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Almost every metal we rely on — the iron in a bridge, the aluminium in a drinks can, the copper in the wires around your home — started life as rock dug from the ground. Only a handful of metals are found as the pure element; the great majority are locked away inside compounds and have to be extracted. What decides how a metal is extracted is a single idea you already know: its position in the reactivity series. This lesson opens Topic C6 (Global challenges) of OCR Gateway Combined Science A by looking at where metals come from and at the two great extraction routes — reduction with carbon and electrolysis — and why the reactivity series tells you which one to use.
By the end of this lesson you should be able to explain why most metals are found as compounds, choose the correct extraction method for a metal from its reactivity relative to carbon, describe reduction in terms of oxygen, and explain why the most reactive metals were the last to be discovered.
This lesson blends AO1 (recalling that reduction is loss of oxygen and why unreactive metals occur native) with AO2, where you apply a metal's position in the reactivity series to decide whether carbon reduction or electrolysis is the right extraction method.
An ore is a rock that contains enough of a metal compound to make extracting the metal worthwhile. Most metals occur in their ores as compounds — very often as oxides (for example iron(III) oxide, Fe2O3, in the ore haematite, or aluminium oxide, Al2O3, in the ore bauxite), though some occur as carbonates or sulfides.
The reason so many metals are found combined is that they are reactive enough to have reacted, over millions of years, with substances around them — especially the oxygen in the air. The only metals found native (as the uncombined element) are the very unreactive ones, chiefly gold. Gold sits so low in the reactivity series that it does not react with oxygen, water or acids, which is why gold nuggets can be panned straight from a river while reactive metals never turn up as the free element.
Exam Tip: "Found as a compound" and "found native" link straight back to reactivity. A metal is found native only when it is very unreactive (gold); the more reactive a metal, the more firmly it is locked inside a compound, usually an oxide.
The central idea of this lesson is that the method used to extract a metal depends on its position in the reactivity series relative to carbon:
The flowchart below summarises the decision.
flowchart TD
A["A metal needs extracting<br/>from its oxide ore"] --> B{"More or less reactive<br/>than carbon?"}
B -->|"LESS reactive than carbon<br/>(zinc, iron, copper)"| C["Reduce the oxide by<br/>heating with carbon<br/>(cheaper)"]
B -->|"MORE reactive than carbon<br/>(aluminium, magnesium,<br/>sodium and above)"| D["Extract by electrolysis of<br/>the molten compound<br/>(expensive)"]
C --> E["e.g. iron in the<br/>blast furnace"]
D --> F["e.g. aluminium from<br/>molten aluminium oxide"]
| Metal's position | Extraction method | Examples |
|---|---|---|
| More reactive than carbon | Electrolysis of the molten compound | Aluminium, magnesium, sodium |
| Less reactive than carbon | Reduction with carbon | Iron, zinc, lead, copper |
| Very unreactive | Found native — little extraction needed | Gold |
Exam Tip: The single dividing line is carbon. Below carbon (iron, copper, zinc) → reduction with carbon; above carbon (aluminium and higher) → electrolysis. Quote carbon's position whenever you justify a method.
A metal below carbon can be extracted by heating its oxide with carbon. The carbon removes the oxygen from the metal oxide, so the oxide is reduced (loses oxygen) while the carbon is oxidised (gains oxygen). Remember the oxygen definitions: oxidation is gain of oxygen; reduction is loss of oxygen.
Iron is extracted on an enormous scale in the blast furnace, where iron(III) oxide is reduced by carbon:
2Fe2O3+3C→4Fe+3CO2
Here the iron(III) oxide is reduced (it loses its oxygen and becomes iron) and the carbon is oxidised (it gains oxygen and becomes carbon dioxide). The carbon is acting as the reducing agent. The same principle is used to extract zinc and copper from their oxides.
| Substance | What happens | Oxidised or reduced? |
|---|---|---|
| Iron(III) oxide, Fe2O3 | Loses oxygen → iron | Reduced |
| Carbon, C | Gains oxygen → carbon dioxide | Oxidised |
Exam Tip: In a carbon-reduction equation, always state both changes: the metal oxide is reduced (loses oxygen) and the carbon is oxidised (gains oxygen). Naming only one half of the change loses marks.
Zinc oxide (ZnO) is reduced by carbon to give zinc and carbon dioxide. Write the balanced equation.
Step 1 — write the products, zinc and carbon dioxide: ZnO+C→Zn+CO2 (not yet balanced for oxygen).
Step 2 — balance the oxygen: carbon dioxide needs 2 oxygen atoms, so use 2 ZnO, which also gives 2 Zn:
2ZnO+C→2Zn+CO2
Step 3 — check: Zn 2=2, O 2=2, C 1=1. Balanced. The zinc oxide is reduced; the carbon is oxidised.
A metal more reactive than carbon cannot be extracted by carbon reduction, because carbon is simply not reactive enough to pull the oxygen away. Instead, electrolysis is used: the compound is melted so that its ions are free to move, and an electric current then forces the metal ions to gain electrons at the negative electrode (the cathode), producing the metal.
Aluminium is the most important example. Aluminium is above carbon, so it is extracted by electrolysis of molten aluminium oxide (obtained from the ore bauxite). The aluminium ions are reduced to aluminium metal at the cathode, and oxygen is given off at the other electrode. (In industry the aluminium oxide is dissolved in molten cryolite, which lowers the temperature needed and saves energy.)
Electrolysis is expensive — not cheap. Melting the compound and supplying a large electric current both use a great deal of energy, which is exactly why electrolysis is reserved for metals that cannot be extracted any other way, and why recycling reactive metals such as aluminium (covered in the next lesson) saves so much energy.
Exam Tip: Electrolysis is used because carbon cannot reduce metals more reactive than itself — not because it is cheaper. In fact it is more expensive (lots of energy), which is a strong reason to recycle.
Magnesium is above carbon in the reactivity series. Explain why magnesium cannot be extracted by heating its oxide with carbon, and state the method that must be used instead.
Step 1 — magnesium is more reactive than carbon, so it holds its oxygen more strongly than carbon does.
Step 2 — therefore carbon cannot remove the oxygen from magnesium oxide — carbon reduction will not work.
Step 3 — magnesium must be extracted by electrolysis of its molten compound (for example molten magnesium chloride).
Answer: carbon is less reactive than magnesium, so it cannot reduce magnesium oxide; magnesium is extracted by electrolysis.
It is worth pausing on a point that examiners test again and again: oxidation and reduction always happen together. You cannot have one without the other. In the blast furnace, the iron oxide cannot simply "lose" its oxygen into thin air — the oxygen has to go somewhere, and it goes to the carbon. So every time a metal oxide is reduced (loses oxygen), something else is oxidised (gains that oxygen). A reaction in which reduction and oxidation happen together like this is called a redox reaction.
This is why, in a carbon-reduction equation, you must always name both halves of the change. Look again at the iron equation and follow where the oxygen goes:
2Fe2O3+3C→4Fe+3CO2
The three oxygen atoms that leave each iron oxide unit do not vanish — they end up bonded to carbon in the carbon dioxide. The carbon is the reducing agent (it does the reducing by grabbing the oxygen), and in the process the carbon is itself oxidised. A neat way to remember the whole scheme is OIL RIG: Oxidation Is Loss (of oxygen), Reduction Is Gain — but at GCSE combined science you can keep it as "oxidation = gain of oxygen, reduction = loss of oxygen".
| In the reaction | Gains or loses oxygen? | Oxidised or reduced? | Role |
|---|---|---|---|
| Iron(III) oxide | Loses oxygen | Reduced | — |
| Carbon | Gains oxygen | Oxidised | Reducing agent |
Exam Tip: Never write that an oxide is reduced without saying what is oxidised. If a question gives you a reduction equation and asks "what is oxidised?", the answer is almost always the carbon (or, more rarely, another element that has taken the oxygen). The two changes are two sides of the same redox reaction.
Extraction questions sometimes ask you to work out the mass of metal you can get from a given mass of ore, using relative formula masses (Mr). This links C6 straight back to the mole and mass-calculation work from earlier chemistry. The key idea is that mass is conserved in a reaction, and the ratio of masses follows the ratio in the balanced equation.
Calculate the maximum mass of iron that can be extracted from 320 tonnes of iron(III) oxide, Fe2O3. (Relative atomic masses: Fe = 56, O = 16.)
Step 1 — find the relative formula mass of Fe2O3: (2×56)+(3×16)=112+48=160.
Step 2 — find the mass of iron inside it. Each Fe2O3 contains 2 iron atoms, so the iron accounts for 2×56=112 out of every 160 mass units:
fraction that is iron=160112=0.70
Step 3 — apply this fraction to the 320 tonnes of ore: 0.70×320=224 tonnes.
Answer: the maximum mass of iron is 224 tonnes. (In practice the real yield is a little lower, because no industrial process is perfectly efficient — some iron is lost in the waste, or the ore is not pure Fe2O3.)
Exam Tip: To find the mass of metal in an oxide, work out Mr of the whole oxidemass of metal in the formula, then multiply by the mass of ore. Watch the number of metal atoms in the formula — Fe2O3 has two iron atoms, so it is 2×56, not 56.
The reactivity series even explains the order in which metals were discovered and first used. The least reactive metals — gold, silver and copper — were known to ancient civilisations because they are found native or are easily extracted. Iron came into widespread use later (the Iron Age), once furnaces hot enough for carbon reduction were available.
The very reactive metals such as aluminium, sodium and potassium were not isolated until the early 1800s, after electricity had been harnessed — because electrolysis was the only way to extract them. So a metal's place in the reactivity series matches, in reverse, the order in which it became available to people: the lower the reactivity, the earlier it was used.
Exam Tip: A neat synoptic point: reactive metals were discovered late because their extraction needed electrolysis, which needed electricity. This links chemistry to the history of technology and is exactly the kind of connection examiners reward.
Tin lies just below iron in the reactivity series, and below carbon. State how tin is likely to be extracted from tin oxide, and justify your choice.
Step 1 — find tin relative to carbon: it is below carbon.
Step 2 — apply the rule: metals below carbon can be reduced by carbon.
Step 3 — conclude: tin is extracted by heating tin oxide with carbon, because carbon is more reactive than tin and can remove its oxygen.
Answer: by reduction with carbon, since tin is less reactive than carbon.
| Misconception | The correct idea |
|---|---|
| "All metals can be extracted with carbon" | Only metals below carbon can; metals above carbon (aluminium and higher) cannot |
| "Electrolysis is used because it is cheaper" | Electrolysis is used because carbon cannot reduce reactive metals; it is actually expensive (lots of energy) |
| "Reduction means the substance gets smaller" | Reduction means loss of oxygen — the oxide loses its oxygen to become the metal |
| "All metals are found as pure elements in the ground" | Most are found as compounds (often oxides); only very unreactive metals like gold are found native |
| "Carbon is reduced in the blast furnace" | Carbon is oxidised (it gains oxygen → carbon dioxide); the iron oxide is reduced |
Question (6 marks): Iron is extracted by heating iron(III) oxide with carbon, but aluminium is extracted by electrolysis of molten aluminium oxide. Explain why two different methods are used, referring to the reactivity series and to oxygen transfer.
Mid-band response: "Iron is below carbon so carbon can reduce it. Aluminium is above carbon so carbon cannot reduce it, so electrolysis is used instead."
Examiner-style commentary: The central rule is correct. To climb a band, explain reduction in terms of oxygen (carbon removes the oxygen from iron oxide) and say why carbon cannot do this for aluminium (aluminium is more reactive and holds its oxygen too strongly).
Stronger response: "Iron is less reactive than carbon, so carbon can remove the oxygen from iron(III) oxide — the oxide is reduced and the carbon is oxidised: 2Fe2O3+3C→4Fe+3CO2. Aluminium is more reactive than carbon, so carbon cannot take its oxygen away, and electrolysis of molten aluminium oxide is used instead. Electrolysis is more expensive because it uses a lot of energy."
Examiner-style commentary: A strong answer that links each method to reactivity and explains the iron reduction in terms of oxygen. To reach the top band, state explicitly that the aluminium ions are reduced at the cathode during electrolysis, and note why the energy cost matters (it makes recycling worthwhile).
Top-band response: "The method depends on each metal's position relative to carbon. Iron is less reactive than carbon, so carbon can remove the oxygen from iron(III) oxide: the oxide is reduced (loses oxygen) and the carbon is oxidised (gains oxygen), 2Fe2O3+3C→4Fe+3CO2. This route is cheaper, so it is used wherever possible. Aluminium is more reactive than carbon, so it holds its oxygen too strongly for carbon to remove — carbon reduction simply will not work. Aluminium is therefore extracted by electrolysis of molten aluminium oxide, in which the aluminium ions are reduced to aluminium metal at the cathode by gaining electrons. Electrolysis is expensive because melting the oxide and supplying the current both use a great deal of energy, which is why recycling aluminium saves so much energy."
Examiner-style commentary: Full marks. It uses carbon as the dividing line, explains the iron reduction in terms of oxygen with a balanced equation, justifies electrolysis for aluminium by its higher reactivity, and adds the energy-cost and recycling point — a complete, well-linked comparison.
This content is aligned with OCR Gateway Combined Science A (J250), Topic C6 Global challenges. Refer to the official OCR specification for exact wording.