Electrical conductivity — copper wiring
Copper is used for the wiring inside buildings and appliances because of its good electrical conductivity. Its metallic structure — a lattice of positive ions in a "sea" of delocalised electrons that are free to move and carry charge — lets current pass with little resistance, so little energy is wasted as heat. Indoor wiring is supported along its whole length, so mass is barely a constraint: conductivity alone is the property to name for copper.
Low density — aircraft and overhead cables
Aluminium () has a low density: about against roughly for steel, about a third. For aircraft, that lower mass means less fuel is needed to lift and move the plane and a greater payload can be carried. For overhead cables, low density is only half the answer: the cable hangs between pylons, so it must not sag or need closely-spaced supports AND must conduct well.
Corrosion resistance — the oxide layer
Aluminium is a reactive metal and does react with oxygen: . The oxide formed is thin, adherent (sticks tightly instead of flaking off) and non-porous, so it seals the surface and stops further oxygen and water reaching the metal beneath. Corrosion effectively stops, making cans, foil and trays durable. The formula follows from charge balance: two (6+) to three (6-).
Drawn from real examiner reports.
Resists corrosion ≠ does not react
Aluminium is reasonably reactive and DOES react with oxygen. "Aluminium doesn't corrode" or "aluminium is unreactive" gives the wrong mechanism even when the use named is correct. The mark-scheme idea is that the product of that first reaction — a thin, adherent oxide layer — then acts as a barrier and stops FURTHER corrosion.
Flagged Nov 2023 P43 Q2a/b (food containers resist corrosion via an adherent oxide layer)
Cables need TWO properties, not one
For overhead cables the answer is low density AND good electrical conductivity: the cable must be light enough to span the gap between pylons without excessive sag and still carry current. Candidates very commonly give conductivity alone and drop density. Contrast aircraft manufacture, where only ONE property (low density) is required.
Flagged Nov 2023 P43 Q2a/b (overhead cables — low density + conductivity)
Saying "light" instead of low density
Mark schemes want the precise property, not a loose synonym — "light", "strong" or "a good metal" do not score. Low density means a small mass for a given volume; a large block of a low-density metal can still have a large total mass, so "light" on its own is not a physical property at all.
Naming the use but not the property
A "describe the use, linked to the property" question needs the property stated explicitly. "Aluminium is used to make aircraft" or "copper is a good metal for wiring" restates the use and gives no reason. Every use in this leaf pairs with a named property: low density; low density AND conductivity; resists corrosion; good electrical conductivity.
Flagged Nov 2023 P43 Q2a/b (uses of aluminium and copper linked to physical properties)
Assuming every electrical use needs both
Density and conductivity are not both required for every electrical use of a metal. Overhead cables hang unsupported between pylons, so weight matters; indoor wiring is supported along its whole length by walls, conduits and casings, so copper is chosen on conductivity alone. The mechanical support context decides which properties count.
Use, property, then link
Three steps: (1) name the use; (2) name the exact property in mark-scheme wording; (3) say HOW it makes the metal suitable ("...so less fuel is needed"). Match the number of points to the mark tariff — a 3-mark cable question wants both properties plus the link.
Density is mass ÷ volume, with units
Density = mass / volume, never volume / mass — the inverted rearrangement is a standard loss. Substitute before simplifying, and always give the unit: , not a bare "2.7". A missing unit costs the mark even when the working is right.
Build ionic formulae by charge balance
Derive from the charges, not from the names: and need two aluminium ions (6+) to three oxide ions (6-). Writing "AlO" matches the number of ion types but leaves the total charges unbalanced, so it scores nothing.
When balancing, oxygen is diatomic
Oxygen gas is , not O. Get the formulae right first, then balance: gives 4 aluminium and 6 oxygen atoms on each side. The usual unbalanced attempt is "Al + O2 gives Al2O3".
Cambridge 0654 spec reference: Section C9 "Metals", sub-topic C9.2 "Uses of metals". Core only (no Supplement for this leaf): describe the uses of metals, linked to their physical properties -- aluminium in aircraft manufacture (low density); aluminium in overhead electrical cables (low density AND good electrical conductivity); aluminium in food containers (corrosion resistance, via a protective oxide layer); copper in electrical wiring (good electrical conductivity).
| Term | Mark-scheme-precise meaning |
|---|---|
| Density | Mass per unit volume of a substance (); aluminium has a low density (about ) compared with other structural metals such as steel (about ) |
| Electrical conductivity | A measure of how easily an electric current flows through a material; metals conduct well because of delocalised (free-moving) electrons in their structure |
| Corrosion resistance | The ability of a metal to resist ongoing chemical attack/degradation from its surroundings -- for aluminium this comes from a protective, adherent oxide layer, NOT from being unreactive |
| Adherent | Sticking tightly to the surface beneath it (does not flake or fall away) -- the key property of aluminium's protective oxide layer |
Define density.
A block of aluminium has a mass of 27 g and a volume of 10 cm3.
Calculate the density of the aluminium block. (2 marks)
