Chemistry • Year 11 • Module 1 • Lesson 4
Separation Techniques — Advanced Methods
Apply your understanding of distillation, chromatography, and Rf calculations to real data sets, scenarios, and a diagram critique.
1. Interpret chromatography data — food dye analysis
A food technologist separated four commercial food dyes (Red, Yellow, Green, Blue) using paper chromatography. The solvent front moved 9.0 cm from the origin. The table below records spot positions. 10 marks
| Dye | Distance moved by spot (cm) | Solvent front distance (cm) | Rf value (show working) |
|---|---|---|---|
| Red | 1.8 | 9.0 | |
| Yellow | 7.2 | 9.0 | |
| Green | 4.5 | 9.0 | |
| Blue | 6.3 | 9.0 |
1.1 Calculate the Rf value for each dye. Show full working in the table above. 4 marks (1 each)
1.2 Which dye has the strongest attraction to the mobile phase? Justify your answer using Rf values. 2 marks
1.3 A reference sample of tartrazine (a yellow food dye) has Rf = 0.80 in the same solvent system. Based on your calculations, which dye in the table is most likely tartrazine? Explain your reasoning. 2 marks
1.4 The technologist repeats the chromatography using a different solvent. State whether the same Rf values would be obtained and explain why. 2 marks
2. Interpret graph — temperature profile during fractional distillation of ethanol/water
A student performed fractional distillation of an ethanol/water mixture (30% ethanol by volume) and recorded the temperature at the top of the fractionating column as successive fractions were collected. 7 marks
Figure 2. Temperature at the head of the fractionating column vs. volume of distillate collected. Initial mixture: 30% ethanol / 70% water by volume. Ethanol BP = 78°C; Water BP = 100°C. Illustrative data.
2.1 Describe the trend shown in the graph between 0 mL and 30 mL of distillate collected. Refer to specific temperature values in your answer. 2 marks
2.2 Explain, using your knowledge of the fractionating column, why the temperature remains near 78°C for the first 15 mL of distillate. 2 marks
2.3 A student argues that simple distillation could have separated this mixture equally well. Use evidence from the graph to refute this claim. 3 marks
3. Compare simple distillation and fractional distillation
Complete the table comparing the two distillation techniques across five features. 10 marks (1 per cell)
| Feature | Simple distillation | Fractional distillation |
|---|---|---|
| Boiling point difference required | ||
| Equipment difference | ||
| Number of fractions collected | ||
| Named Australian / industrial example | ||
| Purity of distillate |
4. Diagram critique — what’s wrong with this student’s chromatography diagram?
A Year 11 student drew the diagram below to explain how to calculate an Rf value. There are three errors. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
4.1 Error 1: What is wrong?
Correction:
4.2 Error 2: What is wrong?
Correction:
4.3 Error 3: What is wrong?
Correction:
Q1.1 — Rf calculations
Red: Rf = 1.8 ÷ 9.0 = 0.20. Yellow: Rf = 7.2 ÷ 9.0 = 0.80. Green: Rf = 4.5 ÷ 9.0 = 0.50. Blue: Rf = 6.3 ÷ 9.0 = 0.70.
Q1.2 — Strongest attraction to mobile phase (2 marks)
Yellow dye (Rf = 0.80) has the strongest attraction to the mobile phase [1]. A higher Rf means the component travelled further with the solvent, indicating it is more soluble in (attracted to) the mobile phase than to the stationary phase [1].
Q1.3 — Identity of tartrazine (2 marks)
Yellow dye is most likely tartrazine [1]. Its calculated Rf (0.80) matches the reference standard for tartrazine (Rf = 0.80) under the same solvent system — Rf values are characteristic per compound under identical conditions, so a matching Rf indicates the same compound [1].
Q1.4 — Different solvent (2 marks)
No, the same Rf values would not be obtained [1]. Rf values depend on the relative attraction of each compound to the specific mobile phase and stationary phase used. A different solvent changes the relative affinities of each dye for the mobile phase, altering how far each migrates and therefore changing all Rf values [1].
Q2.1 — Trend description (2 marks)
The temperature remains nearly constant at approximately 78°C for the first 15 mL of distillate [1]. Between 15 mL and 20 mL the temperature rises sharply through the transition zone, then levels off near 100°C and remains approximately constant for the remaining 10 mL [1].
Q2.2 — Why plateau at 78°C (2 marks)
The fractionating column promotes multiple condensation and vaporisation cycles along its length [1]. This progressively enriches the vapour in ethanol (the lower-boiling component), so that the vapour reaching the column head and condenser is predominantly ethanol — hence the temperature at the top stays near ethanol’s boiling point (78°C) until most of the ethanol is exhausted [1].
Q2.3 — Refuting simple distillation (3 marks)
The boiling point difference between ethanol (78°C) and water (100°C) is only 22°C [1]. With such a small difference, simple distillation (without a fractionating column) would produce a distillate that is a mixture of both ethanol and water, not a pure fraction [1]. The graph shows two distinct temperature plateaus, which can only be achieved because the fractionating column provides the multiple cycles needed to enrich the vapour in ethanol before condensation — simple distillation cannot achieve this separation [1].
Q3 — Compare and contrast table
Boiling point difference: Simple: large difference (>25°C), or one component non-volatile. Fractional: works with small differences (<25°C). Equipment: Simple: flask, condenser, thermometer, receiver. Fractional: same, plus a fractionating column (packed with glass beads/rings). Fractions collected: Simple: one distillate + residue. Fractional: multiple fractions collected at different temperatures. Australian/industrial example: Simple: desalination of sea water; purification of copper sulfate solution. Fractional: separation of ethanol and water in a distillery; crude oil refining (petroleum refinery at Altona or Kurnell). Purity of distillate: Simple: high purity when BP difference is large (non-volatile solute). Fractional: can achieve very high purity for volatile mixtures (~95% ethanol purity achievable).
Q4 — Diagram critique (6 marks)
4.1 Error 1 (Rf formula): The Rf formula uses "distance from solvent front to spot" as numerator, rather than "distance from origin to spot" [1]. Correction: Rf = (distance from origin to spot centre) ÷ (distance from origin to solvent front). Both distances must be measured from the same reference point — the origin [1].
4.2 Error 2 (measurement direction): The arrow measuring the spot distance is drawn from the top of the paper (solvent front) downward to the spot, instead of from the origin upward [1]. Correction: the measurement for both the spot and solvent front distances must start at the origin (baseline) and go upward to the spot centre or solvent front respectively [1].
4.3 Error 3 (Rf > 1): An Rf value of 1.3 is labelled for a spot below the solvent front, implying Rf can exceed 1 [1]. Correction: Rf values are always between 0 and 1 — no component can travel further than the solvent front (which would give Rf = 1). An Rf of 1.3 is physically impossible [1].