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Almost every metal we use — the iron in a bridge, the aluminium in a drinks can, the copper in a wire — began as a rock dug out of the ground. Very few metals are found as the pure element; most are locked up in compounds and must be extracted. How a metal is extracted is decided by one thing you already know well: its position in the reactivity series. This lesson, part of Topic C4 of OCR Gateway Science A, explains where metals are found, why some are extracted by reduction with carbon while others need electrolysis, and how this connects to oxidation and reduction in terms of oxygen.
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, write and interpret reduction equations in terms of oxygen, and explain why reactive metals were discovered later in history.
An ore is a rock that contains enough of a metal compound to make extracting the metal worthwhile. Most metals are found in ores as compounds — very often oxides (such as iron(III) oxide, Fe2O3, in haematite, or aluminium oxide, Al2O3, in bauxite) but also as carbonates and sulfides.
The reason most metals are found combined is that they are reactive enough to have reacted with substances in their environment — especially oxygen — over millions of years. The only metals found native (as the uncombined element) are the very unreactive ones, chiefly gold, which is so low in the reactivity series that it does not react with oxygen, water or acids. This is why gold nuggets can be found in rivers, while reactive metals never occur as the free element.
Exam Tip: "Found as a compound" and "found native" link directly to reactivity. A metal is found native only if it is very unreactive (gold); the more reactive a metal, the more firmly it is locked in a compound (usually an oxide).
The key 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 diagram below summarises the decision:
flowchart TD
A["A metal needs extracting<br/>from its oxide ore"] --> B{"Is the metal more or<br/>less reactive than carbon?"}
B -->|"LESS reactive than carbon<br/>(zinc, iron, copper)"| C["Reduce the oxide<br/>by heating with carbon<br/>(cheaper)"]
B -->|"MORE reactive than carbon<br/>(aluminium, magnesium,<br/>sodium, etc.)"| D["Extract by electrolysis<br/>of 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"]
Exam Tip: The single dividing line is carbon. Below carbon (iron, copper, zinc) → reduction with carbon; above carbon (aluminium and up) → 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 — the oxide is reduced (loses oxygen) while the carbon is oxidised (gains oxygen). Recall the oxygen definitions from C3: oxidation is gain of oxygen; reduction is loss of oxygen.
Iron is extracted on a huge 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 to become iron) and the carbon is oxidised (it gains oxygen to become carbon dioxide). The carbon acts as the reducing agent. The same principle extracts 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 loses marks.
A metal more reactive than carbon cannot be extracted by carbon reduction, because carbon is not reactive enough to take the oxygen away. Instead, electrolysis is used: the compound is melted so its ions are free to move, and an electric current 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 (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 to lower the temperature needed and save 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 is so worthwhile.
| Metal's reactivity | Extraction method | Example |
|---|---|---|
| More reactive than carbon (K, Na, Ca, Mg, Al) | Electrolysis of the molten compound | Aluminium from molten aluminium oxide |
| Less reactive than carbon (Zn, Fe, Cu) | Reduction with carbon | Iron in the blast furnace |
| Very unreactive (Au) | Found native — little or no extraction needed | Gold panned from rivers |
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.
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 humans: 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 the kind of connection examiners reward.
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