HSC Exam Practice

Sample response. Melting point (MP) is the temperature at which a substance changes from solid to liquid. Boiling point (BP) is the temperature at which a substance changes from liquid to gas. For a pure substance, both the MP and BP are sharp and fixed — the transition occurs at a single, precise temperature. For a mixture, the transition occurs over a range of temperatures, because different components require different amounts of energy to change state.

Marking notes. 1 mark for correct definition of MP (solid to liquid); 1 mark for correct definition of BP (liquid to gas); 1 mark for the key difference: pure substance = sharp/fixed; mixture = range/variable.

Sample response. (1) Melting point — temperature of solid–liquid transition. (2) Boiling point — temperature of liquid–gas transition. (3) Electrical conductivity — ability to allow electric current to flow. (4) Hardness — resistance to scratching. (5) Solubility — maximum mass dissolved per volume of solvent at a given temperature. (6) Density — mass per unit volume (g cm−³).

Marking notes. 1 mark per correctly named property. Accept reasonable paraphrases. Do not award marks for properties not listed in the lesson (e.g. colour, smell).

Sample response. In a pure substance, all particles are chemically identical and are arranged in a uniform lattice with identical intermolecular or ionic forces throughout. As the substance is heated, all particles gain enough energy to break free from the lattice at exactly the same temperature, producing a sharp, fixed melting point and a flat plateau on the heating curve. In a mixture, different particles have different bonding strengths. Some particles require less energy (melt at lower temperatures) and others require more, so melting begins at one temperature and finishes at a higher one — producing a gradual transition over a range of temperatures with no flat plateau.

Marking notes. 1 mark for correctly explaining the sharp MP for a pure substance (identical particles, identical bond strengths, all melt at same temperature); 1 mark for correctly explaining the melting range for a mixture (different components, different bond strengths, melt at different temperatures); 1 mark for using particle-level language (e.g. identical particles / lattice / intermolecular forces / bonding strengths) in at least one part.

Sample response. Copper (metallic element): conducts as a solid (free electrons throughout the metallic lattice can flow freely); conducts when molten (electrons remain free in the liquid metal); N/A / does not dissolve in water under normal conditions. NaCl (ionic compound): does NOT conduct as a solid (Na&sup+ and Cl&sup− ions are locked in a rigid lattice and cannot move); DOES conduct when molten (ions become mobile and can carry charge); DOES conduct when dissolved in water (ions dissociate and move freely through the solution as charge carriers).

Marking notes. Award marks for the contrast, not for individual entries. 1 mark for correctly stating copper conducts in all accessible states with reason (free electrons); 1 mark for correctly stating NaCl does NOT conduct as solid (ions locked in lattice); 1 mark for correctly stating NaCl conducts when molten or in solution (mobile ions); 1 mark for correctly distinguishing the mechanism between the two (free electrons for metal vs mobile ions for ionic compound).

Sample response. The student is incorrect because compounds do not inherit, retain, or average the properties of their constituent elements. When Na and Cl&sub2; react, completely new ionic bonds form between Na&sup+ and Cl&sup− ions, creating a new three-dimensional lattice structure — NaCl. This new structure has entirely different properties: NaCl is a white, non-reactive crystalline solid that simply dissolves in water; it does not explode in water like Na, is not toxic like Cl&sub2;, and melts at 801°C rather than 98°C (Na) or −101°C (Cl&sub2;). The properties of a compound are determined by its new bonding structure, not by the properties of its elements.

Marking notes. 1 mark for identifying the error (compounds do not retain element properties); 1 mark for explaining why (new ionic bonds / new structure / new substance); 1 mark for applying to NaCl specifically (e.g. non-reactive with water, not toxic, very different MP) to demonstrate the contrast.

Sample response. Step 1 — Measure: determine the MP, BP, density, conductivity and solubility of the unknown white powder. Step 2 — Check sharpness: determine whether the MP/BP is sharp (pure substance) or gradual (mixture). For the powder to be NaCl, the MP must be sharp at 801°C. Step 3 — Compare: match all measured values against NaCl reference data (MP = 801°C, density = 2.16 g cm−³, solubility ~360 g/L, no conductivity as solid, excellent conductivity in solution). Step 4 — Confirm: at least two independent properties must match before concluding it is NaCl, because a single matching property could be coincidental.

Marking notes. 1 mark for correctly describing the “measure and check sharpness” steps (distinguishing pure from mixture first); 1 mark for the “compare against reference data” step applied to NaCl; 1 mark for the “confirm with at least two independent properties” requirement.

Sample response (a). Batch 1 is a pure substance: the cooling curve shows a clear flat plateau at 162°C (the phase change from liquid to solid), consistent with all particles solidifying at the same temperature because the compound is uniform in composition. Batch 2 is a mixture: the cooling curve shows a continuous downward slope with no flat plateau; solidification occurs gradually over a range from approximately 150°C to below 112°C, indicating mixed composition with components solidifying at different temperatures. [1 Batch 1 classification with curve evidence; 1 Batch 2 classification with curve evidence; 1 explicit reference to flat plateau vs gradual slope as the distinguishing feature.]

Sample response (b). Batch 2 is a mixture containing the target compound and at least one contaminant with a different molecular structure. The contaminant and the target compound have different intermolecular forces and therefore different energies required to solidify [1]. As the mixture cools, some particles (those requiring less energy to solidify) begin to crystallise at approximately 150°C, while others require more cooling before they solidify [1]. Because the composition is not uniform, there is no single temperature at which all particles simultaneously transition from liquid to solid, so no flat plateau forms; the curve slopes continuously as each successive fraction solidifies at a lower temperature [1].

Sample response (c). The claim is not supported by the evidence [1]. The cooling curve for Batch 2 shows a gradual solidification range rather than a sharp plateau, confirming it is a mixture, not a pure compound. A pharmaceutical drug must be pure to ensure a known, reliable dosage and predictable pharmacological effect. The presence of contaminants (as evidenced by the gradual solidification range) means the concentration of the active compound is uncertain and the contaminants’ effects are unknown — making Batch 2 unsuitable for medicinal use regardless of whether it “contains” the correct compound [1].

Marking notes. Part (a): 1 mark Batch 1 = pure; 1 mark Batch 2 = mixture; 1 mark citing specific graph features (flat plateau vs continuous slope). Part (b): 1 mark identifying different components with different intermolecular forces; 1 mark explaining differential solidification temperature; 1 mark explaining why no plateau forms (no single temperature for all particles). Part (c): 1 mark for stating the claim is not supported; 1 mark for linking the gradual range to mixed composition and explaining the pharmaceutical implication.

Sample response. Physical properties are highly useful for classifying substances as elements, compounds, or mixtures because they are measurable, reproducible, and require no knowledge of chemical identity. The most reliable single property is melting/boiling point sharpness: a sharp, fixed transition indicates a pure substance (element or compound), while a gradual range indicates a mixture. This is reliable because it directly reflects compositional uniformity at the particle level. For example, the salt (NaCl) extracted from Western Australian salt lakes — a major Australian export — can be tested for purity by measuring its melting point: pure NaCl melts sharply at 801°C, while contaminated batches will soften over a range. Similarly, aluminium (Al) produced at the Portland aluminium smelter in Victoria melts sharply at 660°C, distinguishing it from aluminium alloys (mixtures), which melt over a range. However, melting point alone cannot distinguish an element from a compound — both can have sharp MPs. This is where the conductivity pattern becomes essential. Metallic elements (e.g. Al, Cu) conduct electricity as solids and when molten, because free electrons are present throughout. Ionic compounds (e.g. NaCl, MgO) do not conduct as solids (ions immobilised in lattice) but do conduct when molten or dissolved (mobile ions). Covalent compounds (e.g. H&sub2;O, C&sub6;H&sub1;&sub2;O&sub6;) do not conduct in any state (no free charges). Combining melting point sharpness with this conductivity pattern allows classification of a pure substance into element, ionic compound, or covalent compound with high confidence. Relying on a single property is insufficient: a white crystalline solid that does not conduct as a solid could be NaCl (ionic) or glucose (covalent), but the in-solution conductivity test resolves this immediately. Density and solubility provide additional confirmation. In summary, physical properties are powerful classification tools when used systematically in combination; the combination of sharp melting point + conductivity pattern is the most discriminating approach available without chemical testing.

Marking criteria (7 marks). 1 = correctly identifies melting/boiling point sharpness as the most reliable single purity test, with explanation of why (particle uniformity). 1 = identifies and explains a limitation of melting point alone (cannot distinguish element from compound). 1 = correctly describes the conductivity pattern for metallic elements (conducts in all states, free electrons). 1 = correctly describes the conductivity pattern for ionic compounds (does not conduct as solid; does conduct when molten or dissolved) with mechanism. 1 = named Australian industrial example used correctly to illustrate a classification test (NaCl from WA salt lakes, Al from Portland smelter, or equivalent). 1 = second named example (same or different context, demonstrating a different classification). 1 = reaches an explicit evaluative judgement that integrates multiple properties as the optimal classification strategy, contrasting this with the limitation of a single-property approach.