Chemistry • Year 11 • Module 1 • Lesson 8
Metallic Bonding and Properties
Build HSC Band 5–6 extended-response technique on the electron sea model, alloy design, and multi-criteria evaluation of metallic materials in engineering contexts.
1. Data + scenario: designing the Sydney Harbour Bridge — steel vs pure iron (Band 5–6)
8 marks Band 5–6
Scenario. When the Sydney Harbour Bridge was designed in the 1920s, engineers chose structural steel (iron + ~0.2% carbon) rather than pure iron for its arches and load-bearing members. The table below compares key properties of pure iron and the structural steel used in the bridge.
| Property | Pure iron (Fe) | Structural steel (Fe + 0.2% C) |
|---|---|---|
| Tensile strength (MPa) | ~270 | ~400 |
| Yield strength (MPa) | ~130 | ~250 |
| Malleability | High — deforms easily | Moderate — resists deformation better |
| Hardness (Brinell) | ~60–70 | ~120–140 |
| Melting point (°C) | 1538 | ~1430–1540 (varies with C%) |
Illustrative data based on ASM Metals Handbook. Tensile strength values are approximate.
Q1. Analyse and evaluate the data above to explain why structural steel was chosen over pure iron for the Sydney Harbour Bridge. In your response you must:
- Explain the electron sea model of metallic bonding in pure iron, including what happens to valence electrons and how the bonding produces malleability.
- Use the alloy structure model (foreign atoms, lattice distortions) to explain why adding 0.2% carbon produces a steel that is harder and stronger than pure iron.
- Interpret the data table: identify which two properties most clearly justify the use of steel over pure iron for a load-bearing bridge arch, and justify your selection using values from the table.
- Evaluate one trade-off of using structural steel over pure iron (i.e. one property that is better in pure iron or worsened in the alloy).
- State one real-world consequence of that trade-off in the context of the Sydney Harbour Bridge.
2. Experimental design — testing whether 24-carat gold or 18-carat gold is harder (Band 5–6)
7 marks Band 5–6
Research question. A jewellery manufacturer claims that 24-carat gold (pure gold) and 18-carat gold (75% Au, 25% copper and silver) have “essentially the same hardness.” Based on your knowledge of the electron sea model and alloy structure, you believe 18-carat gold should be significantly harder. Design a scientific investigation to test this claim.
Constraints: You have access to standard Year 11 laboratory equipment including a calibrated Vickers or Brinell hardness tester, a digital balance, a ruler, standard mass sets, and two sets of sample gold discs (24-carat and 18-carat, same dimensions, 10 mm diameter, 3 mm thick, supplied by the manufacturer). You have one school lesson period (60 minutes).
Q2. Design the investigation and present it in the format below.
- State your hypothesis (a testable prediction including the independent and dependent variables) and link it explicitly to the electron sea model.
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, including how you will measure hardness and how you will obtain a reliable result.
- Explain what result would falsify your hypothesis and what that would suggest about the alloy structure model.
- State two limitations of your design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Electron sea model in pure iron (2 marks): Pure iron has a regular lattice of positive Fe2+ cations surrounded by a sea of delocalised valence electrons released from each Fe atom. The metallic bond is the electrostatic attraction between the Fe2+ cation lattice and the electron sea. This bonding is non-directional — there is no preferred bond direction — which means that when a shear force is applied, layers of Fe ions can slide past each other without breaking the overall bonding structure. The electron sea simply redistributes to maintain attraction in the new ion positions. This layer sliding is why pure iron is highly malleable and ductile [1 electron sea model; 1 malleability explanation].
Alloy structure explanation (2 marks): When 0.2% carbon is added to iron, the smaller carbon atoms occupy interstitial spaces between the Fe ions in the lattice, distorting the regular arrangement at those sites [1]. These lattice distortions act as obstacles to layer sliding: when a shear force is applied, Fe ion layers cannot slide smoothly past the sites where C atoms are lodged (because the C atom’s size difference blocks dislocation movement). Greater force is needed to deform the steel, producing higher tensile strength, yield strength, and hardness compared to pure iron [1].
Data interpretation (2 marks): The two properties that most clearly justify choosing steel over pure iron for a load-bearing bridge arch are tensile strength (400 MPa vs 270 MPa — 48% higher) and yield strength (250 MPa vs 130 MPa — 92% higher) [1]. A bridge arch must bear enormous loads without permanently deforming (high yield strength) or fracturing (high tensile strength). The data show steel outperforms pure iron on both these critical safety-relevant properties [1].
Trade-off and real-world consequence (2 marks): One trade-off is that structural steel is less malleable than pure iron (moderate malleability vs high malleability in pure Fe) [1]. In the context of the Sydney Harbour Bridge, the reduced malleability of steel meant that fabricating the large curved arch sections required more precise hot-forming and riveting techniques during construction rather than simple cold bending. The steel had to be heated to high temperature to be shaped, adding fabrication complexity and cost. Accept also: welding structural steel is more difficult than pure iron due to carbon content [1].
Marking criteria summary (8 marks): 1 = correct electron sea model description (cation lattice + electron sea + electrostatic attraction); 1 = explains malleability using non-directional bonding + layer sliding; 1 = explains carbon atoms occupy interstitial sites creating distortions; 1 = links distortions to impeded layer sliding → harder/stronger; 1 = identifies tensile or yield strength as key property with specific values; 1 = justifies why these properties matter for a bridge arch; 1 = identifies one valid trade-off (reduced malleability or other valid property); 1 = states real-world consequence of that trade-off in Sydney Harbour Bridge context.
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis (1 mark): If 18-carat gold is an alloy where Cu and Ag atoms (different sizes from Au) disrupt the regular gold lattice and prevent smooth layer sliding, then 18-carat gold will have a significantly higher hardness number (Vickers HV or Brinell HB) than 24-carat gold. Independent variable: gold type (24-carat / pure gold vs 18-carat / alloy). Dependent variable: hardness number (Vickers HV or Brinell HB score). Controlled variables: disc dimensions (10 mm diameter, 3 mm thick), same hardness tester, same applied mass and dwell time, same test location (centre of disc face) [1].
Procedure (1 mark): (1) Label each disc and record its mass and dimensions. Place a 24-carat gold disc on the hardness tester stage, align to the specified test location. (2) Apply the standard indenter mass per the Vickers protocol; allow full dwell time; measure the diagonal of the indentation under the microscope and record the Vickers hardness number (HV). Repeat this measurement at 5 different positions across the disc face and calculate a mean HV value. (3) Repeat Steps 1–2 with each of the 18-carat gold discs using identical settings. Record all HV values in a results table. (4) Compare mean HV values: if mean HV (18-carat) > mean HV (24-carat), the hypothesis is supported [1 for four clear steps with repeated measurements].
Falsification (1 mark): If the mean HV of the 18-carat gold is equal to or lower than the mean HV of the 24-carat gold, the hypothesis would be falsified. This would suggest that the Cu/Ag atoms in the 18-carat gold do not significantly distort the Au lattice (perhaps because their sizes are not sufficiently different from Au to impede layer sliding), casting doubt on the simple lattice-distortion model for this alloy system [1].
Limitations (2 marks): (1) The manufacturer’s discs may have surface work-hardening from cutting or polishing, artificially elevating the hardness reading at the disc surface relative to the bulk material [1]. (2) The composition of the 18-carat gold alloy (ratio of Cu to Ag) is not specified; different Cu:Ag ratios produce different degrees of lattice distortion, so the result applies only to this particular alloy composition, not to all 18-carat golds [1].
Improvement (1 mark): Repeat the entire experiment with three different sets of 24-carat and 18-carat gold discs from different batches to improve reliability and confirm that any observed hardness difference is a consistent property of the alloy type rather than a batch-specific artefact. Polish the surface of all discs to a standard finish (e.g. 1200-grit sandpaper) before testing to remove surface work-hardening [1].
Marking criteria summary (7 marks): 1 = testable hypothesis naming IV and DV and explicitly linking to alloy/electron sea model; 1 = four clear procedure steps including repeated measurements; 1 = states what would falsify the hypothesis and what it would imply; 1 = first valid limitation; 1 = second valid limitation; 1 = one specific and actionable improvement; 1 = precise chemical terminology throughout (delocalised electrons, lattice distortion, independent/dependent/controlled variable, Vickers/Brinell hardness).