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.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
A chemist wants to separate a mixture of ethanol (boiling point 78 C) and water (boiling point 100 C). Another chemist wants to separate water from dissolved salt. Which technique simple distillation or fractional distillation should each chemist choose, and why?
Before reading on, write your best answer. What is the key difference between these two mixtures that determines the correct technique?
Core Content
Wrong: Simple distillation can separate any mixture of two liquids.
Right: Simple distillation only works for mixtures with large boiling point differences (>25 degrees C). For liquids with close boiling points, fractional distillation with a fractionating column is required to achieve adequate separation.
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 a & Scondensation cycles, allowing separation of liquids with close boiling points (e.g. ethanol BP 78 a C and water BP 100 a C, or crude oil fractions).
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.
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.
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.
| Component | Distance from origin (cm) | Solvent front distance (cm) | Rf value |
|---|---|---|---|
| Spot 1 | 1.8 | 9.0 | a |
| Spot 2 | 4.5 | 9.0 | a |
| Spot 3 | 7.2 | 9.0 | a |
| Standard A | a | a | 0.50 |
| Standard B | a | a | 0.80 |
| Standard C | a | a | 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.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Multiple Choice
5 random questions from a replayable lesson bank a feedback shown immediately
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. CuSO a & a & is a non-volatile solid (it doesn't boil at any reasonable temperature). Water (BP 100 a C) vaporises and can be condensed as pure distillate. The BP difference is enormous a no fractionating column is needed.
2. Rf = 5.1 a 8.5 = 0.60
3. Fractional distillation. The BP difference is 98 a 69 = 29 a C. This is relatively small a 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 a 9.0 = 0.20 | Spot 2: 4.5 a 9.0 = 0.50 | Spot 3: 7.2 a 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 a so the sample does contain Standard A. The analyst's claim that Standard A is absent is incorrect.
1. C a 22 a C BP difference requires fractional distillation and a fractionating column.
2. B a Rf = 3.6 a 9.0 = 0.40
3. D a Strong attraction to stationary phase a a slow movement a a low Rf.
4. A a Rf values are only comparable under identical conditions. Student used hexane a a compare to hexane standard only a a Standard X.
5. C a Yellow moved furthest (highest Rf = 6.5/8.0 = 0.81) a a greatest affinity for mobile phase (water) a a most soluble in water.
Q6 (3 marks): Simple distillation is used when there is a large BP difference between components (typically >25 a C) or when one component is non-volatile a 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 a C and 100 a C) a 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 a 12.0 = 0.30 (1 mark). Rf(B) = 9.6 a 12.0 = 0.80 (1 mark). Rf(C) = 7.2 a 12.0 = 0.60 (1 mark). Amino acid B has the greatest affinity for the mobile phase a 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 a C for gases to above 350 a C for heavy oils/bitumen) a 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).
Return to your Think First response. You should now be able to justify the correct technique for each separation:
Distillation and Chromatography
Tick when you've finished all activities and checked your answers.