Chemistry • Year 11 • Module 1 • Lesson 2
Properties of Elements, Compounds and Mixtures
Build HSC Band 5–6 extended-response technique by evaluating property data, designing investigations, and arguing why compound properties cannot be predicted from elements.
1. Data + scenario: classifying five unknown substances in a forensic laboratory (Band 5–6)
8 marks Band 5–6
Scenario. A forensic chemist receives five white powder samples seized at a Sydney warehouse. Each sample must be classified and, where possible, identified. The table below summarises the property measurements made in the laboratory.
| Sample | Melting behaviour | Density (g cm−³) | Conductivity (solid) | Conductivity (in water) | Solubility in water |
|---|---|---|---|---|---|
| P | Sharp at 801°C | 2.16 | None | Excellent | Soluble (360 g/L) |
| Q | Gradual, 58–79°C | Variable | None | None | Insoluble |
| R | Sharp at 1455°C | 8.91 | Excellent | N/A (insoluble) | Insoluble |
| S | Sharp at 160°C | 1.33 | None | None | Insoluble |
| T | Gradual, 135–162°C | Variable | None | Low | Slightly |
Illustrative data for educational use.
Q1. Analyse and evaluate the data above to classify each sample and, where the evidence supports it, suggest an identity. In your response you must:
- Classify each sample as element, ionic compound, covalent compound, or mixture, with justification citing specific data.
- For at least two samples, explain how the conductivity data (solid vs in water) was used to narrow the classification.
- Explain why the melting behaviour is the single most reliable indicator of purity, and assess its limitations in distinguishing between an element and a compound.
- Identify which sample is most likely a metallic element and justify with reference to two specific pieces of data.
- Propose one additional property measurement the forensic chemist could perform to resolve any remaining uncertainty.
2. Experimental design — testing whether NaCl retains any properties of Na or Cl&sub2; (Band 5–6)
7 marks Band 5–6
Background. Sodium (Na) is a soft silvery metal that reacts explosively with water. Chlorine (Cl&sub2;) is a toxic yellow-green gas. A Year 11 student claims: “Sodium chloride (NaCl) must contain some of the reactivity of sodium and some of the toxicity of chlorine, because it is made from those two elements.”
Research question. Does NaCl retain any of the physical or chemical properties of its component elements (Na and Cl&sub2;)?
Constraints: You have access to standard Year 11 laboratory equipment: digital balance, conductivity meter, pH meter, dilute acids, distilled water, a fume cupboard, and samples of Na (stored under mineral oil), Cl&sub2; water solution, and NaCl crystals. Your investigation must take one double period (2 hours).
Q2. Design the investigation and present it in the format below.
- State your hypothesis (a testable prediction).
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, choosing property comparisons that directly test whether NaCl’s properties come from its elements.
- Explain what result would support the student’s claim, and what result would refute it.
- State two limitations of your design and one way to improve validity.
Q1 — Sample Band 6 response (8 marks), annotated
Classification of each sample:
P — Ionic compound (NaCl): Sharp MP at 801°C confirms pure substance [1 for classification with justification]. No conductivity as solid = ions locked in lattice. Excellent conductivity when dissolved = free-moving Na&sup+ and Cl&sup− ions. This conductivity contrast is the defining ionic test [1 for conductivity reasoning].
Q — Mixture: Gradual melting 58–79°C (21°C range) confirms mixture. Variable density supports mixed composition. No conductivity in any form indicates no free charges in any component [1 classification].
R — Metallic element (nickel, Ni): Sharp MP at 1455°C confirms pure substance; excellent conductivity as solid confirms metallic bonding (free electrons) [1 for identifying metallic element with two data pieces]. Density 8.91 g cm−³ matches nickel reference data.
S — Covalent molecular compound: Sharp MP at 160°C confirms pure substance; no conductivity in any state confirms absence of free charges or mobile ions — consistent with a covalent molecular structure [1 classification].
T — Mixture: Gradual melting over 27°C range (135–162°C) confirms mixture. Low conductivity when dissolved suggests one component may be slightly ionic [1 classification].
Conductivity contrast analysis (2 samples): For P, the contrast between “no conductivity as a solid” and “excellent conductivity in water” is the key ionic compound test: in the solid state, ions are immobilised in the lattice; dissolution releases them as free-moving charge carriers [1]. For R, excellent conductivity as a solid with no solubility in water is the key metallic element signature: metallic bonding provides free electrons that can flow regardless of dissolution [1].
Melting behaviour as purity indicator, and limitation: A sharp, fixed MP is the most reliable purity indicator because it reflects uniform particle composition throughout the sample — all particles have identical bonding environments and require identical energy to melt [1]. However, it cannot distinguish between an element and a compound: both can have sharp MPs. To distinguish these, at least one additional property (such as conductivity pattern or chemical tests) is needed [1].
Most likely metallic element — R: (1) Sharp MP at 1455°C indicates pure substance; (2) Excellent conductivity as a solid indicates metallic bonding (free electrons). No other class of substance (ionic or covalent compound) conducts as a solid through electron flow. Sample R is most likely nickel (Ni).
Additional property test: A flame test or X-ray diffraction pattern — flame tests produce element-characteristic emission spectra, allowing identification; XRD gives the crystal structure and lattice parameters, confirming identity of both elements and compounds uniquely. [1]
Marking criteria summary (8 marks): 1 = classifies all 5 samples correctly with justification citing specific data; 1 = conductivity solid vs in water used to classify P as ionic compound; 1 = conductivity in solid used to classify R as metallic element; 1 = correctly explains sharp MP as most reliable purity test; 1 = correctly identifies limitation (cannot distinguish element from compound by MP alone); 1 = identifies R as metallic element with two data points; 1 = proposes a valid additional property test with reason; 1 = uses precise chemical terminology throughout (ionic lattice, free electrons, mobile ions, pure substance, melting range).
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If NaCl does not retain the properties of Na or Cl&sub2;, then NaCl will react calmly with water (unlike Na), will not have an odour (unlike Cl&sub2;), and will not conduct electricity as a solid (unlike Na). [1 — testable hypothesis]
Variables: IV = substance tested (Na metal / Cl&sub2; water solution / NaCl crystals). DV = measured properties (conductivity, reactivity with water, pH, odour). Controlled: mass used (0.5 g or 5 mL each); temperature (room temperature); same equipment for each measurement. [1 — IV, DV, 2+ controlled]
Procedure: (1) In a fume cupboard, place 0.5 g of Na (stored under mineral oil) into 10 mL distilled water and observe reaction intensity (vigour, heat, gas evolved). Record. (2) Place 0.5 g NaCl in 10 mL distilled water; observe and record. Compare vigour of dissolution vs Na reaction. (3) Using a conductivity meter, measure conductivity of: (a) dry Na metal; (b) dry NaCl crystals. Record. (4) Using a pH meter, measure the pH of: (a) the NaCl solution; (b) the Cl&sub2; water solution. Record any difference in pH and odour (observed safely from a distance in fume cupboard). [1 — four steps including property comparison tests]
Result that would support the student’s claim: If NaCl solution were strongly acidic or alkaline (like Cl&sub2; in water, which forms HCl and HClO), or if NaCl reacted vigorously with water, this would support the claim that NaCl retains element properties. [1]
Result that would refute the student’s claim: If NaCl simply dissolves in water without visible reaction, its solution is neutral (pH ~7), it has no odour, and it shows no conductivity as a solid (unlike Na), then NaCl does NOT retain element properties — confirming that compound formation creates an entirely new substance [1].
Limitations: (1) Na is highly reactive and dangerous; performing Step 1 requires strict fume cupboard use and small quantities — any error in handling could be hazardous, limiting the scale of comparison [1]. (2) The Cl&sub2; water solution only approximates chlorine gas behaviour; true comparison would require gaseous Cl&sub2;, which is too dangerous for a school lab [1].
Improvement: Use pre-prepared 0.1 mol/L NaOH solution (formed by Na + H&sub2;O) as the comparison for the Na reaction, rather than using Na directly, to improve safety while retaining a valid property comparison [1].
Marking criteria summary (7 marks): 1 = testable hypothesis naming what NaCl should NOT show; 1 = IV, DV, 2+ controlled variables; 1 = four steps including at least one direct property comparison (conductivity, water reactivity, pH, or odour); 1 = result that supports the student’s claim; 1 = result that refutes it; 1 = valid limitation; 1 = valid improvement to the design.