survive the weather.

The oxide layer: why a reactive metal behaves as an unreactive one

Here is the single most misunderstood fact in Topic 9. Aluminium is above zinc and iron in the reactivity series — it is a genuinely reactive metal. It has to be extracted by electrolysis precisely because carbon is not reactive enough to displace it. Yet an aluminium window frame lasts fifty years in monsoon rain.

The resolution: aluminium reacts immediately with oxygen in the air to form a thin, strong, impermeable layer of aluminium oxide (Al₂O₃) that is firmly bonded to the surface. The layer is only about 10 nm thick and transparent, so the metal still looks shiny — but water and oxygen cannot get through it to reach the metal underneath. If it is scratched, it re-forms instantly.

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

Aluminium is reactive, but appears unreactive because it is protected by its oxide layer. The layer is impermeable and self-repairing — a scratch is sealed within moments. Anodising deliberately thickens this layer by electrolysis, making the protection even better (and allowing dye to be absorbed for coloured finishes).

The oxide layer is why a reactive metal survives outdoors. It is a barrier — the same principle as paint or grease, except the metal makes it itself.

Exam Tip

If a question asks "why does aluminium appear to be unreactive?" the mark scheme wants the words oxide layer, impermeable / prevents water and oxygen reaching the metal, and ideally bonded to the surface. Writing "aluminium is unreactive" contradicts the question stem and contradicts the fact that it is extracted by electrolysis. Examiners specifically look for candidates who can hold both ideas at once.

Copper

Copper was the first metal humans used in quantity, because it can be found native (as the element) and is easy to extract from its ore. It is still irreplaceable in two areas: carrying electricity and carrying water.

Use	Property that matters	Why that property does the job
Electrical wiring in buildings and appliances	Excellent electrical conductor; very ductile	Delocalised electrons move freely and carry charge with very little energy wasted as heat. Being ductile, it can be drawn into thin, flexible wires and bent round corners without snapping.
Water pipes and plumbing	Unreactive (below hydrogen in the reactivity series); malleable; does not corrode in water	It does not react with water or with the dilute acids in tap water, so it does not contaminate drinking water and the pipe does not weaken. Malleability allows it to be bent round obstacles instead of being joined at every corner.
Cooking pans and heat exchangers	Best common thermal conductor after silver	Mobile electrons transfer energy rapidly, so the base heats quickly and evenly and food does not scorch in one spot.
Roofing on old buildings	Corrosion resistance; malleable	Forms a green patina of basic copper carbonate that then protects the metal underneath — and looks deliberate.

Supplement

Aluminium versus copper for cables — a genuine engineering trade-off

Copper is the better conductor per unit volume. But conductors are sold and hung by mass, and aluminium's density (2.7 g/cm³) is less than a third of copper's (8.9 g/cm³). An aluminium cable of the same electrical resistance as a copper one is thicker but roughly half the mass. For a cable hanging 300 m between pylons on the Korba–Raipur transmission line, mass is what decides how many pylons you must build, so aluminium wins outdoors.

Inside a house the calculation reverses. Space in a conduit is tight, joints must be reliable, and mass is irrelevant — so copper wins indoors. Cambridge likes this comparison because it rewards candidates who can weigh two properties against each other rather than reciting one.

Other Uses Worth Knowing

Metal	Use	Property justifying it
Iron / mild steel	Girders, bridges, car bodies, rails	Strong and cheap; alloying with carbon distorts the layers so it does not bend under load
Stainless steel	Cutlery, sinks, chemical plant	Hard and corrosion-resistant, thanks to the chromium oxide layer
Zinc	Galvanising steel; roofing sheet	More reactive than iron, so it also protects sacrificially (see 9.3)
Tungsten	Light bulb filaments, cutting tools	Highest melting point of any metal (3422 °C) — it glows without melting
Gold / silver	Jewellery, electrical contacts	Very unreactive, so they do not tarnish; silver is the best conductor of all
Lead	Radiation shielding, roof flashing	Very dense and extremely malleable
Titanium	Hip joints, aircraft engines	Low density, very strong, unreactive with body fluids because of an oxide layer

Memory Trick

"Aluminium = Aircraft, Aerials, Aluminium foil" — all three start with the same sound and all three are chosen for low density plus corrosion resistance. And for copper: "Copper = Cables, Cooking, Coppers (pipes)" — conductivity of electricity, conductivity of heat, and being unreactive with water. Two metals, six uses, three properties each.

Worked Example 1 Overhead electricity cables carrying power from the Korba thermal power station are made of aluminium wound around a steel core, not of copper. Give three reasons for the choice of aluminium, and suggest why a steel core is included. [5]

Step 1: The property that beats copper

Aluminium has a much lower density than copper (2.7 against 8.9 g/cm³), so for the same electrical performance the cable is far lighter. A lighter cable sags less, needs fewer and cheaper pylons, and can span a longer distance.

Step 2: The property it must still have

Aluminium is a good conductor of electricity, because its delocalised electrons are free to move and carry charge. It is not quite as good as copper, but it is more than good enough.

Step 3: The property that keeps it working outdoors

Aluminium is ductile, so it can be drawn into long wires, and it resists corrosion because of its protective oxide layer — important for a cable exposed to monsoon rain for decades with no maintenance.

Step 4: Why the steel core

Aluminium is not very strong in tension — a long span would stretch and snap under its own weight and under wind loading. Steel is much stronger, so the core bears the mechanical load while the aluminium carries the current. Each material does the job it is best at.

Aluminium: low density so the cable is light and needs fewer pylons [1]; good electrical conductor because of delocalised electrons free to move [1]; ductile so it can be drawn into wire, and corrosion resistant because of its oxide layer [1]. The steel core provides the tensile strength [1] that aluminium lacks, preventing the cable stretching or breaking over a long span [1].

Worked Example 2 A student writes: "Aluminium is used for saucepans and window frames because it is an unreactive metal." Explain what is wrong with this statement and give the correct explanation. [4]

Step 1: Establish the true reactivity

Aluminium is reactive — it sits above zinc, iron and carbon in the reactivity series. The proof is in the extraction: aluminium must be obtained by electrolysis because carbon is not reactive enough to reduce its oxide. An unreactive metal would never need that.

Step 2: Explain the apparent contradiction

Aluminium reacts very quickly with oxygen in the air to form a layer of aluminium oxide, Al₂O₃, on its surface: 4Al + 3O₂ → 2Al₂O₃.

Step 3: Explain why that stops further attack

The layer is thin, strong, impermeable and firmly bonded to the metal, so water and oxygen cannot reach the aluminium underneath and no further reaction takes place. If the layer is scratched it re-forms immediately, so the protection is self-repairing.

Step 4: Answer the actual question

So the correct justification is "aluminium resists corrosion because of its protective oxide layer", combined with low density (light frames and pans) and good thermal conductivity for the pan.

The statement is wrong because aluminium is a reactive metal, above zinc and iron in the reactivity series, which is why it must be extracted by electrolysis [1]. It appears unreactive because it reacts at once with oxygen to form a thin, impermeable, strongly bonded layer of aluminium oxide [1] which prevents water and oxygen reaching the metal beneath and re-forms if scratched [1]. The correct reason for the uses is therefore corrosion resistance due to the oxide layer, together with low density [1].

Worked Example 3 A plumber in Manchester replaces a corroded galvanised steel pipe with a copper one, but uses plastic pipe for the outdoor garden supply. Justify the use of copper indoors and explain, using two different properties, why copper is preferred to iron for water pipes. [5]

Step 1: Reactivity — the decisive property

Copper is below hydrogen in the reactivity series, so it does not react with water or with the weak acids dissolved in tap water. Iron is above hydrogen and rusts when it meets water and oxygen, which weakens the pipe and stains the water brown.

Step 2: Malleability — the practical property

Copper is malleable (its layers of identical ions slide), so a pipe can be bent round corners and under floors without cracking, and soldered joints are easy to make. Fewer joints means fewer leaks.

Step 3: Health and taste

Because copper does not react with water, no dissolved metal compounds contaminate the supply. Rusty water from an iron pipe is not only unpleasant but also blocks the pipe as the flaky hydrated iron(III) oxide builds up.

Step 4: Why plastic outdoors

Plastic is even cheaper, cannot corrode at all, and tolerates freezing better because it flexes. This is a reminder that the "best" material depends on the job — the exam answer must always be tied to the specific use described.

Copper is used because it is unreactive — below hydrogen in the reactivity series, so it does not react with water or dilute acids and does not corrode or contaminate the water [2]. It is also malleable, so pipes can be bent to shape without breaking and joined easily [1]. Iron is unsuitable because it is above hydrogen and rusts in the presence of water and oxygen [1], which weakens the pipe and discolours the water [1].

Exam Tips for 9.2

1. Never say "aluminium is unreactive". Say "aluminium resists corrosion because of its protective oxide layer". The examiner is testing whether you know the difference between being unreactive and being protected.

2. Pair each property with a consequence. "Low density" earns a mark only when followed by "so the aircraft is lighter and uses less fuel" or "so the cable needs fewer pylons".

3. Count the marks and give that many properties. A 3-mark "explain the choice" question wants three different properties, not one property described three ways.

4. Copper conducts better; aluminium is lighter. That is the whole indoor-versus-overhead argument. Do not claim aluminium conducts better than copper — it does not.

5. Ductile is for wires, malleable is for sheets. Ductile = drawn into a wire. Malleable = hammered or rolled into a sheet. Using the wrong word in the wrong context loses the mark.

6. Link plumbing to the reactivity series. Copper is below hydrogen, so it does not react with water or dilute acid. That phrase converts a vague "it does not rust" into a proper chemical reason.

7. Non-toxic and does not affect the taste are legitimate marking points for food containers and water pipes — but only alongside a chemical property, never instead of one.

8. Watch out for the "suggest" command word. It means the answer is not in the textbook, so apply the property table to the new situation. The marks are for the reasoning, not for recall.

9. Alloys appear in 9.2 too. Aircraft bodies are aluminium alloy, not pure aluminium, because the pure metal is too soft. Say so — it is often worth a mark.

10. Recycling arguments earn credit. Recycling aluminium uses about 5% of the electricity needed to extract it from bauxite, because there is no electrolysis to run. That single number answers most "evaluate" questions on aluminium.

🌎 Apply It: Real-World Chemistry

Every one of these decisions was made by an engineer holding a table of properties and a budget — which is exactly what an IGCSE question asks you to do.

The Delhi Metro runs on an overhead line of aluminium alloy, while the wiring inside each carriage is copper. A student asks why the engineers did not simply use the better conductor, copper, for both.

Explain the two different decisions.

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Overhead: Mass Is the Constraint

The line hangs unsupported over long spans. Copper is more than three times as dense as aluminium, so a copper line of equal performance would be far heavier, sag more, and need many extra support masts. Aluminium's low density outweighs its slightly poorer conductivity, and its oxide layer means it survives outdoors without maintenance.

Inside: Space and Reliability Are the Constraints

Inside a carriage, mass hardly matters but space does. Copper's higher conductivity means a thinner wire carries the same current, so more circuits fit in a conduit. Copper is also more ductile for tight bends, and it does not form an insulating oxide film at connections the way aluminium can.

Chemistry Connection

Aluminium's oxide layer is a blessing outdoors and a nuisance at a screw terminal, because Al₂O₃ is an insulator. Loose aluminium house wiring caused a wave of fires in 1970s America for exactly this reason. The same chemistry can be an advantage or a hazard depending on the job.

An Indian food company packs ready meals in aluminium trays and sells mango pulp in aluminium-lined cartons. A rival tried plain steel trays and found that acidic tomato-based dishes attacked them within weeks, while the aluminium trays were unaffected.

Explain the difference, given that aluminium is the more reactive of the two metals.

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The Apparent Paradox

In the reactivity series aluminium is above iron, so on paper the aluminium tray should corrode faster. Reactivity alone predicts the wrong answer — you must think about what the corrosion product does.

The Corrosion Products Behave Completely Differently

Aluminium oxide forms as a thin, impermeable layer that sticks to the surface, sealing the metal beneath. Rust (hydrated iron(III) oxide) is flaky and porous: it falls off and exposes fresh iron, so corrosion continues until the tray is eaten through. One metal armours itself; the other peels.

Plus Two Practical Properties

Aluminium is malleable, so trays are pressed from sheet in one operation, and it is a good thermal conductor, so the meal heats evenly in an oven. It is also non-toxic in this form and does not affect the taste.

Chemistry Connection

This is why "which metal corrodes faster?" is not answered by the reactivity series alone. Chromium, aluminium, titanium and zinc are all reactive metals that form adherent oxide layers; iron is unusual in forming one that does not stick. Very acidic foods can still dissolve the aluminium oxide layer, which is why long-term storage cans are lacquered inside.

Zambia's Copperbelt produces around 800,000 tonnes of copper a year, and roughly 60% of the world's copper goes into electrical applications. Copper prices tripled between 2003 and 2023, prompting engineers to ask whether anything cheaper could take its place in domestic wiring.

Why has copper proved so hard to replace inside buildings?

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Conductivity Sets a Hard Floor

Only silver conducts better, and silver is far too expensive. Copper's delocalised electrons move very freely, so little energy is wasted heating the wire — important both for efficiency and for fire safety in a wall cavity.

Ductility and Joint Reliability

Copper is exceptionally ductile, so it is drawn into fine flexible wire and bent repeatedly without work-hardening and snapping. It also solders easily and does not build an insulating oxide film at screw terminals, so joints stay reliable for decades.

The Recycling Answer

Because copper is unreactive, scrap copper is still metallic copper, not a heap of corroded oxide. It can be melted and re-drawn almost indefinitely with no loss of quality, which softens the price problem and reduces the amount of ore that must be mined.

Chemistry Connection

Copper's low reactivity is doing three jobs at once here: it is why copper survives in pipes, why joints stay clean, and why recycling works so well. In Topic 9 terms, being low in the reactivity series is itself a valuable engineering property.

Recycling one tonne of aluminium cans uses about 5% of the electrical energy needed to extract one tonne of aluminium from bauxite, and saves roughly four tonnes of bauxite. India's smelters at Korba and Angul consume so much electricity that they are built next to their own power stations.

Explain, in chemical terms, why aluminium recycling saves so much more energy than steel recycling does.

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Where the Energy Goes in the First Place

Aluminium is above carbon in the reactivity series, so its oxide cannot be reduced by carbon. It must be extracted by electrolysis of molten aluminium oxide, which needs both a very high temperature and an enormous continuous electric current to force the reduction.

Recycling Skips the Chemistry Entirely

A used can is already aluminium metal. Recycling it needs only enough energy to melt it (660 °C) — a physical change. No electrolysis, no reduction, no oxide to break apart. Hence roughly a twentyfold energy saving.

Why Steel Saves Less

Iron is below carbon, so extracting it in a blast furnace is much cheaper in energy terms to begin with. The gap between "extract" and "recycle" is therefore far smaller — though steel recycling still saves energy, ore and land.

Chemistry Connection

The whole economics of recycling is written into the reactivity series. The higher a metal sits above carbon, the more energy its extraction demands, and the bigger the prize for recycling it. That is the link between 9.2 and 9.3 — and a favourite "evaluate" question.

A surgeon in Chennai fits a titanium hip joint. Titanium is above zinc in the reactivity series and burns brightly in oxygen when powdered, yet an implant sits in warm salty body fluid for thirty years without corroding, and the bone grows against it without rejection.

Explain the apparent contradiction and identify the properties that make titanium suitable.

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Same Trick as Aluminium

Titanium is genuinely reactive, but it forms an extremely tough, impermeable and self-repairing layer of titanium oxide the instant it meets air. Body fluid and dissolved oxygen cannot reach the metal beneath, so no corrosion occurs and no metal ions leak into the patient.

The Mechanical Properties

A hip joint carries several times body weight with every step, so the metal must be strong and must not fatigue. Titanium alloys are as strong as steel at roughly half the density, which also means the patient is not carrying unnecessary mass.

Why Not Stainless Steel or Aluminium?

Stainless steel is used for temporary plates but is denser and can slowly release nickel ions, which some patients react to. Aluminium is far too soft for a load-bearing joint. Titanium is the one metal that has all three: corrosion resistance, strength and low density.

Chemistry Connection

Powdered titanium burns fiercely because grinding exposes a huge surface area of fresh metal faster than the oxide layer can seal it. Same element, same reactivity — but change the surface area and the protection fails. Structure decides behaviour, every time.

Practice Questions: 9.2

20 multiple choice questions. Click an option to check your answer.

Your Score 0 / 20

Question 1

Which property of aluminium is the main reason it is used for aircraft bodies?

A It conducts electricity

B It has a low density

C It has a very high melting point

D It is the most abundant metal in the crust

Low density means less mass to lift, so less fuel is burned. Abundance affects price, not suitability — and aircraft use an aluminium alloy for the strength that pure aluminium lacks.

Question 2

Aluminium does not corrode away in the rain because

A it is below hydrogen in the reactivity series

B it does not react with oxygen at all

C it forms a thin impermeable layer of aluminium oxide that protects the metal beneath

D rainwater is not acidic enough to attack it

Aluminium is a reactive metal above zinc and iron; it reacts with oxygen immediately, and the product is what saves it. Answers A and B contradict its position in the reactivity series.

Question 3

Which pair of properties best explains the use of copper for electrical wiring?

A Low density and high melting point

B Good electrical conductivity and ductility

C Hardness and magnetism

D High reactivity and low cost

Conductivity does the electrical job; ductility allows the metal to be drawn into thin, flexible wire. Copper is denser than aluminium, is not magnetic, and is one of the least reactive common metals.

Question 4

Overhead power cables are made of aluminium rather than copper mainly because aluminium

A conducts electricity better than copper

B is much less dense, so the cable is lighter for the same performance

C is stronger than copper in tension

D is unreactive with oxygen

Copper is the better conductor; aluminium wins on mass. A steel core is added precisely because aluminium is not strong in tension, ruling out C.

Question 5

Which statement about the reactivity of aluminium is correct?

A It is unreactive, which is why it is found as the free metal

B It is reactive, and is extracted by electrolysis because carbon cannot reduce its oxide

C It is less reactive than copper

D It is below hydrogen, so it does not react with acids

The extraction method is the proof of reactivity. Anything above carbon must be extracted by electrolysis, and aluminium sits well above it.

Question 6

Copper is used for water pipes rather than iron because copper

A is cheaper than iron

B has a lower density than iron

C is below hydrogen in the reactivity series, so it does not react with water

D is harder than iron

Copper is more expensive, denser and softer than iron — but it does not rust, and that single chemical property outweighs the other three.

Question 7

Aluminium foil is used to wrap food because it is malleable and

A conducts electricity

B resists corrosion, so it does not contaminate the food

C has a high melting point

D is magnetic

Malleability lets it be rolled to 0.01 mm and folded round a dish; the oxide layer keeps the metal out of the food. Aluminium is not magnetic.

Question 8

Which of these is not a valid reason for choosing a metal for saucepan bases?

A Good thermal conductivity

B High melting point

C Good electrical conductivity

D Resistance to corrosion by acidic foods

Electrical conductivity is irrelevant to a saucepan — a classic trap for students who list every metallic property instead of choosing the ones the job requires. (Induction hobs are the one exception, and they need magnetism, not conductivity.)

Question 9

The formula of the oxide layer that protects aluminium is

A AlO

B AlO₂

C Al₂O₃

D Al₃O₂

Al³⁺ and O²⁻ balance as 2:3, giving Al₂O₃. The balanced equation is 4Al + 3O₂ → 2Al₂O₃.

Question 10

Why is aluminium alloy, rather than pure aluminium, used for aircraft frames?

A The alloy has a lower density than pure aluminium

B Pure aluminium is too soft, and alloying stops the layers sliding

C The alloy conducts electricity better

D Pure aluminium melts at too low a temperature

The alloy is slightly denser than pure aluminium, but the strength gained is worth it. Different-sized copper and magnesium atoms distort the layers so they cannot slide.

Question 11

Recycling aluminium saves about 95% of the energy of extracting it because recycling

A uses a cheaper ore

B only needs the metal to be melted, with no electrolysis required

C uses carbon to reduce the oxide instead of electricity

D produces a purer metal

Scrap is already the metal, so only a physical change (melting at 660 °C) is needed. Option C is impossible — carbon can never reduce aluminium oxide, since aluminium is above carbon.

Question 12

A metal is needed for a light bulb filament. The most important property is

A low density

B a very high melting point

C malleability

D resistance to acids

A filament must glow white-hot without melting, so tungsten (3422 °C) is used. It must also be ductile enough to be coiled, but the melting point is the deciding property.

Question 13

Which statement explains why the protection given by aluminium's oxide layer is so effective?

A The layer is thick and flakes off, exposing fresh oxide

B The layer is impermeable, bonded to the surface, and re-forms if scratched

C The layer dissolves in water, carrying contaminants away

D The layer conducts electricity, so charge cannot build up

Flaking (A) is what rust does, which is why iron keeps corroding while aluminium does not. Al₂O₃ is in fact an electrical insulator, ruling out D.

Question 14

Gold is used for the connectors on high-quality electronic plugs mainly because it

A is the best conductor of all metals

B is very unreactive, so the contact does not tarnish and stays conducting

C has a high density

D is very hard

Silver is the best conductor, but it tarnishes. Gold sits at the very bottom of the reactivity series, so a gold-plated contact never grows an insulating oxide or sulfide film. Pure gold is also very soft.

Question 15

Which use is correctly matched to its property?

A Copper for aircraft — low density

B Aluminium for cutlery — hardness

C Stainless steel for surgical instruments — corrosion resistance and hardness

D Lead for overhead cables — low density

Copper is dense, aluminium is soft, and lead is one of the densest common metals. Only C pairs a real property with a real use.

Question 16

In a 3-mark question asking you to justify a metal's use, the best strategy is to

A list as many properties of the metal as you can remember

B give three different properties, each linked to what the object must do

C describe the extraction of the metal in detail

D state one property and explain it three ways

Marks are awarded for distinct property–consequence pairs. An unlinked list scores badly, and repeating one property in three sentences earns just one mark.

Question 17

Iron food cans are coated with a thin layer of tin. The best reason is that tin

A is more reactive than iron, so it is sacrificed

B is less reactive than iron and forms a barrier that keeps water and oxygen off the steel

C conducts heat better than iron

D is magnetic, so the can can be sorted for recycling

Tin sits below iron, so it works purely as a barrier, not sacrificially. That is why a scratched tin can rusts faster than bare steel — a favourite comparison with galvanising in 9.3.

Question 18

Which statement comparing copper and aluminium is correct?

A Aluminium is the better conductor and the denser metal

B Copper is the better conductor; aluminium is the less dense

C Both are below hydrogen in the reactivity series

D Both must be extracted by electrolysis

Aluminium is above hydrogen and above carbon (electrolysis); copper is below hydrogen and below carbon (carbon reduction). C and D each mistakenly lump the two metals together.

Question 19

A student writes: "Copper is used in wiring because it is a metal, and metals conduct." The examiner gives 1 mark out of 3. The best improvement is to add

A that copper is shiny

B that delocalised electrons are free to move and carry charge, and that copper is ductile so it is drawn into wire

C the price of copper per tonne

D that copper is found in Zambia

Two extra marks are available: the mechanism of conduction, and a second property matched to the use. Facts that do not bear on the job earn nothing.

Question 20

Titanium is used for hip replacements although it is a reactive metal. This is possible because titanium

A is coated in gold before implantation

B forms a tough, self-repairing oxide layer that stops body fluids reaching the metal

C becomes unreactive when it is alloyed

D is below copper in the reactivity series

Titanium uses exactly the same protection mechanism as aluminium and chromium. Alloying is a physical mixing and does not change an element's reactivity, so C is wrong.

9.3 Reactivity Series, Extraction and Corrosion ▼

The Reactivity Series

The reactivity series is a list of metals in order of how vigorously they react. It is the most useful single list in IGCSE Chemistry, because it predicts reactions with water, with acid and with oxygen, it predicts which metal displaces which, it decides how each metal must be extracted, and it explains rust prevention. Carbon and hydrogen are non-metals but are placed in the list because they are the two benchmarks everything else is measured against.

K Na Ca Mg Al (C) Zn Fe (H) Cu Ag Au

Potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold. Most reactive at the top, least reactive at the bottom. Carbon is the extraction dividing line. Hydrogen is the acid dividing line.

Memory Trick

"King Nandu Called Magnificent Alfred, Chirping Zestfully Farewell, Having Curious Silver Gold." K, Na, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au. Whichever sentence you invent, make sure C and H are inside it — students who memorise only the metals lose the two most useful landmarks on the list.

Reactions of Metals with Water, Steam, Acid and Oxygen

Read the next table downwards and watch every reaction get gentler. This single table answers the majority of "describe what you would see" questions in Topic 9.

Metal	Cold water	Steam	Dilute acid (HCl)	Oxygen / air
Potassium	Violent; ignites, lilac flame → KOH + H₂	—	Dangerously explosive	Burns instantly; tarnishes in seconds
Sodium	Vigorous; melts into a ball, fizzes → NaOH + H₂	—	Explosive — never done	Burns with a bright orange flame
Calcium	Steady fizzing, sinks then rises → Ca(OH)₂ + H₂	—	Very vigorous fizzing	Burns with a brick-red flame
Magnesium	Very slow (a few bubbles in days)	Reacts readily → MgO + H₂	Rapid fizzing; tube warms	Burns with a brilliant white flame
Aluminium	No visible reaction (oxide layer)	Slow, once the oxide is removed	Slow at first, then rapid	Oxide layer forms at once; powder burns
Zinc	No reaction	Reacts when heated → ZnO + H₂	Steady fizzing	Burns with a blue-green flame when heated
Iron	No reaction (but rusts slowly with water + oxygen)	Reacts slowly, red hot → Fe₃O₄ + H₂	Slow fizzing; solution turns pale green	Burns only as wire wool or filings
Copper	No reaction	No reaction	No reaction — below hydrogen	Does not burn; forms black CuO on heating
Silver, gold	No reaction	No reaction	No reaction	No reaction — do not tarnish (gold)

The three reaction patterns you must be able to write

metal + cold water → metal hydroxide + hydrogen

Only for the reactive metals K, Na, Ca (and very slowly Mg). 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g) — the solution is alkaline, so red litmus turns blue.

metal + steam → metal oxide + hydrogen

For the middle metals Mg, Zn, Fe — note the product is an oxide, not a hydroxide. Mg(s) + H₂O(g) → MgO(s) + H₂(g) 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g) — reversible, which is why it is done in a stream of steam.

metal + dilute acid → salt + hydrogen

Only metals above hydrogen react. Copper, silver and gold do not. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) · Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) Test for the gas: it burns with a squeaky pop.

Exam Tip

Water gives a hydroxide; steam gives an oxide. Writing "Mg + steam → magnesium hydroxide" is one of the most common single errors in this topic. And do not forget the hydrogen — every one of these three reactions produces H₂.

Displacement Reactions — The Practical Test of the Order

A more reactive metal displaces a less reactive metal from a compound of that metal — whether the compound is a solution of a salt or a solid oxide. The more reactive metal takes the oxygen (or the anion) for itself.

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Zinc is above copper, so the reaction happens. Observation: the blue solution fades to colourless, a pink-brown solid coats the zinc, and the mixture warms up. Ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). The sulfate is a spectator ion. The reverse, Cu(s) + ZnSO₄(aq), gives no reaction — copper is below zinc and cannot displace it.

2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l)

The thermite reaction: aluminium is above iron, so it displaces iron from iron(III) oxide. So much heat is released that the iron is produced molten — used to weld railway lines in place. The same rule with oxides: a metal displaces the oxide of any metal below it.

Displacement is how the order is established experimentally. Set up a grid: each metal against each salt solution, and record where a reaction occurs. A metal that displaces everything is at the top; a metal that displaces nothing is at the bottom.

Metal added ↓ / Solution →	MgSO₄(aq)	ZnSO₄(aq)	FeSO₄(aq)	CuSO₄(aq)
Magnesium	—	Reaction	Reaction	Reaction
Zinc	No reaction	—	Reaction	Reaction
Iron	No reaction	No reaction	—	Reaction
Copper	No reaction	No reaction	No reaction	—

Read the staircase: all the reactions sit above the diagonal. That pattern alone proves the order Mg > Zn > Fe > Cu, without knowing anything else about the metals. Cambridge sets this as a data question almost every year — sometimes with metals labelled only as W, X, Y and Z.

Supplement

What reactivity actually is: the loss of electrons

Everything above is description. Here is the explanation, and it is the sentence that separates a grade A from a grade C.

The reactivity of a metal is related to how readily its atoms lose their outer-shell electrons to form positive ions. A metal high in the series loses electrons easily; a metal low in the series loses them only with difficulty.

So in every one of these reactions, the metal atom is oxidised — it loses electrons:

M → Mⁿ⁺ + ne⁻ (oxidation — loss of electrons)

Zn → Zn²⁺ + 2e⁻ (zinc is oxidised) Cu²⁺ + 2e⁻ → Cu (copper ions are reduced) In a displacement reaction the more reactive metal loses electrons more readily, so it hands them to the ions of the less reactive metal, which are reduced back to the metal. OIL RIG: Oxidation Is Loss, Reduction Is Gain.

This also explains why the reactivity series looks so much like the left of the Periodic Table. Group I metals have one loosely held outer electron and sit at the very top; the transition metals hold their electrons more tightly and sit lower down. And it explains extraction: the harder a metal holds its electrons, the harder it is to put them back, which is why the top metals need the brute force of electrolysis.

Exam Tip

If a question says "explain why magnesium is more reactive than copper", the answer is not "because it is higher in the reactivity series" — that just restates the question. Write: "magnesium atoms lose their outer electrons more readily than copper atoms to form positive ions". Mention electrons and you get the mark; describe the fizzing and you do not.

Extraction of Metals — Carbon Is the Dividing Line

Only the very unreactive metals — gold, and sometimes silver and copper — are found native, as the element itself. Everything else is locked up in an ore: a rock containing enough of a metal compound to be worth mining. Extraction means reducing the metal compound back to the metal, and how you do that depends entirely on where the metal sits relative to carbon.

The ladder that runs the whole of 9.3. Above the carbon line: electrolysis. Below it: carbon reduction. Below the hydrogen line: no reaction with acid, and often found native.

Position in series	Metals	Extraction method	Reason
Above carbon	K, Na, Ca, Mg, Al	Electrolysis of the molten compound	Carbon is not reactive enough to displace them; electricity supplies the electrons directly
Below carbon	Zn, Fe, (Pb, Cu)	Reduction by carbon (heating the oxide with coke)	Carbon is more reactive, so it displaces the metal and takes the oxygen
Very low	Ag, Au (and some Cu)	Found native; physical separation only	So unreactive that they exist uncombined in the ground

Aluminium: electrolysis of bauxite

Bauxite is the ore, an impure form of aluminium oxide, Al₂O₃. It is purified and then melted so that the ions become free to move. (Cryolite is added to lower the melting point and save energy, but the essential chemistry is the electrolysis of molten aluminium oxide.)

At the cathode (−): Al³⁺ + 3e⁻ → Al At the anode (+): 2O²⁻ → O₂ + 4e⁻

Aluminium ions gain electrons (reduction) at the negative electrode and molten aluminium collects at the bottom. The carbon anodes burn away in the oxygen produced and must be replaced regularly — a standard exam point. This is why aluminium was once more valuable than gold: it could not be extracted at all until electricity was available.

Iron: The Blast Furnace

Iron is below carbon, so it can be reduced by carbon — and that is done on an enormous scale at works such as Jamshedpur, Bhilai and Rourkela. Three raw materials go in at the top, hot air is blasted in at the bottom, and molten iron and slag are tapped off separately.

Cross-section of a blast furnace. Note the temperature gradient: hottest at the bottom where the coke burns in the hot air blast, coolest at the top where the charge enters.

The role of each raw material

Raw material	What it is	Its job
Iron ore (haematite)	Fe₂O₃	The source of the iron. It is the substance that is reduced.
Coke	Carbon, C	Two jobs. (i) It burns in the hot air to release heat, giving the very high temperature needed. (ii) It reacts with the CO₂ produced to make carbon monoxide, which is the reducing agent.
Limestone	Calcium carbonate, CaCO₃	Removes the acidic impurity. It decomposes to calcium oxide, a base, which reacts with the sandy silicon dioxide impurity to form molten slag, calcium silicate.
Hot air	Oxygen (plus nitrogen)	Supplies the oxygen for the coke to burn, releasing the heat. Blasting it in hot saves fuel — hence the name.

C(s) + O₂(g) → CO₂(g) · CO₂(g) + C(s) → 2CO(g)

Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g)

CaCO₃(s) → CaO(s) + CO₂(g) · CaO(s) + SiO₂(s) → CaSiO₃(l)

Reduced: the iron(III) oxide — it loses oxygen (and Fe³⁺ gains electrons). Reducing agent: carbon monoxide, CO — it removes the oxygen and is itself oxidised to CO₂. Oxidised: the carbon monoxide. Reduction and oxidation always happen together. The molten iron is denser than the slag, so the iron collects at the very bottom and the slag floats on top, letting them be tapped off separately.

Exam Tip

The classic trap: "which substance is reduced?" The answer is iron(III) oxide — not carbon monoxide. CO is the reducing agent, the thing that does the reducing, and it is itself oxidised to CO₂. If the wording is "name the reducing agent", answer carbon monoxide. If it is "which substance is reduced", answer iron(III) oxide. Read the question twice; the two answers are never the same substance.

Zinc

Zinc is also below carbon, so the same principle applies. Zinc blende (zinc sulfide) is first roasted in air to convert it to the oxide, and the oxide is then reduced by carbon:

2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g) then ZnO(s) + C(s) → Zn(g) + CO(g)

Sulfide ores are roasted first because carbon reduces oxides, not sulfides. The sulfur dioxide produced must be captured — released into the air it causes acid rain. Zinc boils at 907 °C, so it leaves the furnace as a vapour and is condensed.

Rusting — and How to Stop It

Rusting is the corrosion of iron and steel. It costs the world economy billions every year: bridges, ships, cars, railway lines and reinforced concrete all suffer. Chemically it is straightforward, and the whole of rust prevention follows from one sentence.

Rusting needs BOTH water AND oxygen

iron + water + oxygen → hydrated iron(III) oxide (rust) Remove either one and rusting stops. That is why every barrier method works. Salt does not cause rusting — it speeds it up. So do acids. Coastal cities such as Mumbai and Chennai are hard on cars for exactly this reason. Rust is flaky and porous: it falls off and exposes fresh iron underneath, so corrosion continues until the object is destroyed. Compare aluminium's adherent oxide layer.

The standard rusting experiment. Tube A is the control with both conditions present; B removes oxygen; C removes water. Only A rusts, which proves both are needed.

Preventing Rust: Two Completely Different Strategies

Strategy 1: Barrier methods — keep water and oxygen out

These work by physically covering the iron. If water and oxygen cannot reach the surface, no rusting occurs.

Method	Where it is used	Limitation
Painting	Cars, bridges, railings, ships	Scratches expose the iron and rusting starts there
Oiling or greasing	Moving parts: bicycle chains, tools, machinery	Must be renewed; collects dust
Coating with plastic	Dish racks, garden furniture, fridge shelves	Cracks or peels if damaged
Plating with a less reactive metal (e.g. tin, chromium)	Food cans, car bumpers, taps	If scratched, rusting is made worse — the iron is now the more reactive metal in contact with tin, so it corrodes faster

Strategy 2: Sacrificial protection — let something else corrode instead

This is chemically far more interesting, and it is where marks are won and lost.

Attach a MORE reactive metal — it corrodes instead of the iron

Zinc and magnesium are above iron in the reactivity series, so their atoms lose electrons more readily to form positive ions. The more reactive metal is therefore oxidised in preference to the iron: Zn → Zn²⁺ + 2e⁻. The electrons released flow into the iron, so the iron is kept as the metal and cannot be oxidised to Fe²⁺/Fe³⁺. The protecting metal is gradually eaten away — it is sacrificed — and must be replaced periodically. It works even when the coating is scratched, because contact, not coverage, is what matters. This is the key advantage over a barrier.

Same scratch, opposite outcomes. Position in the reactivity series — above or below iron — decides everything.

Galvanising

Galvanising is coating iron or steel with a layer of zinc, usually by dipping it into molten zinc. It is the best of both worlds and is used on roofing sheets, buckets, water tanks, nails, motorway crash barriers and the steel in reinforced concrete.

While the coating is whole, the zinc acts as a barrier, keeping water and oxygen away from the iron.
If the coating is scratched, the zinc protects sacrificially: being more reactive, it loses electrons and is oxidised in preference to the iron, so the exposed iron still does not rust.
Zinc also forms its own adherent oxide/carbonate layer, so it corrodes slowly and lasts for decades.

Sacrificial blocks

On ships' hulls, oil rigs, underground steel pipelines and pier legs, blocks of zinc or magnesium are bolted directly to the steel. They corrode away and are unbolted and replaced at the next inspection — far cheaper than replacing a hull. The same trick protects the steel legs of jetties at Kandla and Visakhapatnam.

Exam Tip

Three phrases must appear in a full-mark sacrificial protection answer: (1) the protecting metal is more reactive than iron; (2) it loses electrons / is oxidised in preference to the iron; (3) it is gradually used up and must be replaced. Adding "and it works even if the coating is scratched" usually earns a fourth mark. Answers that say only "the zinc covers the iron" describe a barrier and miss the point of the question entirely.

Worked Example 1 Four metals W, X, Y and Z were tested. W reacts steadily with cold water. X does not react with cold water but reacts with dilute hydrochloric acid. Y does not react with dilute acid but is displaced from its sulfate solution by X. Z reacts violently with cold water. Place the four metals in order of decreasing reactivity and state which could be copper. [5]

Step 1: Use the water tests to split the group

Reaction with cold water means a metal near the top. Z is violent and W is steady, so Z is above W, and both are above X and Y, neither of which touches cold water.

Step 2: Use the acid test

X reacts with dilute acid, so X is above hydrogen. Y does not, so Y is below hydrogen. That already places X above Y.

Step 3: Confirm with the displacement result

X displaces Y from its sulfate solution, which independently confirms X is more reactive than Y. Two pieces of evidence pointing the same way — always say so, it is worth a mark.

Step 4: Assemble and identify

Order: Z > W > X > Y. Y is below hydrogen and is displaced by another metal, so Y fits copper (or silver). Z might be potassium or sodium, W calcium, X zinc or iron.

Decreasing reactivity: Z > W > X > Y [3]. Z and W react with cold water so are the most reactive, with Z the more violent [1]. X reacts with acid so is above hydrogen; Y does not react with acid and is displaced by X, so Y is below hydrogen and is the least reactive — Y could be copper [1].

Worked Example 2 Explain why iron is extracted in a blast furnace using carbon, but aluminium must be extracted by electrolysis, even though aluminium ore is far more abundant. Include the equation for the main reduction in the blast furnace and state which substance is reduced. [6]

Step 1: Locate both metals relative to carbon

The series runs K, Na, Ca, Mg, Al, C, Zn, Fe. Aluminium is above carbon; iron is below it. That single comparison decides both methods.

Step 2: Iron — carbon is more reactive, so it works

Because carbon is more reactive than iron, it can displace iron from its oxide and take the oxygen for itself. In practice the reducing agent is carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂.

Step 3: Aluminium — carbon is not reactive enough

Carbon is less reactive than aluminium, so it cannot displace aluminium from aluminium oxide. No amount of heating will make that reaction go. The only way is to force electrons on to the Al³⁺ ions electrically, so the molten oxide is electrolysed: Al³⁺ + 3e⁻ → Al at the cathode.

Step 4: Answer the "which is reduced" part carefully

The substance reduced is iron(III) oxide — it loses oxygen, and Fe³⁺ gains electrons. Carbon monoxide is the reducing agent and is itself oxidised to carbon dioxide. Abundance is irrelevant to the method; it affects only cost.

Iron is below carbon in the reactivity series, so carbon (as carbon monoxide) is more reactive and can displace / reduce the iron from its oxide [2]: Fe₂O₃ + 3CO → 2Fe + 3CO₂ [1]. Aluminium is above carbon, so carbon is not reactive enough to displace it from aluminium oxide [1], and the compound must instead be electrolysed when molten, with Al³⁺ + 3e⁻ → Al at the cathode [1]. The substance reduced in the blast furnace is iron(III) oxide; carbon monoxide is the reducing agent [1].

Worked Example 3 A steel bucket is galvanised and a steel food can is coated with tin. Both coatings get scratched. Explain, in terms of the reactivity series and electron loss, why the bucket is still protected while the can rusts faster than bare steel would. [6]

Step 1: Compare each coating metal with iron

Zinc is above iron in the reactivity series; tin is below it. Write this down first — everything else follows.

Step 2: The galvanised bucket

Being more reactive, zinc atoms lose electrons more readily than iron atoms: Zn → Zn²⁺ + 2e⁻. So the zinc is oxidised in preference to the iron, and the electrons released pass into the iron, keeping it as the metal. This is sacrificial protection, and it works because the two metals are in contact — the coating does not have to be complete. The zinc is slowly eaten away.

Step 3: The tin-plated can

Tin is less reactive than iron, so it can only ever act as a barrier. Once scratched, the iron is the more reactive metal in electrical contact with the tin, so the iron loses electrons preferentially and corrodes — and because the exposed area is small compared with the tin around it, the attack is concentrated and rusting is faster than on bare steel.

Step 4: Say why tin is used at all

Tin is non-toxic and unreactive with food acids, whereas dissolved zinc compounds would be harmful. For a food can, safety beats corrosion performance — which is why cans are also lacquered and are not meant to be dented.

Zinc is above iron, so it loses electrons more readily (Zn → Zn²⁺ + 2e⁻) and is oxidised in preference to the iron [2]; this sacrificial protection works even where the coating is scratched, and the zinc is gradually used up [1]. Tin is below iron, so it can only act as a barrier [1]; once scratched, the iron is the more reactive metal and loses electrons preferentially [1], so the exposed steel corrodes faster than it would on its own [1].

Exam Tips for 9.3

1. Learn the series with C and H in it. K Na Ca Mg Al C Zn Fe H Cu Ag Au. Carbon decides the extraction method; hydrogen decides whether the metal reacts with acid.

2. Explain reactivity with electrons. "Reactivity depends on how readily the atoms lose electrons to form positive ions." Saying "because it is higher in the series" restates the question and scores nothing.

3. Water → hydroxide; steam → oxide. Both give hydrogen. Getting the product wrong loses the equation mark even if the balancing is perfect.

4. Displacement runs downwards only. A metal displaces one below it. Zn + CuSO₄ works; Cu + ZnSO₄ does nothing. Always write the reactivity order at the top of a displacement question before predicting anything.

5. Reduced ≠ reducing agent. In the blast furnace, iron(III) oxide is reduced; carbon monoxide is the reducing agent and is itself oxidised. Read which one the question asks for.

6. Give coke two jobs. It burns to supply heat and it forms carbon monoxide, the reducing agent. Only one of those is usually written, and the second mark is left on the table.

7. Limestone removes the impurity. CaCO₃ → CaO + CO₂, then CaO + SiO₂ → CaSiO₃ (slag). Do not say limestone "makes the iron pure" without naming the silicon dioxide.

8. Rusting needs BOTH water and oxygen. Never write "air" alone, and never write "water" alone. Salt is a catalyst-like accelerator, not a requirement.

9. Barrier versus sacrificial. Paint, grease, plastic and tin are barriers — useless once scratched. Zinc and magnesium are sacrificial — they still work when scratched, because the more reactive metal is oxidised instead.

10. Aluminium is reactive. It is protected by its oxide layer, and it is extracted by electrolysis because it is above carbon. Both facts are consistent, and questions often test whether you can hold them together.

11. Give observations, not deductions. "Hydrogen is produced" is a deduction. "Bubbles of gas / effervescence", "the blue solution fades", "a pink-brown solid forms" are observations — and observations earn the marks.

12. Electrolysis needs a MOLTEN (or aqueous) compound. The ions must be free to move. "Electrolysis of solid aluminium oxide" scores zero.

🌎 Apply It: Real-World Chemistry

Bridges, ships, pipelines and steel plants are all just the reactivity series applied at scale — usually by someone trying not to spend money.

Tata Steel's Jamshedpur works consumes iron ore from Noamundi, coke made from Jharia coal and limestone quarried nearby. Each tonne of iron produced also generates roughly 300 kg of slag, which is sold to cement makers rather than dumped.

Explain the job of each raw material, and why the slag is worth selling.

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Ore, Coke and Air

The haematite ore (Fe₂O₃) is the source of iron and is the substance reduced. The coke does two jobs: it burns in the hot air blast (C + O₂ → CO₂) to give the very high temperature, and it then reacts with that CO₂ (CO₂ + C → 2CO) to make carbon monoxide, the reducing agent, which strips the oxygen from the ore: Fe₂O₃ + 3CO → 2Fe + 3CO₂.

Limestone and the Sandy Impurity

The ore contains silicon dioxide (sand), an acidic oxide. Limestone decomposes in the heat (CaCO₃ → CaO + CO₂) to give calcium oxide, a base, which neutralises it: CaO + SiO₂ → CaSiO₃. The molten calcium silicate slag floats on the denser molten iron, so the two are tapped off separately.

Why Sell the Slag

Slag is a calcium silicate very like the material in cement clinker, so it can replace part of the cement in concrete — which saves the enormous CO₂ emissions of making that cement. A waste stream becomes a revenue stream and the landfill disappears.

Chemistry Connection

The slag reaction is pure Topic 7: an acidic oxide plus a basic oxide gives a salt. Blast furnace chemistry is not a special topic to be memorised — it is neutralisation, redox and thermal decomposition, all happening in one very large tube.

A cargo ship docked at Kochi has slabs of zinc bolted to its steel hull below the waterline. During dry-dock inspection every few years, engineers unbolt the badly corroded slabs and bolt on fresh ones. The hull itself is also painted, but the paint is chipped in dozens of places.

Explain how the zinc slabs work, why the paint alone is not enough, and why the slabs must be replaced.

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Why Paint Alone Fails

Paint is a barrier — it stops water and oxygen reaching the iron only while it is intact. A hull is constantly scraped by ropes, quays and floating debris, so chips are inevitable, and at every chip the bare steel meets salt water and oxygen and rusts. Salt water accelerates rusting because dissolved ions carry charge and speed up the electron transfer.

How the Zinc Works

Zinc is above iron in the reactivity series, so zinc atoms lose electrons more readily: Zn → Zn²⁺ + 2e⁻. The zinc is therefore oxidised in preference to the iron, and the electrons it releases flow through the hull, keeping the iron as metal. This is sacrificial protection, and crucially it protects the whole hull, including the chipped spots, because the metals are in electrical contact — coverage is not required.

Why They Must Be Replaced

The protection works precisely because the zinc is being consumed. Once a slab has corroded away, there is nothing left to be oxidised and the hull starts to rust. Replacing a few slabs is trivially cheap compared with replacing steel plate.

Chemistry Connection

Notice that the ship uses both strategies at once: paint as a barrier for the 95% of the hull that is undamaged, zinc as sacrificial insurance for the 5% that is not. Magnesium is used instead of zinc for buried pipelines, because it is even more reactive and works in low-conductivity soil.

Copper was smelted in the Middle East over 7000 years ago, iron reached India around 1800 BCE, but aluminium was not isolated until 1825 and was so precious in the 1850s that Napoleon III served his most honoured guests on aluminium plates while everyone else made do with gold.

Explain this historical order using the reactivity series.

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Gold and Copper First

Gold is so unreactive that it is found native — as the metal — needing no chemistry at all. Copper is only just above it, so its ores are reduced by carbon at the modest temperature of a wood or charcoal fire. Both were available to people with no theory whatsoever.

Iron Next — Same Chemistry, More Heat

Iron is below carbon too, so charcoal can reduce its oxide, but it is higher in the series than copper, so a much higher temperature is needed. The Iron Age had to wait for the technology of bellows and better furnaces, not for new chemistry.

Aluminium Last — Different Chemistry Entirely

Aluminium is above carbon, so no furnace at any temperature can make carbon displace it from its oxide. It cannot be extracted until electricity is available, because only electrolysis can force electrons on to Al³⁺ ions. Once the Hall–Héroult process and cheap generation arrived in the 1880s, the price collapsed from precious-metal levels to that of a drinks can.

Chemistry Connection

Human history follows the reactivity series in reverse: the lower a metal sits, the earlier we learned to extract it. The dividing line at carbon is visible in the archaeological record as a 7000-year gap. Potassium and sodium, higher still, were isolated by Humphry Davy in 1807 — also by electrolysis, also as soon as batteries existed.

Railway maintenance crews on the Konkan Railway weld rails together on site using a thermite charge: powdered aluminium mixed with iron(III) oxide in a crucible, ignited with a magnesium fuse. Within seconds, white-hot molten iron pours into the gap between the rails.

Write the equation, identify what is oxidised and reduced, and explain why the iron comes out molten.

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The Reaction

Aluminium is above iron in the reactivity series, so it displaces iron from iron(III) oxide: 2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l). It is a displacement reaction between a metal and a metal oxide — the same rule as Zn + CuSO₄, just with an oxide instead of a solution.

Oxidation and Reduction

Aluminium is oxidised: it loses electrons, Al → Al³⁺ + 3e⁻, and gains oxygen. Iron(III) oxide is reduced: it loses oxygen, and Fe³⁺ gains electrons to become Fe. Aluminium is the reducing agent.

Why Molten Iron

Aluminium is much more reactive than iron, so it holds oxygen far more strongly; the reaction is extremely exothermic and releases enough energy to raise the products above the melting point of iron (1538 °C). The molten iron flows into the gap and solidifies, welding the rails; the less dense aluminium oxide floats off as slag.

Chemistry Connection

The magnesium fuse is there because the thermite mixture needs a lot of activation energy — magnesium burns hot enough to supply it. And note the elegance: the reaction is the exact reverse of the extraction problem. Aluminium is so hard to extract because it grips oxygen so tightly, and that same grip is what makes thermite work.

The iron pillar at Mehrauli in Delhi was forged around 400 CE and stands seven metres tall, yet it has barely rusted in 1600 years. Meanwhile a mild steel gate in coastal Chennai visibly rusts within one monsoon. Analysis of the pillar shows an unusually high phosphorus content and a thin, even, protective film on its surface.

Explain both observations using what you know about the conditions required for rusting.

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Rusting Requires Both Water and Oxygen

Iron rusts only when water AND oxygen are both present. Delhi's climate is dry for most of the year and humidity is low, so for long periods one of the two requirements is largely missing and rusting all but stops.

Why Chennai Is Different

Coastal Chennai has high humidity all year (plenty of water) and salt spray from the sea. Dissolved salt does not cause rusting but greatly accelerates it, because the ions in solution make electron transfer between different parts of the metal much easier. Both conditions are permanently present and boosted.

The Pillar's Own Barrier

The phosphorus in the ancient iron helped form a thin, even, adherent film of iron phosphate and oxide on the surface — behaving like aluminium's oxide layer rather than like ordinary flaky rust. Because it is impermeable and does not fall off, water and oxygen cannot reach the metal beneath and corrosion effectively stops.

Chemistry Connection

The whole difference between a metal that survives and one that crumbles is whether the corrosion product sticks. Aluminium, chromium, titanium, zinc and the Delhi pillar all form adherent films; ordinary rust is porous and flakes away, exposing fresh iron. Modern "weathering steel" used in bridge design deliberately copies the pillar's trick.

Practice Questions: 9.3

20 multiple choice questions. Click an option to check your answer.

Your Score 0 / 20

Question 1

Which list places the metals in order of decreasing reactivity?

A Cu, Fe, Zn, Mg

B Mg, Zn, Fe, Cu

C Zn, Mg, Cu, Fe

D Fe, Cu, Mg, Zn

The order is K Na Ca Mg Al C Zn Fe H Cu Ag Au. Read the question carefully — "decreasing" means most reactive first.

Question 2

What determines how reactive a metal is?

A How many electrons it can gain

B How readily its atoms lose electrons to form positive ions

C Its density

D Its position in the Periodic Table alone

Metals always lose electrons, never gain them. This is the sentence that turns a description into an explanation — use it whenever a question says "explain".

Question 3

Magnesium reacts with steam to give

A magnesium hydroxide and oxygen

B magnesium hydroxide and hydrogen

C magnesium oxide and hydrogen

D magnesium and water vapour only

Steam gives an oxide; cold water gives a hydroxide. Mg + H₂O(g) → MgO + H₂. This pair is tested almost every session.

Question 4

Which metal will not react with dilute hydrochloric acid?

A Zinc

B Iron

C Magnesium

D Copper

Only metals above hydrogen displace it from an acid. Copper is below hydrogen, so no reaction — which is exactly why copper is used for plumbing.

Question 5

Which mixture will react?

A Copper + zinc sulfate solution

B Zinc + copper(II) sulfate solution

C Silver + magnesium chloride solution

D Iron + magnesium sulfate solution

Displacement runs downwards only: the added metal must be above the metal in solution. Zinc is above copper, so B works and A (the reverse) does not.

Question 6

When zinc is added to blue copper(II) sulfate solution, the observations include

A the solution turning a deeper blue and the tube cooling

B the blue colour fading, a pink-brown solid forming, and the tube warming

C bubbles of hydrogen and a white precipitate

D no visible change

Cu²⁺(aq) is blue and Zn²⁺(aq) is colourless, so the colour fades as copper metal deposits. Displacement reactions are exothermic. No hydrogen is involved — this is not an acid.

Question 7

Aluminium is extracted by electrolysis rather than by heating with carbon because

A aluminium oxide has too high a melting point to be heated

B aluminium is above carbon in the reactivity series, so carbon cannot displace it

C aluminium is below carbon, so electrolysis is cheaper

D bauxite contains no oxygen

Position relative to carbon is the whole answer. Electrolysis is much more expensive, which is why it is used only when carbon reduction is impossible.

Question 8

In the blast furnace, which substance is reduced?

A Carbon monoxide

B Iron(III) oxide

C Limestone

D Silicon dioxide

Fe₂O₃ loses oxygen, so it is reduced. Carbon monoxide is the reducing agent and is itself oxidised to CO₂. This distinction is examined every year.

Question 9

What is the main role of limestone in the blast furnace?

A To reduce the iron oxide to iron

B To provide the heat for the furnace

C To remove the acidic silicon dioxide impurity as slag

D To supply oxygen to burn the coke

CaCO₃ → CaO + CO₂, then the basic CaO neutralises the acidic SiO₂: CaO + SiO₂ → CaSiO₃. Coke gives the heat and reducing agent; hot air supplies oxygen.

Question 10

Which equation represents the main reduction of iron ore?

A C + O₂ → CO₂

B CaCO₃ → CaO + CO₂

C Fe₂O₃ + 3CO → 2Fe + 3CO₂

D CaO + SiO₂ → CaSiO₃

All four reactions occur in the furnace, but only C produces the iron. A supplies heat, B and D deal with the impurity.

Question 11

Rusting of iron requires

A water only

B oxygen only

C both water and oxygen

D water, oxygen and salt

Salt speeds up rusting but is not required — nails rust perfectly well in distilled water and air. Saying "salt is needed" is a common and costly error.

Question 12

In the classic rusting experiment, why is the water in one tube boiled and then covered with a layer of oil?

A To warm the nail so it reacts faster

B To remove dissolved air and stop more dissolving back in, so oxygen is absent

C To sterilise the water so bacteria do not interfere

D To make the water more acidic

That tube tests whether oxygen is necessary, so all oxygen must be excluded. The third tube uses anhydrous calcium chloride to remove water instead.

Question 13

Which method protects iron sacrificially?

A Painting

B Coating with tin

C Attaching blocks of magnesium

D Coating with grease

Sacrificial protection needs a more reactive metal. Paint, grease and tin are all barriers — and tin, being below iron, actually makes matters worse once scratched.

Question 14

Why does galvanising still protect steel after the coating has been scratched?

A The zinc flows into the scratch and seals it

B Zinc is more reactive, so it loses electrons and is oxidised in preference to the iron

C Zinc repels water from the surface

D Zinc reacts with the rust and removes it

Only electrical contact is needed, not coverage — which is precisely what makes sacrificial protection better than a barrier. The zinc is consumed in the process.

Question 15

A scratched tin-plated steel can rusts faster than a plain steel can because

A tin catalyses the rusting reaction

B iron is more reactive than tin, so the exposed iron loses electrons preferentially

C tin absorbs water and holds it against the steel

D the tin coating makes the can hotter

This is sacrificial protection working in reverse. When two metals are in contact, the more reactive one corrodes — and with tin plating that is the iron itself.

Question 16

Metal Q does not react with cold water or steam, but does react with dilute sulfuric acid. Metal Q is most likely to be

A sodium

B magnesium

C lead

D copper

Reacting with acid puts Q above hydrogen, ruling out copper; not reacting with steam puts it below iron, ruling out sodium and magnesium. Lead sits in exactly that narrow band, just above hydrogen.

Question 17

In the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), the zinc is

A reduced, because it gains electrons

B oxidised, because it loses electrons

C neither oxidised nor reduced

D acting as a catalyst

Zn → Zn²⁺ + 2e⁻ is a loss of electrons, so oxidation (OIL RIG). The copper ions gain those electrons and are reduced. The more reactive metal is always the one oxidised.

Question 18

Gold and silver are sometimes found as the uncombined element because they

A are very dense

B are so unreactive that they do not readily form compounds

C decompose out of their ores when heated by the sun

D are found only in meteorites

Being at the bottom of the series means their atoms hold their electrons very tightly, so they resist oxidation. This is also why they need no chemical extraction — only physical separation.

Question 19

In the thermite reaction 2Al + Fe₂O₃ → Al₂O₃ + 2Fe, aluminium acts as

A an oxidising agent, and is reduced

B a reducing agent, and is oxidised

C a catalyst

D a solvent for the iron oxide

Aluminium removes the oxygen from Fe₂O₃, so it is the reducing agent; in doing so it gains oxygen and loses electrons, so it is oxidised. The reducing agent is always the substance oxidised.

Question 20

Which statement about the extraction of metals is correct?

A All metals can be extracted by heating their oxides with carbon

B Electrolysis is used for metals below carbon because it is cheaper

C The higher a metal is in the reactivity series, the more difficult and expensive its extraction

D Zinc must be extracted by electrolysis because it is above carbon

Zinc is below carbon, so it is reduced by carbon after its sulfide ore is roasted to the oxide. Electrolysis is far more expensive and is used only where carbon reduction is impossible.