Chemistry • Year 11 • Module 1 • Lesson 7
Ionic Bonding and Properties
Build HSC Band 5–6 extended-response technique by evaluating multi-compound data sets, designing an investigation, and constructing evidence-based arguments about lattice structure and properties.
1. Data + scenario: Choosing ionic materials for the Snowy 2.0 pump turbine (Band 5–6)
8 marks Band 5–6
Scenario. Snowy 2.0 is a pumped-hydro expansion of the Snowy Mountains Hydroelectric Scheme in New South Wales. The pump turbine machinery operates at high temperatures and requires electrically insulating ceramic linings that will not melt or deform. An engineer is comparing four ionic compounds for use as ceramic insulation. The table below summarises relevant data.
| Compound | Ions (charges) | Melting point (°C) | Conducts electricity (solid)? | Relative hardness |
|---|---|---|---|---|
| NaCl | Na¹¹ (+1), Cl¯ (−1) | 801 | No | Moderate |
| MgO | Mg²¹ (+2), O²¯ (−2) | 2852 | No | Very high |
| Al2O3 | Al³¹ (+3), O²¯ (−2) | 2072 | No | Extremely high |
| CsI | Cs¹¹ (+1), I¯ (−1) | 632 | No | Low |
Illustrative data for problem-solving purposes.
Q1. Analyse and evaluate the data above to recommend the most suitable ionic compound for the Snowy 2.0 insulating ceramic lining. In your response you must:
- Rank the four compounds by melting point and link this to lattice energy using ion charge and ion size reasoning.
- Explain why a high melting point alone is not sufficient justification and identify at least one other property that makes the recommended compound suitable.
- Explain why CsI is unsuitable for this application, referring specifically to its lattice properties.
- Account for why Al2O3 might be preferred over MgO in some applications, despite MgO having a higher melting point.
- State one limitation of using the data table alone to make this engineering decision.
2. Experimental design — verifying the conductivity rule for ionic compounds (Band 5–6)
7 marks Band 5–6
Research question. A Year 11 student claims: “All ionic compounds conduct electricity in every state.” Design an investigation using two ionic compounds (NaCl and MgO) to test this claim across three states — solid, molten, and dissolved — and identify which states conduct electricity and why.
Constraints: You have access to standard Year 11 laboratory equipment (a conductivity meter with electrodes, a Bunsen burner, crucibles, a balance, distilled water, and 5 g each of NaCl and MgO powder).
Q2. Design the investigation and present it in the format below.
- State your hypothesis as a testable prediction including the independent variable (state of the compound) and dependent variable (conductivity).
- Identify two controlled variables and explain why controlling them is important.
- Describe the procedure in at least five numbered steps covering all three states for at least one compound.
- Predict the result for each state (solid / molten / dissolved) and link each prediction to whether ions are mobile.
- State one result that would falsify the original student’s claim and one limitation of your design.
Q1 — Sample Band 6 response (8 marks), annotated
Ranking by melting point: CsI (632 °C) < NaCl (801 °C) < Al2O3 (2072 °C) < MgO (2852 °C). This rank reflects increasing lattice energy [1]. CsI and NaCl have ±1 ion charges; their electrostatic force is proportional to (1×1) = 1 (Coulomb’s Law). CsI has larger ions (Cs+, I−) than NaCl (Na+, Cl−), so ions are further apart and attraction is weaker — lower lattice energy, lower MP. MgO has ±2 charges, giving a force proportional to (2×2) = 4 — four times stronger than NaCl — hence much higher MP. Al2O3 has Al3+ (+3) and O2− (−2) ions; Coulomb force ∝ (3×2) = 6, even larger than MgO’s 4, yet its MP (2072 °C) is lower than MgO’s, likely because Al³¹ is smaller and the Al2O3 crystal structure differs [1].
High MP alone is insufficient; other properties needed: For an insulating ceramic lining, the compound must not melt (high MP) AND must not conduct electricity (so charge cannot leak through the lining). All four compounds do not conduct as solids [1]. Hardness is also important: the lining must resist physical wear. MgO (very high hardness) and Al2O3 (extremely high hardness) are superior on this criterion [1].
Why CsI is unsuitable: CsI has large Cs+ and I− ions, so the distance between ion centres is large. Combined with only ±1 charges, the electrostatic attraction is weak, giving low lattice energy and a low melting point of only 632 °C. This means CsI would melt inside operating turbine machinery, and its low hardness means it would be easily damaged by mechanical stress [1].
Al2O3 preferred over MgO in some applications: Although MgO has a higher melting point (2852 vs 2072 °C), Al2O3 has extremely high hardness (hardness 9 on Mohs scale, close to diamond’s 10), making it more resistant to mechanical wear and abrasion. In turbine applications where physical erosion is a concern, Al2O3 may outperform MgO despite the lower MP [1].
Limitation of data table alone: The table provides no information about chemical stability (e.g. whether MgO reacts with water vapour or acidic gases at high temperature), thermal expansion behaviour (which can cause cracking during heating/cooling cycles), cost, ease of manufacturing into lining shapes, or compatibility with other materials in the machine [1].
Recommendation: MgO is the primary recommendation for a high-temperature insulating ceramic because it has the highest melting point (2852 °C), does not conduct as a solid, and has very high hardness. Al2O3 is a strong alternative if mechanical hardness is the priority [1].
Marking criteria summary (8 marks): 1 = correct rank with lattice energy reasoning linking charge; 1 = Coulomb’s Law applied to compare at least two compounds quantitatively; 1 = high MP alone insufficient — names a second required property (hardness or non-conductivity); 1 = CsI ruled out with specific lattice explanation (large ions + ±1 charge + low MP + low hardness); 1 = Al2O3 vs MgO trade-off discussed with a specific property; 1 = valid limitation of the data table; 1 = clear evidence-based recommendation naming the compound and justification; 1 = uses precise chemical terminology throughout (lattice energy, Coulomb’s Law, electrostatic attraction, lattice, coordination number / ion charge).
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If ionic compounds only conduct electricity when ions are free to move, then NaCl will conduct as a molten liquid and as an aqueous solution but NOT as a solid; the student’s claim that all ionic compounds conduct in every state is false. Independent variable: state of NaCl (solid / molten / dissolved). Dependent variable: conductivity reading (mS/cm) from the conductivity meter [1].
Controlled variables: (1) The same conductivity meter and electrode separation for all tests — so differences in reading reflect the compound’s conductivity, not equipment variation [1]. (2) The same mass of NaCl (2.0 g) in each test — to ensure comparisons are fair (same quantity of ions available in each state) [1].
Procedure: (1) Place 2.0 g of solid NaCl crystals in a dry crucible on the lab bench. Insert the conductivity meter electrodes into the solid and record the reading. (2) Carefully heat the NaCl in a crucible over a Bunsen burner until it melts (MP 801 °C; use a high-temperature crucible and appropriate PPE). Insert electrodes into the molten NaCl and record the conductivity reading. Allow to cool. (3) Dissolve 2.0 g of fresh NaCl in 200 mL of distilled water in a beaker. Stir to ensure full dissolution. Insert electrodes and record the conductivity reading. (4) Repeat steps 1–3 using MgO (note: MgO cannot be melted with a Bunsen burner — only solid and dissolved tests are feasible; this is a limitation). (5) Record all readings in a table and compare solid, molten, and dissolved results [1 — five clear steps].
Predicted results (linked to ion mobility): Solid NaCl: zero conductivity — ions fixed in lattice, immobile [1]. Molten NaCl: high conductivity — lattice broken, Na+ and Cl− ions free to move and carry charge [1]. Dissolved NaCl: high conductivity — water hydrates and mobilises ions into solution [1].
Falsifying the student’s claim: If solid NaCl registers zero conductivity on the meter, this directly falsifies the claim that ionic compounds conduct in every state. The student’s claim is falsified even by a single state with zero conductivity.
Limitation: MgO has a melting point of 2852 °C, far beyond the maximum temperature of a Bunsen burner (~1000 °C). Therefore the “molten” conductivity of MgO cannot be tested with standard Year 11 equipment — only solid and dissolved states can be measured for MgO in this investigation [1].
Marking criteria summary (7 marks): 1 = testable hypothesis naming IV (state) and DV (conductivity); 1 = controlled variable 1 with justification; 1 = controlled variable 2 with justification; 1 = procedure with at least 5 numbered steps covering all three states for NaCl; 1 = all three predictions linked to ion mobility; 1 = states what would falsify the original claim; 1 = one valid limitation (MgO too high MP, or electrode contamination between tests, or dissolved MgO concentration is very low due to low solubility).