9. Metals
- Syllabus
- 0620–2026–2027
- Section
- 9
- Level
- —
Exam analysis
9.1 Properties of metals
- Syllabus
- 0620–2026–2027
- Topic
- 9.1
- Level
- —
Compare metals and non-metals physically
| Physical property | Metals: general pattern | Non-metals: general pattern |
|---|---|---|
| thermal conductivity | good conductors of heat | poor conductors of heat |
| electrical conductivity | conduct when solid and molten | usually do not conduct |
| malleability | malleable: can be hammered or pressed into shape | brittle when solid |
| ductility | ductile: can be drawn into wires | not ductile |
| melting and boiling points | generally high | generally lower |
Use several properties together. A solid that conducts electricity and heat and can be bent or hammered without shattering is likely to be a metal; a brittle insulator is likely to be a non-metal.
Malleability concerns shaping sheets or other forms by hammering or pressing. Ductility concerns pulling a material into a wire; the terms are related but not interchangeable.
These are general patterns, not exception-free definitions. Graphite conducts electricity, and mercury is a liquid metal with a low melting point. Classify using the complete evidence supplied.
Describe the general reactions of metals
| Reagent | General products for a reacting metal | Example |
|---|---|---|
| dilute acid | salt + hydrogen | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| cold water | metal hydroxide + hydrogen | 2Na + 2H₂O → 2NaOH + H₂ |
| steam | metal oxide + hydrogen | Mg + H₂O(g) → MgO + H₂ |
| oxygen | metal oxide | 2Mg + O₂ → 2MgO |
Hydrogen formation is shown by effervescence and is confirmed by a squeaky pop with a lighted splint. A metal may disappear as a salt solution forms in acid or as a hydroxide forms with cold water.
Distinguish the two water conditions: very reactive metals react with cold water to form hydroxides, while some less reactive metals react only with steam and form oxides. Reaction vigour depends on the metal's reactivity.
Acid reactions form salt and hydrogen—not salt and water. Steam reactions form an oxide, whereas cold-water reactions form a hydroxide. Metals below hydrogen in the reactivity series may not release hydrogen from dilute acids.
9.2 Uses of metals
- Syllabus
- 0620–2026–2027
- Topic
- 9.2
- Level
- —
Link metal uses to physical properties
| Metal and use | Required property | Why the property matters |
|---|---|---|
| aluminium in aircraft | low density | reduces aircraft mass |
| aluminium in overhead electrical cables | low density and good electrical conductivity | keeps suspended cables lighter while carrying current |
| aluminium in food containers | resistance to corrosion | prevents rapid reaction with air, water or food |
| copper in electrical wiring | good electrical conductivity and ductility | carries current efficiently and can be drawn into long thin wires |
Answer as a complete causal link: name the material, state the property, then connect it to the use. For example, copper is used for wiring because it conducts electricity well and is ductile, so it can be drawn into wires.
The same metal can be chosen for different reasons. Aluminium's low density controls aircraft and overhead-cable choices; its corrosion resistance controls food-container use; conductivity is additionally essential for cables.
A true property is not automatically a relevant reason. Aluminium's conductivity does not explain a food container, and copper's high density does not explain wiring. Match each stated property to the job it performs.
9.3 Alloys and their properties
- Syllabus
- 0620–2026–2027
- Topic
- 9.3
- Level
- —
Define alloys and name their components
An alloy is a mixture of a metal with one or more other elements. The added elements may be metals or non-metals; an alloy is a mixture, not a compound.
| Alloy | Main metal | Other required elements |
|---|---|---|
| brass | copper | zinc |
| stainless steel | iron | elements such as chromium, nickel and carbon |
Describe composition with 'mixture of'. Brass contains copper and zinc; stainless steel contains iron with other elements and may include both metallic chromium/nickel and non-metallic carbon.
An alloy does not have to contain two metals: stainless steel contains carbon. Because composition can vary, an alloy is not a pure element or a fixed-ratio compound.
Compare alloys with pure metals
Alloys can be harder and stronger than the pure metals from which they are made, so they are often more useful.
| Material form | Typical response to force | Practical consequence |
|---|---|---|
| pure metal | often softer and easier to deform | useful where shaping is important |
| alloy | can resist scratching, bending or breaking more strongly | useful where hardness or strength is needed |
Choose the material by the required property. Alloying changes properties and can improve hardness or strength, but the exact benefit depends on the alloy's composition.
More useful does not mean better for every purpose. A harder alloy may be less malleable; property requirements, not the word 'alloy' alone, control the choice.
Link stainless-steel cutlery to its properties
Stainless steel is used for cutlery because it is hard and resistant to rusting.
| Cutlery requirement | Stainless-steel property | Why it fits |
|---|---|---|
| keeps its shape and resists scratching in use | hardness | withstands repeated contact and force |
| remains serviceable around water and food | resistance to rusting | avoids rapid corrosion and surface damage |
A complete explanation names the use and both properties: stainless steel suits cutlery because it is hard and resists rusting.
Strength alone is not the prescribed explanation. For cutlery, state hardness and resistance to rusting; conductivity is irrelevant to this use.
Identify an alloy structure representation
| Text-first particle representation | Identification | Diagnostic clue |
|---|---|---|
| ● ● ● ● / ● ● ● ● / ● ● ● ● | pure metal | one atom size/type in regular layers |
| ● ● ○ ● / ● ◉ ● ● / ○ ● ● ◉ | alloy | different atom sizes/types mixed through the metallic structure |
Ignore the symbols' names and compare sizes or types. A regular array containing only identical atoms represents a pure metal; a metallic array containing a second size or type represents an alloy.
The different symbols are interspersed rather than arranged in a fixed repeating ratio, consistent with an alloy being a mixture.
Do not label every diagram with two atom types as an alloy. It must represent a metal-based mixture; an ordered repeating pattern of unlike atoms may instead represent a compound.
Explain why alloys are harder and stronger
In a pure metal, equal-sized atoms form regular layers that can slide over one another when a force is applied.
| Structure | Effect on layers | Mechanical result |
|---|---|---|
| pure metal: similar-sized atoms | layers remain regular and slide relatively easily | softer, more malleable |
| alloy: different-sized atoms | regular layers are distorted and cannot slide easily | harder and stronger |
Use the full causal chain: different-sized atoms → distorted layers → layers cannot slide over each other easily → greater hardness and strength.
Do not explain alloy strength by saying the added element is simply 'hard' or makes stronger bonds. The required particle explanation is obstruction of layer sliding by different-sized atoms.
9.4 Reactivity series
- Syllabus
- 0620–2026–2027
- Topic
- 9.4
- Level
- —
Recall the reactivity series
Most reactive → K > Na > Ca > Mg > Al > C > Zn > Fe > H > Cu > Ag > Au → least reactive.
| Position | Meaning |
|---|---|
| above carbon | metal is too reactive for carbon reduction of its oxide |
| below carbon | oxide may be reduced by carbon |
| above hydrogen | metal can release hydrogen from dilute acid |
| below hydrogen | metal does not normally release hydrogen from dilute acid |
Write every member before using a position. Carbon and hydrogen are comparison benchmarks even though they are non-metals; they must remain in the stated order.
Do not omit carbon or hydrogen, and do not swap zinc and iron or copper and silver. The direction is from potassium, the most reactive, to gold, the least reactive.
Predict metal reactions from series position
| Metal and reagent | Expected result | Products |
|---|---|---|
| potassium + cold water | very vigorous | potassium hydroxide + hydrogen |
| sodium + cold water | vigorous | sodium hydroxide + hydrogen |
| calcium + cold water | reacts readily | calcium hydroxide + hydrogen |
| magnesium + steam | reacts on heating | magnesium oxide + hydrogen |
| magnesium + dilute HCl | vigorous | magnesium chloride + hydrogen |
| zinc + dilute HCl | moderate | zinc chloride + hydrogen |
| iron + dilute HCl | slower | iron(II) chloride + hydrogen |
| copper, silver or gold + dilute HCl | no reaction | none |
Examples: 2Na + 2H₂O → 2NaOH + H₂; Mg + H₂O(g) → MgO + H₂; Zn + 2HCl → ZnCl₂ + H₂.
Higher position means greater tendency to react. Metals above hydrogen displace H⁺ from dilute hydrochloric acid; metals below hydrogen do not. With water, the more reactive named metals react under milder conditions.
Keep conditions and products paired: cold water gives a hydroxide, steam gives an oxide. Magnesium is specified with steam; copper, silver and gold do not react with dilute hydrochloric acid.
Deduce a reactivity order from results
| Experimental result | Deduction |
|---|---|
| A displaces B from B ions | A > B |
| A does not displace B | A < B, if conditions are suitable |
| A reacts more vigorously with the same acid/water condition | A is more reactive |
| A reacts with cold water, B only with steam | A > B |
| A releases H₂ from dilute acid, B does not | A > H > B |
| carbon reduces A oxide but not B oxide | B > C > A |
Translate every observation into an inequality, combine overlapping inequalities into one chain, then check every result against the final order. For A > B and C > A, the combined order is C > A > B.
A no-reaction result gives a directional constraint only when the reagents and conditions would allow a displacement. Use multiple results to resolve unknowns and do not infer an exact gap in reactivity.
The deposited metal is the less reactive metal displaced from its ions. Do not reverse the inequality: if zinc deposits copper from Cu²⁺ solution, zinc is more reactive than copper.
Explain aqueous metal displacement
A more reactive metal has a greater tendency to lose electrons and form positive ions. It displaces a less reactive metal from an aqueous solution of that metal's ions.
| Required relative order | What a solid metal can displace below it |
|---|---|
| Mg > Zn > Fe > Cu > Ag | magnesium can displace Zn²⁺, Fe²⁺/Fe³⁺, Cu²⁺ and Ag⁺; zinc can displace iron, copper and silver ions; iron can displace copper and silver ions; copper can displace silver ions; silver displaces none above it |
Example: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s). Magnesium atoms lose electrons and form Mg²⁺; copper ions gain electrons and form copper metal.
Compare the solid metal with the metal named by the aqueous ion. If the solid is higher in the series, reaction occurs; if it is lower, write no reaction.
Do not compare the spectator ion such as sulfate or nitrate. The direction is controlled by the two metals: the more reactive atom forms positive ions while the less reactive ion becomes metal.
Explain aluminium's apparent unreactivity
Aluminium is reactive, but its surface rapidly forms a thin, adherent layer of aluminium oxide.
| Stage | Effect |
|---|---|
| aluminium surface contacts oxygen | Al₂O₃ layer forms |
| oxide layer covers the metal | air, water or reagent cannot easily reach fresh aluminium |
| contact is blocked | reaction appears absent or starts slowly |
| layer is removed or penetrated | the underlying aluminium can react |
The oxide layer explains delayed reaction with dilute hydrochloric acid or copper(II) sulfate and aluminium's resistance to further corrosion in air.
Apparent unreactivity does not mean aluminium is low in the reactivity series. It is above carbon; the protective, unreactive oxide coating creates the misleading observation.
9.5 Corrosion of metals
- Syllabus
- 0620–2026–2027
- Topic
- 9.5
- Level
- —
State the conditions for rusting
Iron and steel rust only when both oxygen and water are present. The rust formed is hydrated iron(III) oxide.
| Test condition | Oxygen present? | Water present? | Rust? |
|---|---|---|---|
| ordinary damp air | yes | yes | yes |
| dry air with a drying agent | yes | no | no |
| boiled water protected by an oil layer | no | yes | no |
Word equation: iron + oxygen + water → hydrated iron(III) oxide.
Oxygen alone or water alone is insufficient. Rust is hydrated iron(III) oxide, not anhydrous iron oxide or iron(III) hydroxide.
Name common barrier methods
A barrier method covers iron or steel with a layer that separates the metal from its surroundings.
| Common barrier method | Applied layer | Typical fit |
|---|---|---|
| painting | solid paint film | exposed structures and objects |
| greasing or oiling | grease/oil film | moving parts or surfaces needing renewal |
| coating with plastic | continuous polymer layer | objects that can be fully covered |
The coating must be continuous and maintained. Choose a method that can stay attached and cover the surface during use.
Galvanising also forms a barrier but has an additional sacrificial effect; it is treated separately from these common barrier-only methods.
Explain how barriers prevent rusting
A complete barrier prevents oxygen or water from reaching the iron surface, so the two reactants required for rusting cannot both contact the metal.
| Barrier state | Contact with iron | Outcome |
|---|---|---|
| intact paint, grease or plastic | oxygen and/or water excluded | rusting prevented |
| scratched, cracked or worn coating | oxygen and water can reach exposed iron | rusting can begin |
State the excluded substance explicitly: painting prevents oxygen and water from reaching the iron. A barrier does not make iron chemically unreactive.
A barrier works only while coverage is effective. Ordinary paint or plastic does not sacrificially protect exposed iron after a scratch.
Explain the two roles of zinc in galvanising
Galvanising coats iron or steel with zinc.
| Zinc coating condition | Protection mechanism |
|---|---|
| intact | barrier: zinc blocks oxygen and water from reaching iron |
| scratched so iron is exposed | sacrificial: zinc is more reactive and is oxidised in preference to iron |
Because zinc provides both mechanisms, galvanised iron can remain protected even when a small area of coating is damaged, unlike paint or plastic.
Do not say galvanising forms an alloy. It is a zinc coating, and the sacrificial action depends on zinc being more reactive than iron.
Explain sacrificial protection with electrons
Attach a metal above iron in the reactivity series, such as zinc or magnesium. The more reactive metal loses electrons and is oxidised instead of the iron.
| Step | Zinc example |
|---|---|
| sacrificial metal is more reactive | Zn is above Fe |
| sacrificial metal loses electrons | Zn → Zn²⁺ + 2e⁻ |
| electrons are supplied to the protected iron system | iron is prevented from losing electrons |
| consequence | zinc corrodes; iron is not oxidised to iron ions |
Magnesium and zinc can protect steel because they are more reactive than iron. Copper, silver and gold cannot: they are below iron and do not lose electrons in preference to it.
Sacrificial protection is not simply blocking oxygen or water. It can protect exposed iron because the attached metal undergoes oxidation and supplies electrons preferentially.
9.6 Extraction of metals
- Syllabus
- 0620–2026–2027
- Topic
- 9.6
- Level
- —
Relate extraction difficulty to reactivity
The more reactive a metal is, the more strongly it is combined in its ore and the harder it is to extract.
| Reactivity position | Typical occurrence or extraction route | Reason |
|---|---|---|
| very unreactive | may occur native as the element | little tendency to form compounds |
| below carbon | heat the oxide with carbon or carbon monoxide | carbon removes oxygen from the less reactive metal oxide |
| above carbon | electrolysis of a molten ionic compound | carbon cannot reduce the stable oxide |
Iron, zinc and copper are below carbon and their oxides can be reduced using carbon or carbon monoxide. Aluminium is above carbon, so aluminium oxide is electrolysed instead.
Electrolysis is not reserved only for aluminium, but it is required for metals above carbon. Do not propose carbon reduction for aluminium, magnesium, calcium, sodium or potassium compounds.
Describe iron extraction in the blast furnace
| Stage | Blast-furnace change | Purpose |
|---|---|---|
| 1 | coke burns in hot air to form carbon dioxide | releases heat |
| 2 | carbon dioxide reacts with more coke to form carbon monoxide | makes the reducing agent |
| 3 | carbon monoxide reduces hematite, iron(III) oxide, to iron | produces molten iron |
| 4 | limestone thermally decomposes to calcium oxide and carbon dioxide | makes a basic oxide |
| 5 | calcium oxide reacts with acidic silicon(IV) oxide impurity | forms calcium silicate slag |
The main inputs are hematite, coke, limestone and hot air. Molten iron and slag leave separately; waste gases include nitrogen, carbon dioxide and some carbon monoxide.
Coke has two linked roles: its combustion supplies heat, and it generates carbon monoxide, which removes oxygen from iron(III) oxide. Limestone removes silica impurity through lime and slag formation.
Carbon dioxide does not reduce hematite; carbon monoxide does. Limestone does not directly become slag: it first decomposes to calcium oxide, which then reacts with silicon(IV) oxide.
State how aluminium is obtained from bauxite
Bauxite is the main ore of aluminium and contains mainly aluminium oxide. Aluminium is extracted from aluminium oxide by electrolysis.
| Ore | Main aluminium compound | Extraction process |
|---|---|---|
| bauxite | aluminium oxide, Al₂O₃ | electrolysis of the molten electrolyte |
Aluminium lies above carbon in the reactivity series, so carbon cannot remove oxygen from aluminium oxide; electrical energy is used to reduce aluminium ions.
Hematite is an iron ore, not an aluminium ore. Bauxite is not itself the lowering agent or electrolyte solvent; purified aluminium oxide supplies the aluminium ions.
Write the five blast-furnace equations
| Process | Required equation |
|---|---|
| coke burns | C + O₂ → CO₂ |
| carbon monoxide forms | C + CO₂ → 2CO |
| hematite is reduced | Fe₂O₃ + 3CO → 2Fe + 3CO₂ |
| limestone decomposes | CaCO₃ → CaO + CO₂ |
| slag forms | CaO + SiO₂ → CaSiO₃ |
The hematite equation is redox: iron(III) oxide is reduced and carbon monoxide is oxidised. Limestone undergoes thermal decomposition. Slag formation is an acid–base reaction between basic CaO and acidic SiO₂.
Check atoms across every equation. In the reduction equation, Fe₂O₃ requires 2Fe and three CO molecules produce three CO₂ molecules.
Keep the two carbon oxides in sequence: coke first forms CO₂, then CO₂ reacts with more carbon to form CO. Do not reverse the slag equation or replace CO with CO₂ as the reducing agent.
Explain aluminium electrolysis
Purified aluminium oxide is dissolved in molten cryolite. Cryolite lowers the operating temperature and improves electrical conductivity, reducing the energy needed compared with melting pure aluminium oxide.
| Electrode | Ion change | Half-equation | Product |
|---|---|---|---|
| negative cathode | Al³⁺ gains electrons; reduction | Al³⁺ + 3e⁻ → Al | molten aluminium |
| positive carbon anode | O²⁻ loses electrons; oxidation | 2O²⁻ → O₂ + 4e⁻ | oxygen |
The oxygen produced at the positive anodes reacts with their carbon: C + O₂ → CO₂. The carbon anodes are therefore consumed and must be replaced regularly.
Aluminium forms at the cathode, not the anode. Cryolite is a solvent and lowers operating temperature; it is not a catalyst, aluminium source or protection for the carbon anodes. Details of bauxite purification are not required.