Metals

• If you can say where a metal sits relative to carbon and relative to hydrogen, you can already predict its acid reaction, its displacement behaviour, whether it can protect iron, and how it is extracted. Those two reference points do more work than any other fact in this chapter.
• Eleven danger zones, one habit that defuses most of them: before writing a sentence about a metal, say silently where it sits relative to carbon, relative to hydrogen, and relative to iron. Those three comparisons decide extraction, acid reaction and protection respectively.
• Alloys can be harder and stronger than the pure metals, and this makes them more useful.
• Optional mnemonic, if you want one: Kings Name Castles Magnificent, Always Calling Zinc Fearless; Heroes Cut Against Authority. Use it only as a scaffold — the chunk meanings above are what you actually reason with.
• The main ore of aluminium is bauxite. Aluminium is extracted from it by electrolysis.
• Recite the chain, not the list: coke burns → heat and carbon dioxide → carbon dioxide over hot coke → carbon monoxide → carbon monoxide reduces the ore → molten iron → and separately, limestone decomposes → calcium oxide → calcium oxide takes the sand → slag.
• If you are on the Core route and revise only one of these, revise number 4: three reagents, three product sets, and the metals that go with each. If you are on the Extended route, revise number 6 as well — barrier versus sacrificial is the single distinction that appears in the most different question types.
• Thirty-eight examples, one route: evidence → position or rule → equation or method → observation or role → audit → answer. If a question ever leaves you stuck, write down what you have been given and turn it into a comparison. That single move unlocks most of Topic 9.

• Core 9.1.1 Compare the general physical properties of metals and non-metals: thermal conductivity, electrical conductivity, malleability and ductility, and melting and boiling points.
• Core 9.1.2 Describe the general chemical properties of metals: their reactions with dilute acids, with cold water and steam, and with oxygen.
• Core 9.2.1 Describe four uses of metals in terms of their physical properties: aluminium in aircraft, in overhead electrical cables and in food containers, and copper in electrical wiring.
• Core 9.3.1 Describe an alloy as a mixture of a metal with other elements, and give the composition of brass and of stainless steel.
• Core 9.3.3 Describe the uses of alloys in terms of their physical properties, including stainless steel in cutlery because it is hard and resists rusting.
• Core 9.4.2 Describe the reactions, if any, of potassium, sodium and calcium with cold water, magnesium with steam, and magnesium, zinc, iron, copper, silver and gold with dilute hydrochloric acid — and explain each one from the metal’s position in the reactivity series.
• Core 9.5.3 Describe how barrier methods prevent rusting, by excluding oxygen or water.
• Core 9.6.1 Describe the ease of obtaining a metal from its ore, related to its position in the reactivity series.
• Core 9.6.2 Describe the extraction of iron from hematite in the blast furnace: burning the coke, reducing carbon dioxide to carbon monoxide, reducing iron(III) oxide with carbon monoxide, decomposing the limestone, and forming the slag. Symbol equations are not required.
• Core 9.3.2 State that alloys can be harder and stronger than the pure metals, and are more useful.
• Core 9.4.1 State the order of the reactivity series: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.
• Core 9.5.1 State the conditions required for rusting of iron and steel, and name the product as hydrated iron(III) oxide.
• Core 9.5.2 State some common barrier methods: painting, greasing and coating with plastic.
• Core 9.6.3 State that the main ore of aluminium is bauxite, and that aluminium is extracted by electrolysis.
• Core 9.3.4 Identify representations of alloys from diagrams of structure.
• Core 9.4.3 Deduce an order of reactivity from a given set of experimental results.
• Supplement 9.4.4 Describe the relative reactivities of metals in terms of their tendency to form positive ions, by displacement reactions with the aqueous ions of magnesium, zinc, iron, copper and silver.
• Supplement 9.5.4 Describe the use of zinc in galvanising as an example of a barrier method and of sacrificial protection.
• Supplement 9.6.5 Describe the extraction of aluminium from purified bauxite / aluminium oxide: the role of cryolite, why the carbon anodes need replacing, and the reactions at the electrodes including ionic half-equations.
• Supplement 9.3.5 Explain, in terms of structure, how alloys can be harder and stronger than the pure metals, because the different-sized atoms mean the layers can no longer slide over each other.
• Supplement 9.4.5 Explain the apparent unreactivity of aluminium in terms of its oxide layer.
• Supplement 9.5.5 Explain sacrificial protection in terms of the reactivity series and in terms of electron loss.
• Supplement 9.6.4 State the five symbol equations for the extraction of iron from hematite.

Why this matters. This is the one process in Topic 9 where the electrode signs, the ion movements and the electron counts all have to be right at once. On the Extended route every part of it is examinable, and each part is a separate thing to get right.

Malleability

The property of a material that allows it to be hammered, rolled or pressed into a new shape without breaking. In metals it arises because layers of identical positive ions can slide over one another while the delocalised electrons keep holding the structure together.

Metal Hydroxide

The compound formed when a reactive metal reacts with cold water, alongside hydrogen gas. A soluble metal hydroxide gives an alkaline solution, which is why universal indicator turns purple when potassium, sodium or calcium reacts with water.

Alloy

A mixture of a metal with one or more other elements. The components keep their own identities and there is no fixed formula, because no chemical reaction has taken place; metallic bonding still holds the structure together, which is why an alloy is still a metal in its behaviour.

Barrier Protection

A method of preventing rust by covering iron or steel with a layer that keeps oxygen and water away from the metal surface. Painting, greasing and coating with plastic all work this way, and all of them stop working at any point where the layer is broken.

Order of Reactivity

An arrangement of metals from most reactive to least reactive, worked out from experimental results rather than recalled. Each result gives one comparison between two things; chaining the comparisons together gives the order, and any pair the results never compare stays undetermined.

Reactivity Series

An order of metals arranged by how readily each one reacts, with the most reactive at the top. Carbon and hydrogen are included as non-metal reference points: carbon marks the boundary for extraction method and hydrogen marks the boundary for reaction with dilute acids.

Metal

An element whose atoms are held in a giant lattice of positive ions surrounded by delocalised electrons. Metals are generally good conductors of heat and electricity, malleable and ductile, and have high melting and boiling points; chemically they tend to lose electrons and form positive ions.

Metal Oxide

A compound formed when a metal combines with oxygen. Metal oxides are generally basic, and most metals occur in the Earth as their oxides or as other compounds, which is why they must be extracted rather than dug up as the element.

Rusting

The corrosion of iron and steel, which happens only when both oxygen and water are present and produces hydrated iron(III) oxide. Salt speeds rusting up but is not one of the required conditions, and the word rust applies to iron and steel alone, not to the corrosion of other metals.

Ductility

The property of a material that allows it to be drawn out into a thin wire without snapping. In metals it has the same structural cause as malleability: layers of identical positive ions slide past one another while the delocalised electrons continue to bond the structure.

Stainless Steel

An alloy of iron with elements such as chromium, nickel and carbon. It is hard and it resists rusting, and those two properties together make it the standard material for cutlery and surgical instruments.

Sacrificial Protection

A method of preventing rust in which a more reactive metal is attached to iron or steel and is corroded in its place. The more reactive metal loses electrons in preference to the iron, so the iron does not form ions and does not rust; the attached metal is gradually eaten away and is replaced.

Bauxite

The main ore of aluminium. It is purified to aluminium oxide before the metal is obtained from it by electrolysis, which is necessary because aluminium is above carbon in the reactivity series and so its oxide cannot be reduced by carbon.

Blast Furnace

The tall furnace in which iron is extracted from its ore. Hematite, coke and limestone are fed in at the top and hot air is blasted in at the bottom; carbon monoxide formed from the coke reduces the iron(III) oxide to molten iron, while calcium oxide from the limestone removes sandy impurities as molten slag.

Aluminium Electrolysis

The extraction of aluminium by passing electricity through purified aluminium oxide dissolved in molten cryolite. Aluminium ions gain electrons at the negative cathode to form molten aluminium, oxide ions lose electrons at the positive carbon anodes to form oxygen, and that oxygen burns the anodes away so they must be replaced.

Density

The mass of a material per unit volume. A low-density material gives a large object for a small mass, which is why aluminium rather than a denser metal is chosen for aircraft bodies and for overhead cables that must not sag under their own weight.

Galvanising

Coating iron or steel with a layer of zinc. The zinc acts as a barrier that keeps oxygen and water away from the iron, and because zinc is more reactive than iron it also acts sacrificially, so the iron is still protected at any point where the coating is scratched.

Metal Ore

A rock containing enough of a metal compound to make extraction of the metal worthwhile. Most metals occur combined with oxygen or other elements rather than as the element itself, and how tightly the metal holds on to that combination is set by its position in the reactivity series.

Protective Oxide Layer

A thin, tough, continuous layer of aluminium oxide that forms on the surface of aluminium in air. It sticks firmly to the metal and keeps air, water and acids away from it, so the metal beneath appears unreactive even though aluminium is high in the reactivity series.

Corrosion Resistance

The ability of a metal to stay unattacked in air, water or food acids. Aluminium resists corrosion well, which is why it is used for food containers and foil; the reason it does so is an Extended-route explanation covered in lesson 9.4F.

• 1. “All metals have high melting points and all non-metals are insulators.” Repair The comparison is general, not universal. Mercury is a liquid at room temperature; graphite is a non-metal that conducts electricity. Write “metals generally have higher melting points” and the mark is safe. Where Lesson 9.1A. Core 9.1.1
• 2. “Malleable means it can be drawn into wires.” Repair Malleable = can be hammered or pressed into shape. Ductile = can be drawn out into a wire. Copper wiring needs ductility; aluminium foil needs malleability. Swapping them answers a different question from the one asked. Where Lesson 9.1A. Core 9.1.1
• 3. “An alloy is a compound of two metals.” Repair An alloy is a mixture of a metal with other elements. There is no fixed formula, no chemical reaction has happened, and the metallic bonding is still there. Also note “other elements”, not “other metals” — carbon in steel is not a metal. Where Lesson 9.3A. Core 9.3.1
• 4. “Copper displaces iron from iron(II) sulfate.” Supplement 9.4.4 Repair Displacement only runs downhill: the more reactive metal takes the place of the less reactive one. Copper is below iron, so nothing happens. Check the direction before you write an equation, every time. Where Lesson 9.4E. Supplement 9.4.4
• 5. “Carbon and hydrogen are in the series, so they must be metals.” Repair They are non-metal reference points. Carbon sits between aluminium and zinc and marks the boundary between electrolysis and reduction. Hydrogen sits between iron and copper and marks the boundary for reaction with dilute acid. Get their positions wrong and two whole subtopics collapse. Where Lesson 9.4A. Core 9.4.1
• 6. “Aluminium does not react with water, so it must be low in the reactivity series.” Supplement 9.4.5 Repair Aluminium is high in the series, above zinc and iron. It behaves as though it were unreactive because a thin, tough, continuous layer of aluminium oxide forms on the surface and stops air and water reaching the metal. Say “oxide layer”, never “low reactivity”. Where Lesson 9.4F. Supplement 9.4.5
• 7. “Iron needs oxygen, water and salt to rust.” Repair The two required conditions are oxygen and water. Salt speeds rusting up but is not necessary — a nail rusts perfectly well in pure water and air. Listing salt as a condition contradicts the very experiment that establishes the conditions. Where Lesson 9.5A. Core 9.5.1
• 8. “Any metal coating protects the iron underneath, even when scratched.” Supplement 9.5.4 Repair Only a more reactive coating keeps protecting after a scratch, because it loses electrons in preference to the iron. Zinc does; tin does not. A tin coating is a barrier only, and a scratched tin can rusts faster than bare iron. Where Lessons 9.5B and 9.5C. Supplement 9.5.4
• 9. “Coke reduces the iron(III) oxide in the blast furnace.” Repair In the route you are required to give, the reducing agent is carbon monoxide: it is the substance that takes the oxygen away from the iron(III) oxide. The coke does two other jobs — it burns to release heat and carbon dioxide, and it then converts that carbon dioxide into the carbon monoxide that does the reducing. Extended Supplement 9.6.4 Extended candidates give the same repair as a symbol equation, \(\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}\). Core candidates are not asked for it — the syllabus states that symbol equations are not required for the furnace. Where Lesson 9.6B. Core 9.6.2
• 10. “Cryolite is a catalyst.” Supplement 9.6.5 Repair It is a solvent for the aluminium oxide. Dissolving the oxide in molten cryolite lets the cell run at a much lower temperature than molten aluminium oxide alone would need, so less energy is used. A catalyst changes the rate of a reaction; cryolite changes what the electrolyte is. Where Lesson 9.6D. Supplement 9.6.5
• 11. “The anode is negative because it attracts positive ions.” Supplement 9.6.5 Repair In electrolysis the cathode is negative and attracts the positive \(\mathrm{Al^{3+}}\) ions; the anode is positive and attracts the negative \(\mathrm{O^{2-}}\) ions. Electrons are gained at the cathode and lost at the anode. Getting the signs backwards turns two correct half-equations into a wrong answer. Where Lesson 9.6D. Supplement 9.6.5
• 1. “The limestone reduces the iron(III) oxide.” Defect Limestone is nowhere near the reduction. It decomposes to calcium oxide and then removes the silica. Reason Reduction means removing oxygen. Limestone gives off carbon dioxide; it does not take oxygen from anything. Repair “Carbon monoxide reduces the iron(III) oxide. Limestone removes the silica impurity as slag.” Transfer If no limestone were added, what would still work and what would fail? Answer: the iron would still be reduced, but the silica would stay in it as an impurity.
• 2. “Coke acts as a catalyst.” Defect A catalyst is not used up and is not a reactant. Coke is consumed continuously and must be fed in constantly. Reason Coke is used up in two of the five stages — it burns in the hot air blast, and it then reacts with the carbon dioxide to make carbon monoxide. Anything that is consumed in the furnace and has to be fed in again is a reactant, not a catalyst. Repair “Coke is a fuel and the source of the reducing agent.” Transfer Name one substance in the furnace that is essentially unchanged. Answer: the nitrogen in the air blast, which passes through and leaves in the waste gases.
• 3. “Carbon dioxide reduces the iron(III) oxide.” Defect Carbon dioxide is a product of the reduction, not the agent of it. Reason Carbon dioxide is already fully combined with oxygen, so it has no appetite for more. Carbon monoxide has room to take one more oxygen, which is exactly what it does. Repair “Carbon monoxide reduces the iron(III) oxide, and is itself oxidised to carbon dioxide.” Carbon monoxide goes in; carbon dioxide comes out. Transfer Which step turns carbon dioxide back into a reducing agent? Answer: step 2 — carbon dioxide passing over hot coke is converted to carbon monoxide, so the furnace recycles it.
• 4. “Slag is the waste iron that did not melt properly.” Defect Slag contains no iron. It is calcium silicate, formed from the limestone and the sandy impurity. Reason If slag were unmelted iron the process would be losing most of its product, and there would be no reason to add limestone at all. Repair “Slag is molten calcium silicate, \(\mathrm{CaSiO_3}\), formed when calcium oxide reacts with silica. It floats on the iron and is tapped off separately.” Transfer Why does slag float? Answer: it is less dense than molten iron.
• 5. “Calcium oxide is one of the raw materials charged into the furnace.” Defect Calcium oxide is made inside the furnace. The raw material is calcium carbonate — limestone. Reason Step 4 exists precisely to produce the calcium oxide. If it were charged directly, step 4 would be pointless. Repair Name the three charged solids as hematite, coke and limestone. Calcium oxide is an intermediate. Transfer Name the other substance produced in step 4. Answer: carbon dioxide, which step 2 can convert into more carbon monoxide.
• 6. “\(\mathrm{Fe_2O_3 + CO \rightarrow Fe + CO_2}\)” Defect Not balanced. Fe: 2 on the left, 1 on the right. O: \(3 + 1 = 4\) on the left, 2 on the right. Reason A correct-looking equation with the right species is still wrong if the numbers do not match. Balance and chemistry are two separate checks. Repair \(\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}\). Fe 2 = 2, C 3 = 3, O 6 = 6. Transfer Balance the equation for the reduction of copper(II) oxide by carbon monoxide. Answer: \(\mathrm{CuO + CO \rightarrow Cu + CO_2}\); Cu 1 = 1, C 1 = 1, O 2 = 2 — already balanced as written.
• 1. “All metals have high melting points and all non-metals are gases.” Core 9.1.1 Defect Two universal claims where the syllabus makes general ones. Reason Mercury is a metal that is liquid at room temperature; sulfur and iodine are solid non-metals. One counter-example destroys a universal claim. Repair “Metals generally have higher melting points than non-metals.” Transfer Name a metal that breaks the pattern and a non-metal that breaks it. Mercury; diamond.
• 2. “Copper is malleable, which is why it is drawn into wires.” Core 9.1.1 Defect The wrong one of the two words. Reason Malleable = hammered or pressed into shape. Ductile = drawn into a wire. Wires need ductility. Repair “Copper is ductile, so it can be drawn into thin wires.” Transfer Which word applies to aluminium cooking foil? Malleable — foil is rolled into a sheet.
• 3. “Non-metals never conduct electricity.” Core 9.1.1 Defect Universal claim again, and this one has a famous exception. Reason Graphite is a non-metal with delocalised electrons between its layers, so it conducts. Repair “Non-metals are generally insulators; graphite is the exception.” Transfer Why does graphite conduct when diamond does not? Graphite has electrons that are free to move; in diamond every electron is held in a bond.
• 4. “An alloy is a compound formed when two metals are melted together.” Core 9.3.1 Defect Calls a mixture a compound. Reason No reaction occurs; there is no fixed formula, and the proportions can be varied while it remains the same alloy. Repair “An alloy is a mixture of a metal with other elements.” Transfer Give one piece of evidence that brass is a mixture. Its composition can be varied; the copper and zinc are not chemically bonded.
• 5. “An alloy is a mixture of two metals.” Core 9.3.1 Defect Too narrow — and it makes stainless steel impossible to describe. Reason Carbon in stainless steel is a non-metal, and it is still part of the alloy. Repair “A mixture of a metal with one or more other elements.” Transfer Name the elements in stainless steel. Iron, with chromium, nickel and carbon.
• 6. “Alloys are harder because the smaller atoms fill the gaps between the big ones.” Supplement 9.3.5 Defect Invents a mechanism the syllabus does not use, and it is not what the diagram shows. Reason The required mechanism is about layers, not gaps: differently sized particles disrupt the regular layers. Repair “Different-sized particles disrupt the regular layers, so the layers cannot slide over each other easily.” Transfer Does an alloy have to contain smaller particles? No — larger ones disrupt the layers just as well.
• 7. “This diagram shows an alloy because the circles are two different colours.” Core 9.3.4 Defect Uses a drawing convention as physical evidence. Reason Colour is chosen by whoever drew the diagram. Size, or a key naming a second element, is what tells you. Repair “The particles are of different sizes, so this is an alloy.” Transfer A diagram has equal circles in two shades, no key. What can you conclude? Nothing about alloying — the evidence does not support a conclusion either way.
• 8. “Aluminium is used for drinks cans because it is unreactive.” Core 9.2.1 Defect Contradicts subtopic 9.4, where aluminium is above zinc and iron. Reason Resisting corrosion and being unreactive are not the same claim. Aluminium is high in the series and still resists corrosion. Repair “Aluminium resists corrosion, so the food acids do not attack it.” That is the whole Core answer. Transfer Which subtopic does the claim contradict? 9.4 — the reactivity series, where aluminium sits above zinc and iron. Extended Supplement 9.4.5 Extended candidates can also give the reason: a thin, tough, continuous oxide layer keeps air, water and acids off the metal. Core candidates are not asked for it.
• 9. “Overhead cables should be copper, because copper is the better conductor.” Core 9.2.1 Defect Optimises the wrong property for the situation. Reason The cable also has to hold itself up over a long span. Aluminium’s low density is what makes that possible. Repair “Aluminium is used, because it has a low density and is a good conductor.” Transfer Why is copper right for house wiring? A short wire inside a wall does not have to support itself, so the better conductivity wins.
• 10. “Carbon and hydrogen are the least reactive metals in the series.” Core 9.4.1 Defect Calls two non-metals metals, and misplaces both. Reason They are reference points. Carbon sits between aluminium and zinc; hydrogen between iron and copper. Repair “Carbon and hydrogen are non-metals included as reference points.” Transfer What does each marker predict? Carbon: the extraction method. Hydrogen: whether the metal reacts with a dilute acid.
• 11. “\(\mathrm{Cu(s) + FeSO_4(aq) \rightarrow CuSO_4(aq) + Fe(s)}\)” Supplement 9.4.4 Defect Displacement written in the wrong direction. Reason Copper is below iron, so it cannot lose electrons to iron(II) ions. Repair “No reaction, because copper is less reactive than iron.” Transfer Which way round does it work? \(\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu(s)}\).
• 12. “\(\mathrm{Zn(s) + Ag^+(aq) \rightarrow Zn^{2+}(aq) + Ag(s)}\)” Supplement 9.4.4 Defect The chemistry is right but the charges do not balance: \(-1\) net difference between the sides. Reason A zinc atom releases two electrons, but a single silver ion accepts only one. Repair \(\mathrm{Zn(s) + 2Ag^+(aq) \rightarrow Zn^{2+}(aq) + 2Ag(s)}\). Charge: \(+2\) on both sides. Transfer Balance copper with silver ions. \(\mathrm{Cu(s) + 2Ag^+(aq) \rightarrow Cu^{2+}(aq) + 2Ag(s)}\).
• 13. “Magnesium reacts with steam to give magnesium hydroxide and hydrogen.” Core 9.4.2 Defect The cold-water product used for the steam reaction. Reason A hydroxide needs liquid water. With steam the product is the oxide. Repair \(\mathrm{Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)}\). Transfer Which state symbol on the water shows it was steam? \(\mathrm{(g)}\).
• 14. “\(\mathrm{Fe + 3HCl \rightarrow FeCl_3 + H_2}\)” Core 9.4.2 Defect Wrong oxidation state, and unbalanced hydrogen as well. Reason Iron with a dilute acid gives iron(II). And three hydrogens on the left cannot form one \(\mathrm{H_2}\). Repair \(\mathrm{Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)}\). Transfer What colour is the solution formed? Pale green, from \(\mathrm{Fe^{2+}(aq)}\).
• 15. “Copper does not react with dilute acid because it has a protective oxide layer.” Core 9.4.2 Defect Borrows aluminium’s explanation for a metal that does not need one. Reason Copper genuinely is unreactive towards dilute acid, because it sits below hydrogen. No barrier is involved. Repair “Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid.” Transfer Which metal does need the oxide-layer explanation, and why? Aluminium — because its position predicts a reaction that is not observed.
• 16. “Iron rusts when oxygen, water and salt are present.” Core 9.5.1 Defect Promotes a rate factor to a required condition. Reason A nail in ordinary water and air rusts perfectly well, so salt cannot be necessary. Repair “Oxygen and water are required. Salt increases the rate.” Transfer Which tube in the four-tube experiment proves salt is not required? Tube A — it rusted without any.
• 17. “Rust is iron oxide, \(\mathrm{Fe_2O_3}\).” Core 9.5.1 Defect Two pieces missing: the oxidation state and the water. Reason Iron forms more than one oxide, so the state must be given; and rust is hydrated, which is why water is one of the conditions. Repair “Hydrated iron(III) oxide, \(\mathrm{Fe_2O_3{\cdot}xH_2O}\).” Transfer Why does the formula contain \(x\)? The amount of water in rust is variable, not fixed.
• 18. “The aluminium window frames have rusted.” Core 9.5.1 Defect Uses “rust” for a metal that cannot rust. Reason Rusting is specific to iron and steel. Other metals corrode. Repair “The aluminium has corroded” — and in fact aluminium resists corrosion well, which is why it is used for food containers (Core 9.2.1). Transfer Name two other metals that corrode without rusting. Copper, which goes green; silver, which tarnishes. Extended Supplement 9.4.5 Extended candidates can add why aluminium resists corrosion: a thin, tough, continuous oxide layer keeps air and water off the metal. Core candidates are not asked for it.
• 19. “A scratched tin can is still protected, because tin is a metal coating.” Supplement 9.5.4 Defect Assumes any metal coating is sacrificial. Reason Tin is below iron, so once the coating is broken the iron loses electrons in preference to the tin. The can rusts faster than bare steel would. Repair “Tin is a barrier only. Only a coating more reactive than iron protects after damage.” Transfer Which coating would still protect? Zinc — it is above iron.
• 20. “Sacrificial protection works because the zinc covers the iron and keeps water off it.” Supplement 9.5.5 Defect Describes the barrier mechanism and calls it sacrificial. Reason A zinc block bolted to a ship’s hull covers almost none of it, yet still protects. Covering is not the mechanism. Repair “Zinc is more reactive than iron, so it loses electrons in preference to the iron, and the iron is not oxidised.” Transfer What must be true of the connection between the two metals? They must remain in contact, so electrons can pass.
• 21. “The coke reduces the iron(III) oxide in the blast furnace.” Core 9.6.2 Defect Names the wrong reducing agent for the required route. Reason The coke burns to carbon dioxide and then converts it to carbon monoxide; it is the carbon monoxide that reduces the ore. Repair “Carbon monoxide reduces the iron(III) oxide”, with coke named as the fuel and as the source of that carbon monoxide. Transfer Which stage makes the reducing agent? The stage in which carbon dioxide passes over hot coke and is converted to carbon monoxide. Extended Supplement 9.6.4 As symbol equations: \(\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}\) for the reduction, and \(\mathrm{CO_2 + C \rightarrow 2CO}\) for the stage that makes the reducing agent.
• 22. “Cryolite is a catalyst that speeds up the electrolysis.” Supplement 9.6.5 Defect Assigns a role the substance does not have. Reason Cryolite is the solvent. Its effect is on the temperature the electrolyte melts at, not on a reaction rate. Repair “Cryolite dissolves the aluminium oxide so the cell runs at a much lower temperature, saving energy.” Transfer Why does that matter industrially? Less energy is needed to keep the electrolyte molten, so the process costs less.
• 23. “The anode is negative because the negative ions go there.” Supplement 9.6.5 Defect Reasons backwards from the ions to the sign. Reason The anode is positive, which is precisely why negative ions are attracted to it. Opposite charges attract. Repair “The anode is positive and attracts the negative oxide ions; the cathode is negative and attracts the positive aluminium ions.” Transfer On which side of the anode half-equation do the electrons go? The right — they are lost.
• 24. “Carbon dioxide is produced at the anode in the aluminium cell.” Supplement 9.6.5 Defect Confuses the electrode product with what happens to it next. Reason Electrolysis discharges the ions present. The only negative ion is the oxide ion, so the product is oxygen. Repair “Oxygen is produced at the anode. It then reacts with the hot carbon anodes to form carbon dioxide.” Transfer What is the practical consequence? The anodes are burnt away and must be replaced.

• Reading the numbers. 0620 numbers its statements continuously inside each subtopic, running through the Core ones and then the Supplement ones. So 9.4 goes Core 9.4.1, 9.4.2, 9.4.3, then Supplement 9.4.4 and 9.4.5 — and a Supplement statement can carry a lower-looking number than a Core statement in the next subtopic. The tag on each block tells you the route; the number alone does not.
• Where these six land. Check 1 carries the whole of 9.4. Check 2 is needed only on the Extended route, for Supplement 9.4.4 and 9.5.5. Check 3 carries every equation in 9.1, 9.4 and 9.6. Checks 4 and 5 are practical-paper skills, for Core and Extended candidates alike. Check 6 is the reading skill that Core 9.3.4 is built on.
• Where the seven Supplement statements sit. One in 9.3, two in 9.4, two in 9.5 and two in 9.6 — and none at all in 9.1 or 9.2, whose syllabus pages print an empty Supplement column. So properties and uses are identical for both routes, and every tier decision in this topic is about alloy structure, displacement, aluminium, rust protection or the extraction equations.
• A structural mistake worth avoiding. It is easy to answer explain questions with observations, and describe questions with theory. If the question gives you an experiment, start with what is seen. If the question gives you a metal and a reagent, start with the position in the series.
• A Core candidate is never penalised for knowing more — but a Core answer that reaches for a Supplement mechanism instead of the required Core reason can miss the point of the question. If you are on the Core route and you find yourself writing about electrons, check whether the question was asking for a position.
• Two-part structure. “Aluminium is used for aircraft because it has a low density” is half the answer. Adding “so the aircraft has a smaller mass and uses less fuel” is the other half. The property alone is never the whole answer.
• Two properties, not one property twice. “It is hard so it does not bend, and it is strong so it does not break” is one idea written twice, so it gives only one of the two properties. Hardness and rust resistance are genuinely different properties, and they are the two the statement names.
• How to write a no-reaction answer. “Nothing happens” is not an answer. “No reaction, because copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid” is. The reason is the answer; the observation is only the evidence.
• On the displacement row. “A displaced B” is a result you may be given, and reading it as “A is more reactive than B” is all this statement needs. Knowing without being told which of magnesium, zinc, iron, copper and silver displaces which — and explaining displacement in terms of forming positive ions — is Supplement 9.4.4, in lesson 9.4E.
• Why silver needs a 2. A silver ion carries only one positive charge, so it takes only one electron. A magnesium, zinc, iron or copper atom releases two. Two silver ions are therefore needed to accept them — hence \(\mathrm{2Ag^+}\) and \(\mathrm{2Ag}\). Check the charge on both sides every time: in \(\mathrm{Cu(s) + 2Ag^+(aq) \rightarrow Cu^{2+}(aq) + 2Ag(s)}\), the left side is \(2+\) and so is the right.
• The naming mark. Rust is hydrated iron(III) oxide. Writing “iron oxide” misses the oxidation state; writing \(\mathrm{Fe_2O_3}\) alone misses the water. The full answer is the name plus \(\mathrm{Fe_2O_3{\cdot}xH_2O}\).
• Read the statement carefully. Core 9.6.2 lists five stages of the blast furnace and then says, in the syllabus itself, “Symbol equations are not required”. Everything in this lesson is therefore in words. Extended candidates write the same five stages as balanced symbol equations under Supplement 9.6.4, in the equation ladder that follows.
• The most valuable single sentence in 9.6B. “The reducing agent is carbon monoxide.” Coke is the source of it and the source of the heat, but coke is not what takes the oxygen off the iron(III) oxide in the route you are required to give.
• How much to write. This statement opens with State. “The main ore of aluminium is bauxite” and “aluminium is extracted by electrolysis” are the two answers, and each stands on its own. If the question adds “explain why electrolysis is used”, add the carbon comparison — and stop there.
• How to use these again. On a second pass, cover the answers and time yourself. A Core candidate should get through R1–R72 in about 20 minutes once the topic is solid; an Extended candidate should add another 10 for R73–R104. Mark only the ones you got completely right — a half-remembered equation is a wrong answer in an exam.
• Marking yourself honestly. On the Extended route, if you wrote “because zinc is more reactive” but did not say so it loses electrons in preference to the iron, that is half the explanation, not all of it. On the Core route the equivalent trap is naming a property without its consequence, or a reaction without the position in the series that explains it. Either way, the second half of the explanation is where most of Topic 9’s marks live.
• If a session goes badly. Do not restart the chapter. Identify which statement failed — the mastery checklist gives you the code and the official reference — and re-read only the lesson that owns it. Topic 9 is unusually well suited to this, because almost every failure traces back to one specific position in the reactivity series.
• If you are on the Core route and short of time, the highest-value 20 minutes in this chapter is the reactivity series plus R32–R50: those prompts carry three Core statements between them — the named reactions, the deduction skill, and the series itself — and they feed straight into 9.5 and 9.6.