Aluminium — properties matched to uses
Low density (~2.7 g/cm3) makes aluminium light — used for aircraft bodies and overhead cables. Soft when pure, it is alloyed with copper, magnesium or zinc for aircraft strength. A self-healing, non-toxic oxide layer resists corrosion (food cans, foil, packaging), and it is malleable/ductile for foil and wire. For overhead cables its low density beats its lower conductivity: lighter cables sag less and need fewer towers.
Copper — properties matched to uses
Copper has the highest electrical conductivity of common metals and is ductile, so it is drawn into wire for household wiring. Its high thermal conductivity suits heat exchangers and pans. It does not react with water or release toxic ions, and bends easily, so it is used for water pipes. Indoors copper beats aluminium because mass is irrelevant in walls: higher conductivity wastes less heat and allows thinner wires (Ar Cu = 63.5).
Metallic bonding explains the properties
A metal is a giant lattice of positive ions in a sea of mobile (delocalised) electrons held by strong electrostatic attraction. The mobile electrons carry charge, giving electrical conductivity, and transfer heat. Layers of ions slide without breaking the bond, so metals are malleable and ductile. The strong attraction takes much energy to break, so melting points are high. Mark schemes demand mobile or delocalised; free electrons alone earns nothing.
Drawn from real examiner reports.
'Free electrons' scores no marks
Explaining a metal's properties by free electrons gains no credit: Cambridge wants mobile or delocalised electrons carrying charge through a lattice of positive ions. A second error is describing ionic bonding (positive and negative ions attracting) instead of metallic bonding. Solid metals have no negative ions — the charge carriers are delocalised electrons.
W22 P41 Q2(a); S22 P41 Q3(a)
Wrong metal blamed for a use
Candidates match the wrong property to the wrong metal. Aluminium is not the best conductor — copper is; aluminium is used for overhead cables for its low density, copper for wiring for its conductivity. Do not say copper is used for aircraft because it is light: copper is about three times denser, so aircraft use aluminium alloys.
'Good for the environment' is vague
Justifying recycling with good for the environment or reduces pollution gains no marks. Give a specific benefit: it conserves finite ore reserves (bauxite for aluminium), uses far less energy than extracting the metal by electrolysis, or reduces landfill waste and mining damage. Recycling aluminium uses about 5% of the energy of extraction.
W22 P31 Q2(c)(v)
Aircraft need the alloy, not pure Al
Pure aluminium is soft, so aircraft use aluminium alloys such as duralumin (aluminium with copper, magnesium and zinc), not pure aluminium. The added atoms are different sizes, so they distort the layers and stop them sliding, making the alloy harder and stronger while keeping the low density. Saying aircraft use pure aluminium loses the mark.
Reactive aluminium resists corrosion
Aluminium is high in the reactivity series, so students expect it to corrode quickly — but it does not. A thin, tightly-held oxide layer forms instantly in air and seals the surface. This self-healing layer, not low reactivity, is why aluminium resists corrosion and is safe for food packaging. Strip it and the metal reacts rapidly.
Aluminium corrodes, it does not rust
Rust is specific to iron: it is hydrated iron(III) oxide formed when iron meets oxygen and water. Aluminium and copper do not rust. Aluminium corrodes (oxidises) to a protective oxide layer; copper tarnishes to a green patina. For aluminium or copper use corrode, oxidise or tarnish — writing that they rust loses the mark.
Two-step property-use answer
For Explain why [metal] is used for [use], give two linked steps: name the property, then say how it fits the use. A bare fact like aluminium is light rarely scores both marks — add the consequence, e.g. a lighter cable sags less and needs fewer towers.
Find the deciding property
Aluminium-versus-copper questions hinge on the deciding property. Indoors (wiring) conductivity wins, so copper; for long overhead cables low mass wins, so aluminium. Fix the property for the context first, and never claim aluminium conducts better than copper.
Bonding answers need both parts
When explaining a property by bonding, name both parts: mobile (delocalised) electrons AND a lattice of positive ions. For conductivity the electrons carry charge; for malleability layers of ions slide. Only free electrons, or only the ions, misses half the marks.
Recycling: one specific benefit
For state an advantage of recycling a metal, give one specific benefit, not good for the environment: resources (finite ore conserved), energy (far less than extraction), or waste (less landfill and mining damage). One clear benefit with a reason scores.
Metallic bonding — the strong electrostatic attraction between a giant lattice of positively charged metal ions and a sea of mobile (delocalised) electrons. The electrons are not attached to any one ion; they move freely through the entire lattice. This single structural feature explains all the characteristic properties of metals.
Ductility — the ability of a metal to be drawn into a wire without breaking.
Malleability — the ability of a metal to be hammered or rolled into thin sheets without cracking.
Corrosion resistance — the ability of a metal to resist attack by oxygen, water or other environmental agents.
Alloy — a mixture of a metal with one or more other elements (usually metals, or carbon), not chemically combined. The atoms of the added element disrupt the regular arrangement of the host metal's lattice, making it harder to slide layers — so alloys are harder and stronger than the pure metal.
State FOUR uses of aluminium and the key property that makes aluminium suitable for EACH use.
Aluminium reacts with oxygen in the air to form a thin protective layer of aluminium oxide on its surface.
(a) Write the formula of aluminium oxide. (Aluminium forms ions; oxygen forms ions.)
(b) Explain why this oxide layer makes aluminium useful for food containers.
