Extracting and Using MetalsAQA GCSE Chemistry: Revision notes
Section 1
Alternative Methods of Extracting Metals (Higher Tier)
Not all metals are extracted by traditional reduction with carbon. Two alternative biological methods are:
- Phytomining — plants absorb metal compounds from the soil as they grow; the plants are then harvested and burned, producing an ash that contains the metal compounds
- Bioleaching — bacteria are used to produce a leachate solution containing dissolved metal compounds
Copper can then be obtained from the resulting solutions in two ways:
- Displacement using scrap iron (more reactive than copper, so it displaces copper from solution)
- Electrolysis
These biological methods are especially useful for low-grade ores, where traditional mining would be uneconomical.
Both phytomining and bioleaching avoid traditional mining and can extract metal from low-grade ores that would otherwise be uneconomical to process.
Section 2
Corrosion and How to Prevent It
Corrosion is the destruction of materials by chemical reactions with substances in the environment. Rusting of iron specifically requires both air and water to be present.
Corrosion can be prevented by:
- Barrier coatings — greasing, painting, or electroplating, which physically stop air and water reaching the metal
- Sacrificial protection — attaching a more reactive metal (e.g. zinc, to galvanise iron)
In sacrificial protection, the more reactive metal corrodes instead of the metal it is protecting, because it loses electrons more readily.
Galvanising iron with a layer of zinc protects the iron because zinc is more reactive and corrodes first.
Section 3
Common Alloys and Their Uses
Pure metals are often too soft for practical use, so they are mixed with other metals to form harder alloys:
| Alloy | Made from | Notable property/use |
|---|---|---|
| Bronze | Copper and tin | Harder than copper; used for statues, bearings |
| Brass | Copper and zinc | Used for fittings, musical instruments |
| Gold alloys | Gold with silver, copper and zinc (measured in carats) | Used in jewellery — pure gold is too soft |
| High carbon steel | Iron with carbon | Strong but brittle |
| Low carbon steel | Iron with carbon | Softer and more easily shaped |
| Stainless steel | Iron with chromium and nickel | Resistant to corrosion |
| Aluminium alloys | Aluminium with other metals | Low density |
Section 4
Recycling Metals
Recycling is an important way of reducing the use of Earth's limited resources. Metals can be recycled by melting and recasting them into new products.
Recycling metals saves both the raw ore that would otherwise need to be mined, and the large amount of energy needed to extract new metal from its ore.
Must Know
- Phytomining (plants absorb metal, then burned for ash) and bioleaching (bacteria produce a leachate solution) are alternative extraction methods (HT); copper can then be obtained by displacement with scrap iron or by electrolysis
- Corrosion/rusting needs both air and water; prevented by barrier coatings or sacrificial protection with a more reactive metal
- Common alloys: bronze (Cu+Sn), brass (Cu+Zn), gold alloys (measured in carats), high/low carbon steel, stainless steel (Fe+Cr+Ni), aluminium alloys
- Metals can be recycled by melting and recasting, saving both raw material and energy
That's the notes covered.
Carry on to the next subtopic.