Topic 9 feels like a topic you can talk your way through — metals are shiny, alloys are hard, rust is bad. Then the challenge paper asks you to explain why an alloy is harder, and half the room writes "because the atoms are held together more strongly", which scores zero. This guide hunts every trap in the topic: the missing sliding layers, sacrificial protection confused with a barrier, aluminium described as "unreactive" when it is merely protected, carbon reduction chosen for aluminium, rusting attributed to water or oxygen instead of both, displacement predicted backwards, the reducing agent muddled with the substance reduced, and reactivity "explained" without ever mentioning electron loss. Fix these twelve things and Topic 9 becomes one of the most reliable scoring topics on the whole paper.
Twelve traps that cost students marks on Topic 9 questions. Every one of them appears on challenge papers regularly.
Six challenging questions broken down step by step. Try each step yourself before revealing the next.
Almost every candidate starts by describing the alloy. Start with pure copper instead: all its ions are the same size, so they pack into perfectly regular layers, and when a force is applied those layers slide over one another easily. That is why pure copper is soft. If you cannot state the baseline, you cannot explain the change.
Zinc atoms are a different size from copper atoms. Note the wording — "different", not "bigger". Either larger or smaller works, because what matters is that they distort / disrupt the regular arrangement of the layers. Do not say "they fill the gaps" or "they make it more tightly packed"; that is a different (and wrong) mechanism.
Because the layers are distorted, they can no longer slide over one another easily. More force is needed to deform the metal, so the brass is harder and stronger. Three sentences, three marks — and notice that the metallic bonding itself has not changed at all.
(i) The copper and zinc are simply mixed together; no chemical reaction occurs and no new substance is formed, so there is no fixed formula. (ii) The proportions can be varied — brass can be made with anything from 5% to 40% zinc, each with different properties. A compound could never do that.
A plug pin must conduct electricity (delocalised electrons free to move and carry charge) and must resist corrosion so the contact stays clean and the connection reliable. Both are creditworthy. "It is shiny" is not — it has nothing to do with the job.
Only reactive metals (K, Na, Ca, and slowly Mg) react with cold water. Y does, and gives an alkaline solution — the hydroxide. So Y is the most reactive, and W, X and Z are all below it. Never start with the fiddly displacement data; start with the test that separates the group most cleanly.
W and Z both react with dilute acid, so both are above hydrogen. X does not, so X is below hydrogen — the least reactive of the four. The rate is informative too: W is vigorous and Z is slow, which suggests W is above Z.
W displaces Z from its sulfate, which proves W is above Z — rate of fizzing on its own is only suggestive, since surface area and concentration also affect it. W does not displace X, consistent with X being the lowest. Always say "confirmed by two independent tests"; examiners award for the quality of the reasoning.
Y reacts with cold water → calcium (or sodium). W is vigorous with acid but not with cold water → magnesium or zinc. Z is slow with acid → iron. X is below hydrogen → copper (or silver). Check each suggestion against every observation before committing.
"More reactive" means the atoms lose their outer electrons more readily to form positive ions. Y's atoms lose electrons most readily, X's least readily. In the displacement W + ZSO₄, W is oxidised (W → W²⁺ + 2e⁻) and Z²⁺ ions are reduced to Z metal.
Iron ore (haematite, Fe₂O₃) — the source of the iron. Coke (carbon) — two jobs, see step 2. Limestone (calcium carbonate, CaCO₃) — removes the impurity. And blasted in at the bottom, hot air supplying the oxygen. Give the chemical name and formula each time; examiners often demand both.
(i) Coke burns in the hot air: C + O₂ → CO₂. This is strongly exothermic and provides the very high temperature the furnace needs. (ii) Further coke reacts with that carbon dioxide: CO₂ + C → 2CO, producing carbon monoxide, the reducing agent. Most candidates write only one of these and lose a guaranteed mark.
Check the balancing: 2 Fe each side, and oxygen 3 + 3 = 6 on the left against 6 in 3CO₂ on the right. The iron is produced molten (state symbol l) because the furnace is above 1538 °C, and it runs to the bottom because it is denser than the slag.
Reduced: iron(III) oxide, because it loses oxygen (and Fe³⁺ gains electrons). Reducing agent: carbon monoxide, because it removes the oxygen and is itself oxidised to CO₂. Write both justifications — the marks are usually for the reasons, not the names.
The ore contains silicon dioxide (sand), an acidic oxide. Limestone thermally decomposes: CaCO₃ → CaO + CO₂. The calcium oxide is a base and neutralises the sand: CaO + SiO₂ → CaSiO₃. The molten calcium silicate slag floats on the denser molten iron, so the two are tapped off separately — and the slag is sold for road building and cement.
Rusting needs water AND oxygen — both, every time. The product is hydrated iron(III) oxide. Do not write "air" (it hides which component matters) and do not add salt to the list (it accelerates, it is not required).
Tube A (control): nail in tap water, open to the air — both present. Tube B: nail in boiled water with a layer of oil on top — water present, oxygen removed. Tube C: nail in dry air over anhydrous calcium chloride, sealed with a bung — oxygen present, water removed. Identical nails, same temperature, same time.
Water is boiled to drive out dissolved air/oxygen; the oil layer stops air dissolving back in. Anhydrous calcium chloride is a drying agent that absorbs water vapour, and the bung stops moist air entering. Results: only tube A rusts; B and C show no rust. Conclusion: both water and oxygen are necessary.
Paint is a barrier: it keeps water and oxygen away from the iron. It works only while it is intact. Once scratched, water and oxygen reach the bare steel at the scratch and rusting begins there — and because rust is flaky and porous it lifts the surrounding paint and the damage spreads.
Zinc is a barrier too, but it is also more reactive than iron, so its atoms lose electrons more readily (Zn → Zn²⁺ + 2e⁻) and the zinc is oxidised in preference to the iron. The electrons flow into the steel, keeping it as metal. This sacrificial protection works even at a scratch, because only contact is needed; the zinc is gradually consumed and eventually needs renewing.
Aluminium is ABOVE carbon → carbon cannot displace it → electrolysis. Iron is BELOW carbon → carbon is the more reactive element and displaces it → reduction with carbon in the blast furnace. Draw the line through carbon on your paper before writing anything; the whole of part (a) then falls out.
"Aluminium is extracted by electrolysis" is one mark; "because aluminium is above carbon in the reactivity series, so carbon is not reactive enough to displace it from its oxide" is the second. Similarly for iron: "because iron is below carbon, so carbon (as carbon monoxide) can reduce the ore". Two methods, two reasons, four marks.
The oxide must be molten so the ions are free to move (cryolite is added to lower the melting point and save energy). Cathode (−): Al³⁺ + 3e⁻ → Al — reduction, gain of electrons. Anode (+): 2O²⁻ → O₂ + 4e⁻ — oxidation, loss of electrons. Check the electrons balance when you scale the two half-equations.
The anodes are made of carbon, and oxygen is produced at them at over 900 °C. The carbon therefore reacts with the oxygen to form carbon dioxide (C + O₂ → CO₂), so the anodes are burnt away and must be replaced regularly — adding cost and CO₂ emissions to an already expensive process.
Recycling aluminium skips an electrolysis that needs both a very high temperature and a huge continuous current — scrap is already the metal, so only melting (660 °C, a physical change) is needed: about a 95% saving. Recycling steel skips a blast furnace, which was far cheaper in energy to begin with because iron is below carbon, so the saving is real but much smaller. The energy saved mirrors the position in the reactivity series.
(i) The blue colour fades to colourless, because Cu²⁺(aq) is blue and Zn²⁺(aq) is colourless. (ii) A pink-brown solid (copper) is deposited on the zinc. (iii) The mixture warms up — displacement reactions are exothermic. Also acceptable: the grey zinc dissolves / gets smaller. "Copper is formed" is a deduction, not an observation.
Both metals form 2+ ions, so the equation balances 1:1 with no coefficients. State symbols matter: the zinc and copper are solids, the two sulfates are aqueous.
The sulfate ion is a spectator — it is SO₄²⁻(aq) before and after, so it is cancelled. Check that both atoms and charges balance: 2+ on the left, 2+ on the right.
Zn → Zn²⁺ + 2e⁻ — zinc loses electrons, so zinc is OXIDISED (and is the reducing agent). Cu²⁺ + 2e⁻ → Cu — copper ions gain electrons, so Cu²⁺ is REDUCED. Note it is the copper ion that is reduced, not "copper".
For Cu + ZnSO₄ to work, copper atoms would have to lose electrons to Zn²⁺ ions. But zinc is the more reactive metal, meaning zinc atoms lose electrons more readily than copper atoms. Copper therefore cannot reduce Zn²⁺, and no reaction occurs. Displacement runs downwards only.
Pairs of questions that look nearly identical but have different answers. Spot the key distinction.
Click each node to see how the subtopics connect.
Spot the error in each student's answer. Think before revealing.
Ten Cambridge-style challenge questions. Write your answer, then reveal the model answer with mark scheme and examiner's notes.