Chemistry • Year 11 • Module 1 • Lesson 4
Separation Techniques — Advanced Methods
Build HSC Band 5–6 extended-response technique on choosing and justifying separation methods, evaluating data, and designing experiments involving distillation and chromatography.
1. Data + scenario: crude oil refining at Kurnell (Band 5–6)
8 marks Band 5–6
Scenario. The Kurnell oil refinery in New South Wales historically processed crude oil into useful petroleum fractions. Crude oil is a complex mixture of hydrocarbons; selected fractions and their approximate boiling point ranges are shown below. After the refinery closed in 2014 and was converted to a fuel import terminal, Australia now imports refined petroleum. A chemical engineer is evaluating whether Australia could economically re-establish domestic fractional distillation of crude oil to increase fuel security.
| Fraction | Approximate boiling point range (°C) | Primary use |
|---|---|---|
| Liquefied petroleum gas (LPG) | Below 40 | Camping gas, bottled fuel |
| Petrol (gasoline) | 40–200 | Motor vehicle fuel |
| Kerosene / jet fuel | 150–300 | Aircraft fuel, heating |
| Diesel | 250–350 | Trucks, trains, ships |
| Fuel oil / heavy fuel | 300–400 | Power stations, marine engines |
| Bitumen / asphalt | Above 400 | Road surfacing |
Data adapted from standard petroleum chemistry references. Boiling point ranges are approximate and vary with crude oil source.
Q1. Analyse and evaluate the use of fractional distillation to separate crude oil into useful petroleum fractions. In your response you must:
- Explain the principle by which fractional distillation separates crude oil fractions, with reference to the role of the fractionating column and boiling point.
- Use the data in the table to justify why fractional rather than simple distillation is required, with specific reference to boiling point overlap between at least two named fractions.
- Evaluate the effectiveness of the technique in producing pure fractions, including a discussion of one limitation shown in the data.
- Assess whether simple distillation could be used as an alternative for any one fraction; justify your assessment.
- State one social or industrial benefit and one environmental cost of domestic crude oil fractional distillation in Australia.
2. Experimental design — testing whether a mystery substance is a pure compound or a mixture (Band 5–6)
7 marks Band 5–6
Research question. A forensic chemist receives a coloured liquid sample labelled “Unknown X” and needs to determine whether it is a pure coloured liquid or a mixture of at least two differently coloured compounds dissolved in a solvent. Design an investigation using paper chromatography to answer the research question, and explain how you would use Rf values to identify the components.
Constraints: You have access to standard Year 11 laboratory equipment including filter paper, a range of solvents (water, ethanol, propanone), a pencil, a ruler, a UV lamp, and reference standards (pure samples) of eight known coloured compounds. Your investigation must be completed within one laboratory session.
Q2. Design the investigation and present it in the format below.
- State a hypothesis (a testable prediction that includes the independent and dependent variables).
- 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 use reference standards and Rf values to identify components.
- Explain what result would falsify your hypothesis (i.e. confirm Unknown X is a pure compound).
- State two limitations of paper chromatography as an identification technique, and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Principle (fractional distillation of crude oil): Fractional distillation separates crude oil by exploiting the different boiling points of its hydrocarbon components [1 — correct principle]. When crude oil is heated in a distillation tower, the fractions with the lowest boiling points vaporise first and rise to the top of the fractionating column where they are condensed and collected; higher-boiling fractions condense at lower positions on the column. The fractionating column creates a temperature gradient — cooler at the top, hotter at the bottom — so each fraction condenses at the appropriate level according to its boiling point [1 — fractionating column role and BP gradient].
Why fractional rather than simple distillation: The data show significant overlap in boiling point ranges between fractions — for example, kerosene (150–300°C) and diesel (250–350°C) overlap between 250°C and 300°C [1 — specific data reference with named fractions]. Simple distillation could not separate these overlapping fractions because both would vaporise at similar temperatures and contaminate each other’s collection flask. Only the multiple condensation/vaporisation cycles provided by the fractionating column allow adequate separation [1 — justification linked to multiple cycles].
Effectiveness and limitation: Fractional distillation is highly effective for separating fractions with large enough BP differences (e.g. LPG <40°C vs. bitumen >400°C). However, a key limitation is that each fraction is a range of hydrocarbons with overlapping BPs, not a single pure compound — petrol (40–200°C) still contains dozens of different hydrocarbons [1 — limitation from data].
Could simple distillation be used for any fraction? Simple distillation could not substitute for fractional distillation for most fractions due to overlapping BPs. However, for the purpose of separating LPG (BP <40°C) from bitumen (BP >400°C), the boiling point difference exceeds 360°C — which is large enough for single-stage distillation to produce LPG vapour while leaving bitumen as residue [1 — justified assessment with specific fraction].
Social benefit and environmental cost: Social/industrial benefit: domestic fractional distillation would reduce Australia’s dependence on imported fuels, improving fuel security and reducing supply chain vulnerability to global disruptions (e.g. shipping blockades or price shocks) [1 — valid benefit]. Environmental cost: crude oil refining produces significant CO⊂2; emissions and generates toxic by-products (sulphur compounds, heavy metals) that require careful disposal; operating a refinery near Botany Bay (Kurnell) poses risks of hydrocarbon spills and air pollution affecting coastal ecosystems [1 — valid environmental cost with Australian context].
Marking criteria summary (8 marks): 1 = correct principle (boiling point + fractionating column temperature gradient); 1 = role of fractionating column (multiple vaporisation/condensation cycles); 1 = specific data reference with named overlapping fractions (kerosene/diesel or petrol/kerosene); 1 = justification of why fractional distillation is needed (overlapping BPs); 1 = limitation from data (fractions are ranges, not pure compounds); 1 = valid assessment of simple distillation for one fraction with justification; 1 = valid social or industrial benefit; 1 = valid environmental cost with Australian context.
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If Unknown X is a mixture of two or more compounds, then paper chromatography in ethanol solvent will produce more than one spot on the chromatogram, each with a different Rf value matching a different reference standard. Independent variable: whether the sample is Unknown X or a reference standard compound. Dependent variable: the number of spots produced and their Rf values. Controlled variables: same solvent (ethanol), same stationary phase (same grade of filter paper), same development distance (solvent front allowed to travel 10.0 cm). [1 — hypothesis with IV and DV]
Procedure: (1) Using a pencil (not pen), draw a light baseline 2.0 cm from the bottom of a strip of filter paper. With a fine capillary tube, place a small spot of Unknown X at the centre of the baseline; place spots of each of the eight reference standards at equally-spaced positions along the same baseline. Allow spots to dry. (2) Pour enough ethanol solvent into a developing jar to reach 1.0 cm depth — below the baseline. Lower the chromatography strip into the jar so it stands upright without touching the sides; seal the lid. Allow the solvent to rise until the front reaches 10.0 cm above the baseline, then remove and immediately mark the solvent front in pencil. Allow to dry. (3) Using a ruler, measure the distance from the baseline to the centre of each spot and to the solvent front. Calculate Rf = (distance from baseline to spot) ÷ (distance from baseline to solvent front) for each spot produced by Unknown X and each reference standard. (4) Compare the Rf values of spots from Unknown X to the Rf values of reference standards. Spots with matching Rf values (within ±0.02) are tentatively identified as the same compound. Count the number of spots in the Unknown X lane to determine whether it is a pure compound or mixture. [1 — four clear steps with Rf calculation and identification method]
Falsification: If Unknown X produces exactly one spot and its Rf matches that of exactly one reference standard (within ±0.02), the hypothesis would be falsified — there is no evidence of a mixture, and Unknown X is consistent with being that single pure compound [1].
Limitations: (1) Two different compounds could have the same or very similar Rf values in a given solvent system, meaning a single spot does not conclusively confirm a pure compound — a second solvent must be used to confirm identification [1]. (2) Paper chromatography is a qualitative technique; it cannot determine the concentration or relative proportion of each component in Unknown X [1].
Improvement: Repeat the chromatography at least three times in the same solvent to check reproducibility of Rf values; additionally, run a second chromatogram using a different solvent (e.g. propanone) to confirm the identification by cross-referencing two independent Rf values for each suspected component [1].
What the results would show (if mixture): Two or more spots in the Unknown X lane, each matching a reference standard Rf, confirming at least two different compounds are present and Unknown X is a mixture [1].
Marking criteria summary (7 marks): 1 = testable hypothesis naming IV and DV; 1 = four numbered steps with correct Rf measurement method (from baseline, not from solvent front); 1 = correct Rf formula applied; 1 = identifies what would falsify the hypothesis (one spot = pure compound); 1 = first valid limitation (same Rf values possible for different compounds); 1 = second valid limitation (quantitative limitation or sensitivity); 1 = one specific improvement increasing reliability (repeat or second solvent).