Imagine trying to separate two invisible gases that are perfectly mixed. How would you even begin? Distillation and chromatography are the chemist's answer — two elegant techniques that exploit subtle differences in physical properties to pull apart what looks inseparable.
Core Content
Principle: boiling point difference. When a solution is heated, the more volatile component (lower BP) vaporises first. The vapour is cooled in a condenser, converting it back to liquid (distillate) in a separate container. The less volatile component remains behind.
Principle: same as simple distillation, but uses a fractionating column packed with glass beads or rings. The column creates many successive vaporisation–condensation cycles, allowing separation of liquids with close boiling points (e.g. ethanol BP 78°C and water BP 100°C, or crude oil fractions).
| Feature | Simple distillation | Fractional distillation |
|---|---|---|
| BP difference needed | >25°C (ideally much more) | Works with small differences (<25°C) |
| Equipment | Flask, condenser, thermometer, receiver | Same + fractionating column |
| Example | Salt water → pure water | Ethanol/water mixture, crude oil |
| Result | One distillate + residue | Multiple fractions collected separately |
Insert two labelled side-by-side diagrams. Simple: round-bottom flask, thermometer, Liebig condenser, receiver flask. Fractional: same but add fractionating column (with packing) between flask and condenser head. Label all components and show vapour pathway.
Chromatography separates components based on how strongly each component is attracted to the stationary phase vs how well it dissolves in the mobile phase. Components that are more strongly attracted to the stationary phase move slowly; those more attracted to the mobile phase move faster.
Stationary phase = filter paper (cellulose). Mobile phase = solvent (e.g. water, ethanol). A spot of the mixture is placed near the bottom of the paper; the solvent travels up by capillary action, carrying components at different rates.
Stationary phase = silica or alumina coated on a glass/aluminium plate. More sensitive than paper chromatography; components often appear as UV-visible spots under UV light. Otherwise operates on the same principle.
Insert diagram of a developed chromatography strip: show baseline (origin), two spots at different heights, solvent front line. Label: origin, solvent front, distance moved by component A, distance moved by component B, distance moved by solvent front. Show Rf calculation beside each spot.
Chromatography is ideal when: (a) separating a mixture of dissolved substances with different polarities or sizes, (b) identifying components of a mixture by comparison with known standards, (c) monitoring the purity of a product.
| Mixture type | Best technique | Why |
|---|---|---|
| Solvent + non-volatile dissolved solid (e.g. salt water) | Simple distillation | Large BP difference; collect pure solvent |
| Two miscible liquids, similar BPs (e.g. ethanol + water) | Fractional distillation | Need multiple vaporisation cycles to separate |
| Multiple dissolved compounds in solution (e.g. dyes in ink) | Chromatography | Separates by differential attraction, not BP |
| Two liquids + need to identify components | Chromatography + compare Rf to standards | Rf values are characteristic per compound |
Worked Examples
Activities
1 A student wants to obtain pure water from a solution of copper sulfate in water. Which distillation technique is most appropriate? Justify your choice.
2 On a chromatography strip, a spot travels 5.1 cm and the solvent front travels 8.5 cm. Calculate the Rf value and show your working.
3 A mixture of hexane (BP 69°C) and heptane (BP 98°C) needs to be separated. Which distillation technique should be used, and why? Consider the boiling point difference in your answer.
| Component | Distance from origin (cm) | Solvent front distance (cm) | Rf value |
|---|---|---|---|
| Spot 1 | 1.8 | 9.0 | — |
| Spot 2 | 4.5 | 9.0 | — |
| Spot 3 | 7.2 | 9.0 | — |
| Standard A | — | — | 0.50 |
| Standard B | — | — | 0.80 |
| Standard C | — | — | 0.20 |
A Calculate the Rf value for each of Spots 1, 2, and 3. Show full working for each.
B Using your calculated Rf values, identify which spot matches which standard. Explain your reasoning.
C The mixture being analysed was a sample of food colouring. The analyst concluded that the sample contained Standard B and Standard C but not Standard A. Is this conclusion supported by the data? Explain.
Multiple Choice
Click to check. One attempt only.
1. Which technique is most appropriate for separating ethanol (BP 78°C) from water (BP 100°C)?
2. A component travels 3.6 cm on a chromatography strip where the solvent front travels 9.0 cm. What is its Rf value?
3. In paper chromatography, a component with a strong attraction to the stationary phase will:
4. A student uses chromatography to identify an unknown compound and finds its Rf = 0.65. A reference chart lists Standard X (Rf = 0.65, hexane solvent) and Standard Y (Rf = 0.65, ethanol solvent). The student used hexane as the mobile phase. Which conclusion is valid?
5. A chromatography experiment separates a mixture of three food dyes (Red, Blue, Yellow) using water as the mobile phase. The results show Red spot at 2.0 cm, Yellow at 6.5 cm, Blue at 4.0 cm (solvent front 8.0 cm). Which dye has the greatest attraction to the mobile phase (water)?
Short Answer
6. Explain the difference between simple distillation and fractional distillation. In your answer, specify when each technique is appropriate and the role of the fractionating column. 3 MARKS
7. A student separates a mixture of three amino acids using paper chromatography. The solvent front moves 12.0 cm. Amino acid A moves 3.6 cm, B moves 9.6 cm, C moves 7.2 cm. Calculate the Rf value for each amino acid and identify which amino acid has the greatest affinity for the mobile phase. 4 MARKS
8. Crude oil is a mixture of hydrocarbons with different boiling points. Evaluate the use of fractional distillation to separate crude oil into useful fractions, including a discussion of what makes this technique effective and any limitations. 4 MARKS
1. Simple distillation. CuSO₄ is a non-volatile solid (it doesn't boil at any reasonable temperature). Water (BP 100°C) vaporises and can be condensed as pure distillate. The BP difference is enormous — no fractionating column is needed.
2. Rf = 5.1 ÷ 8.5 = 0.60
3. Fractional distillation. The BP difference is 98 − 69 = 29°C. This is relatively small — both components are volatile and will compete for the vapour phase. A fractionating column provides multiple condensation/vaporisation cycles to adequately separate the two liquids. Simple distillation would give a mixture of both compounds in the distillate.
A: Spot 1: 1.8 ÷ 9.0 = 0.20 | Spot 2: 4.5 ÷ 9.0 = 0.50 | Spot 3: 7.2 ÷ 9.0 = 0.80
B: Spot 1 = Standard C (Rf 0.20). Spot 2 = Standard A (Rf 0.50). Spot 3 = Standard B (Rf 0.80). Identification by matching calculated Rf to known reference Rf values.
C: The conclusion is not supported. The data shows Spots matching Standard C (0.20), Standard A (0.50), and Standard B (0.80). Standard A has Rf = 0.50 and Spot 2 matches it — so the sample does contain Standard A. The analyst's claim that Standard A is absent is incorrect.
1. C — 22°C BP difference requires fractional distillation and a fractionating column.
2. B — Rf = 3.6 ÷ 9.0 = 0.40
3. D — Strong attraction to stationary phase → slow movement → low Rf.
4. A — Rf values are only comparable under identical conditions. Student used hexane → compare to hexane standard only → Standard X.
5. C — Yellow moved furthest (highest Rf = 6.5/8.0 = 0.81) → greatest affinity for mobile phase (water) → most soluble in water.
Q6 (3 marks): Simple distillation is used when there is a large BP difference between components (typically >25°C) or when one component is non-volatile — only the more volatile component vaporises and is collected as distillate (1 mark). Fractional distillation is needed when two or more miscible liquids have similar boiling points (e.g. 78°C and 100°C) — both would partially vaporise in simple distillation, giving an impure distillate (1 mark). The fractionating column provides multiple condensation/vaporisation cycles along its length, gradually enriching the vapour in the lower-boiling component, so that the vapour reaching the condenser is predominantly the more volatile substance (1 mark).
Q7 (4 marks): Rf(A) = 3.6 ÷ 12.0 = 0.30 (1 mark). Rf(B) = 9.6 ÷ 12.0 = 0.80 (1 mark). Rf(C) = 7.2 ÷ 12.0 = 0.60 (1 mark). Amino acid B has the greatest affinity for the mobile phase — it moved furthest (highest Rf = 0.80), meaning it was most attracted to the mobile phase and least attracted to the stationary phase (1 mark).
Q8 (4 marks): Fractional distillation is effective for crude oil because different hydrocarbon fractions have significantly different boiling points (ranging from below 20°C for gases to above 350°C for heavy oils/bitumen) — the fractionating column allows these to be separated into distinct fractions collected at different temperature zones (1 mark). Each fraction contains hydrocarbons with similar chain lengths and similar properties (e.g. petrol, kerosene, diesel), making them useful directly or as feedstocks for further processing (1 mark). Limitations: the process requires large energy input to maintain high temperatures; fractions are not pure single compounds but mixtures of similar hydrocarbons; very closely-boiling components are difficult to fully separate even with tall columns (1 mark). Additionally, crude oil composition varies between sources, meaning fractionation conditions must be adjusted for each batch (1 mark).
Tick when you've finished all activities and checked your answers.