Cambridge IGCSE Chemistry · Syllabus 0620 · Metals
Metal
What is 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.
This definition is part of the Metals chapter in Cambridge IGCSE Chemistry.
Metal in context
Everything in Topic 9 hangs on one chain. A metal is a lattice of positive ions in a sea of delocalised electrons, and that structure gives the physical properties. Those properties decide what each metal is used for. Separately, metals can be put in order of how readily they react — the reactivity series — and that one position predicts a metal’s reactions with water, steam and acid, whether it can protect iron from rusting, and how hard it is to pull out of its ore. Extended candidates add a second layer to the same chain: reactivity is the tendency of an atom to form a positive ion, and that is what displacement reactions and sacrificial protection actually measure.
Common mistakes with Metal
- 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
- 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
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Examiner tips on Metal
- 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.