HSC Exam Practice

Sample response. Simple distillation separates a solvent from a dissolved solid (or a liquid from a non-volatile solute) by heating the mixture so the more volatile component vaporises, then cooling the vapour in a condenser to collect the distillate. Fractional distillation separates two or more miscible liquids with similar boiling points using the same apparatus plus a fractionating column, which provides multiple successive condensation and vaporisation cycles. Key equipment difference: fractional distillation requires a fractionating column (packed with glass beads or rings); simple distillation does not.

Marking notes. 1 mark for a correct definition of simple distillation (one component non-volatile or large BP difference; condenser collects distillate). 1 mark for a correct definition of fractional distillation (miscible liquids with similar BPs; fractionating column). 1 mark for explicitly identifying the fractionating column as the key equipment difference.

Sample response. (a) Simple distillation, because NaCl is a non-volatile solid with a boiling point far exceeding that of water (100°C); one evaporation/condensation cycle is sufficient to collect pure water as distillate. (b) Fractional distillation, because hexane (BP 69°C) and heptane (BP 98°C) have a small boiling point difference of 29°C; both are volatile and a fractionating column is needed for adequate separation. (c) Chromatography (paper or TLC), because the technique separates components based on differential attraction to the stationary and mobile phases rather than boiling point, allowing identification by Rf value comparison with known standards.

Marking notes. 1 mark per correct technique with a correct one-sentence justification referencing the relevant physical property (BP, volatility, or differential affinity).

Sample response. The fractionating column is packed with glass beads or rings that create a large surface area. As mixed vapour rises through the column, it repeatedly condenses on the cooler packing surfaces and re-vaporises — each cycle enriches the vapour in the lower-boiling (more volatile) component. By the time the vapour reaches the top of the column, it is predominantly the more volatile substance. This allows separation of liquids with close boiling points that would remain as a mixed vapour in simple distillation.

Marking notes. 1 mark for the structural function (large surface area for repeated condensation/vaporisation). 1 mark for explaining the enrichment mechanism (each cycle enriches vapour in more volatile component). 1 mark for linking to the outcome (vapour at top is predominantly the lower-boiling component, enabling separation of close BPs).

Sample response (a). Rf(P) = 3.0 ÷ 12.0 = 0.25.   Rf(Q) = 9.6 ÷ 12.0 = 0.80.   Rf(R) = 6.0 ÷ 12.0 = 0.50.

Sample response (b). Amino acid Q (Rf = 0.80) has the greatest affinity for the mobile phase. It moved the furthest from the origin, indicating it was most strongly carried by (attracted to) the mobile phase relative to the stationary phase. A higher Rf value reflects weaker attraction to the stationary phase and stronger attraction to the mobile phase.

Marking notes. 1 mark each for correct Rf calculations showing working: Rf(P) = 0.25, Rf(Q) = 0.80, Rf(R) = 0.50 (3 marks). 1 mark for correctly identifying Q as having the greatest affinity for the mobile phase with a valid explanation (highest Rf = travels furthest = most attracted to mobile phase).

Sample response. The stationary phase is the fixed material (filter paper / cellulose) through which the mobile phase moves; it does not move. The mobile phase is the solvent that flows through the stationary phase, carrying dissolved components with it. Components with stronger attraction to the stationary phase are held back and migrate more slowly, resulting in a low Rf value (small distance from origin). Components with stronger attraction to the mobile phase (greater solubility in the solvent) are carried further, resulting in a high Rf value. The Rf value therefore reflects the balance between the two opposing attractions for each component.

Marking notes. 1 mark for correctly defining and distinguishing stationary vs mobile phase (fixed vs moving). 1 mark for correctly explaining how strong attraction to stationary phase → low Rf. 1 mark for correctly explaining how strong attraction to mobile phase → high Rf (or equivalently, Rf reflects the balance of two attractions).

Sample response. The student’s claim is incorrect because Rf values are not universal constants — they depend on the specific solvent (mobile phase) and stationary phase used. Different solvents have different polarities and interact differently with each compound, changing how far it migrates and therefore changing its Rf. Similarly, different types of paper or TLC plate (e.g. cellulose vs. silica) alter the Rf. The correct conditions under which Rf values can be used for identification are: the same solvent, same stationary phase, same temperature, and same plate/paper type must be used as in the reference standard being compared. Only under identical conditions can a match in Rf values be used as evidence of the same compound.

Marking notes. 1 mark for identifying that Rf values change with different solvents (mobile phases). 1 mark for identifying that Rf values change with different stationary phases. 1 mark for correctly stating the conditions under which Rf can be used for identification (same solvent, same stationary phase, same conditions).

Sample response (a) — Rf calculations (solvent front = 8.5 cm): Spot 1: Rf = 1.7 ÷ 8.5 = 0.20.   Spot 2: Rf = 3.4 ÷ 8.5 = 0.40.   Spot 3: Rf = 5.1 ÷ 8.5 = 0.60.   Spot 4: Rf = 6.8 ÷ 8.5 = 0.80.

Marking notes (a). 1 mark per correct Rf value with working shown. 4 marks total. Accept correctly shown division even if rounding differs by ±0.01.

Sample response (b) — identification: Spot 1 (Rf = 0.20) = Standard β kaempferol. Spot 2 (Rf = 0.40) = Standard γ rutin. Spot 3 (Rf = 0.60) = Standard α quercetin. Spot 4 (Rf = 0.80) = Standard δ luteolin. All four spots match a reference standard, so all four flavonoid compounds are tentatively present in the wattle leaf extract. No spot is unmatched.

Marking notes (b). 1 mark for correctly matching all four spots to standards (accept any 3–4 correct matches for 1 mark). 1 mark for clearly explaining reasoning (Rf values match the reference standard Rf values). 1 mark for noting that all four spots are accounted for / no unmatched spot. Award 2 of 3 if all four are matched but no explanation given.

Sample response (c) — evaluate the claim: The chemist’s statement contains two flaws. First, the TLC experiment shows that this particular wattle leaf sample contains a compound with Rf = 0.80, matching luteolin under these conditions — but matching Rf values only provide evidence of the same compound under the same conditions; TLC cannot confirm chemical identity without additional tests (e.g. mass spectrometry). Second, the claim that “all wattle leaf samples from any location” contain luteolin is an unjustified generalisation; different populations of wattle trees, different species, or different growth conditions may produce different chemical profiles. The chemist would need to test multiple wattle species from multiple locations to support such a broad claim.

Marking notes (c). 1 mark for identifying that Rf match is not conclusive chemical identification (same Rf ≠ same compound without further testing). 1 mark for identifying that a single sample cannot be generalised to “all wattle samples from any location” — requires broader sampling or different species tested. Accept any two valid flaws in the chemist’s reasoning.

Sample response. Selecting an appropriate separation technique requires identifying which physical property differs between the components of the mixture. Distillation and chromatography exploit fundamentally different properties and are therefore suited to different types of mixtures. Distillation separates components based on differences in boiling point (volatility). In simple distillation, the mixture is heated so the more volatile component vaporises first; the vapour is cooled in a Liebig condenser and collected as the distillate, leaving the less volatile residue behind. This is highly effective when the boiling point difference is large (>25°C) or when one component is non-volatile. For example, desalination of sea water at the Wonthaggi desalination plant (which uses reverse osmosis, but simple distillation is the same principle) involves exploiting the enormous BP difference between water (100°C) and dissolved salts (non-volatile), producing pure water as distillate. When two miscible liquids have close boiling points — such as ethanol (78°C) and water (100°C) produced in Australian distilleries — fractional distillation with a fractionating column is required. The column creates multiple condensation/vaporisation cycles that progressively enrich the vapour in the lower-boiling component, allowing high-purity ethanol (~95%) to be collected. A key limitation of distillation is that it cannot separate components with identical boiling points (azeotropic mixtures), and it is unsuitable for thermally unstable compounds that decompose on heating. Chromatography separates components based on differential attraction to a stationary phase vs a mobile phase. A component strongly attracted to the mobile phase moves quickly (high Rf); one strongly attracted to the stationary phase moves slowly (low Rf). This makes chromatography ideal for identifying unknown compounds, monitoring product purity, or separating dissolved mixtures where BP differences are negligible. For example, forensic scientists at Australian crime laboratories use TLC and HPLC to identify drug compounds by comparing Rf values (or retention times) against reference standards — a task that distillation could not perform. Paper chromatography is used in schools and quality control labs to verify food dye composition. Key limitations of chromatography include: Rf values are not universal and only permit identification under identical conditions; it is primarily qualitative unless coupled with quantitative instruments; and two compounds can have coincidentally similar Rf values in a given solvent system. In conclusion, choosing between distillation and chromatography depends entirely on the mixture: distillation is selected when components differ in boiling point and the goal is physical separation; chromatography is selected when components differ in affinity for a phase system and the goal is identification or separation of dissolved substances. Neither technique is universally superior — the decision must be driven by the mixture’s properties.

Marking criteria (7 marks). 1 = correctly explains the physical principle of distillation (BP / volatility difference) including role of condenser. 1 = correctly explains when fractional distillation is preferred over simple distillation (close BPs, fractionating column), with mechanism. 1 = names and correctly uses an Australian or industrial example for distillation (desalination, ethanol production, crude oil refining at Kurnell/Altona, Australian distillery). 1 = correctly explains the physical principle of chromatography (differential attraction to stationary and mobile phases; Rf value). 1 = names and correctly uses an Australian or industrial example for chromatography (forensic TLC, food dye analysis, quality control). 1 = analyses at least one limitation of each technique (azeotropes/thermal instability for distillation; non-universal Rf / qualitative nature for chromatography). 1 = reaches an explicit evaluative judgement: the choice of technique is driven by the mixture’s physical properties, not a universal ranking.