Extraction and Uses of Metals
5 marks · EasyA company is designing a new bridge. The main parts of the bridge can be made either from an aluminium alloy or from mild steel. The table below gives key properties of these materials:
| Material | Density (g/cm^3) | Typical use | Ability to resist corrosion |
|---|---|---|---|
| Aluminium alloy | 2.7 | Aircraft, cables, building | Yes |
| Mild steel | 7.8 | Car bodies, bridges, nails | No |
The bridge must be strong, corrosion-resistant, and lightweight.
State one advantage and one disadvantage of using aluminium alloy instead of mild steel for the main structure of the bridge. Explain your choices.
Calculate the mass of a metal beam of volume 2 m^3 made from (i) aluminium alloy and (ii) mild steel. Give your answers in kilograms.
Explain why mild steel is preferred over high-carbon steel for making bridge structures.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show One advantage with explanation (e.g. lower density = lighter structure, corrosion resistance = less maintenance), One disadvantage with explanation (e.g. cost, lower strength compared to steel)
(b) — · must show Correct use of density = mass/volume formula, Accurate conversion of units if necessary
(c) Mild steel is less brittle (more malleable/ductile) and can better withstand stress in bridges than high-carbon steel, which is harder but more brittle. · must show Correct comparison of malleability/ductility
Common mistakes: Mixing up densities; Giving an incomplete explanation of corrosion resistance
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States and explains one advantage of using aluminium alloy (e.g. lower density/less weight, corrosion resistance, or suitable property for bridges).
B2 (a) · 1 mark · States and explains one disadvantage of using aluminium alloy (e.g. cost, strength, or other material property).
M1 (b) · 1 mark · Uses correct density = mass/volume formula and substitutes correctly for each material.
A1 (b) · 1 mark · Calculates both masses correctly and gives answers in kg (to 2 significant figures or better).
B3 (c) · 1 mark · Explains that mild steel is less brittle/more ductile/malleable than high-carbon steel, making it better for bridge structures.
Worked solution
a) Advantage: Aluminium alloy is less dense than mild steel, so structures made from it are much lighter. This reduces the load the bridge needs to support, making construction easier and possibly cheaper overall. It also resists corrosion, so it will not rust and needs less maintenance.
Disadvantage: Aluminium alloys are generally more expensive than mild steel and may not be as strong as steel in some applications, increasing cost or requiring thicker sections.
b) First, convert the volumes from m^3 to cm^3: 1 m^3 = 1,000,000 cm^3; 2 m^3 = 2,000,000 cm^3.
For aluminium alloy: mass = 2.7 g/cm^3 × 2,000,000 cm^3 = 5,400,000 g = 5,400 kg.
For mild steel: mass = 7.8 g/cm^3 × 2,000,000 cm^3 = 15,600,000 g = 15,600 kg.
c) Mild steel is less brittle (more workable, malleable, and ductile) than high-carbon steel, so it is less likely to crack under stress and can be easily shaped, which makes it more suitable for bridge structures.