Distillation and Chromatography
In 1952, British chemists Archer Martin and Richard Synge won the Nobel Prize for inventing paper chromatography — a technique now used by Australian forensic labs to separate samples as small as 0.001 g from crime scenes.
Printable Worksheets
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Q1 · If you wanted to get pure water from muddy river water, what would you do? Describe your steps.
Q2 · Why do you think different colours in a felt-tip pen end up at different heights when you dip it in water?
● Know
- What distillation is and when it is used to separate mixtures
- The role of each piece of equipment in a distillation setup
- What chromatography is and how Rf values work
● Understand
- Why different boiling points allow distillation to separate two liquids
- Why substances travel different distances in chromatography
- How Rf values can be used to identify an unknown substance
● Can do
- Label the key parts of a distillation apparatus
- Calculate an Rf value from given measurements
- Choose whether distillation or chromatography is better for a given separation task
- Distillation
- Condenser
- Boiling point
- Chromatography
- Rf value
- Temperature at which a liquid turns to vapour
- Uses boiling point differences to separate liquid mixtures
- Distance spot moved ÷ distance solvent moved
- Cool tube that turns vapour back into liquid
- Separates dissolved substances by how far they travel in a solvent
Imagine you have a glass of saltwater. You can't filter out the salt — it's dissolved. But if you heat the water, the water molecules escape as steam long before the salt does. Catch that steam, cool it back into liquid, and you've collected pure water. You've just done distillation.
Distillation separates a mixture by exploiting the fact that different substances have different boiling points. The substance with the lower boiling point vaporises first; the vapour is channelled away, cooled in a condenser, and collected as a pure liquid in a separate container.
Key Australian applications:
- Petroleum refining — refineries at Botany Bay (NSW) and Kwinana (WA) use fractional distillation to separate crude oil into petrol, diesel, kerosene and bitumen. Each fraction has a different boiling point range.
- Desalination — in remote communities and on ships, distillation turns salty or brackish water into safe drinking water.
- Eucalyptus oil production — steam distillation separates the aromatic oil from plant material. Australia is a world-leading producer of eucalyptus oil.
| Substance | Boiling point | Use in distillation |
|---|---|---|
| Water (H₂O) | 100 °C | Desalination, lab separations |
| Ethanol | 78 °C | Separating alcohol from water (lower boiling point → vaporises first) |
| Petrol fraction | 35–75 °C | First fraction to come off in crude oil distillation |
| Salt (NaCl) | 1413 °C | Never vaporises at normal distillation temps — always stays behind |
- Flask A is heated
- Vapour enters the condenser
- Liquid drips into flask B
- Higher boiling point substance stays in flask A
- The separated, purified liquid is collected
- The lowest boiling point component begins to vaporise
- It has not yet reached its boiling point so it cannot escape
- Cool water outside the condenser turns the vapour back to liquid
Here is a standard simple distillation setup:
Step-by-step sequence:
- Flask A holds the mixture (e.g., ethanol + water).
- The mixture is heated gently. The component with the lowest boiling point vaporises first (ethanol, 78 °C).
- Vapour travels along the glass tube into the condenser. Cold water flowing around the outside removes the heat.
- The vapour condenses back into liquid and drips into Flask B.
- The substance with the higher boiling point (water, 100 °C) remains in Flask A.
- Result: Flask A has water; Flask B has ethanol. Separated.
A thermometer at the top of Flask A monitors the temperature of the vapour. When the thermometer reads 78 °C, ethanol is coming over. When it jumps to 100 °C, water starts to vaporise — time to change collection flasks or stop the experiment.
In distillation, the component with the boiling point vaporises first. The vapour travels into the , where it is cooled and turns back into a . This liquid is collected in Flask B and is now .
Chromatography works on a completely different principle to distillation. Instead of heat, it uses a moving liquid (solvent) to carry dissolved substances across a stationary material. Different substances travel different distances because they have different solubilities in the solvent — the more soluble a substance, the further it travels.
In paper chromatography:
- The mobile phase is the solvent (water, or a specific liquid). It moves up the paper.
- The stationary phase is the paper (or silica plate). It doesn't move.
- Substances that are more attracted to the mobile phase (more soluble) travel further. Substances more attracted to the paper travel less far.
After the experiment, each substance leaves a spot at a different height. You can calculate its Rf value (retention factor):
Rf = distance travelled by spot ÷ distance travelled by solvent
Rf values range from 0 (didn't move at all) to 1 (moved as far as the solvent). Every substance has a unique Rf value in a given solvent — this acts like a fingerprint to identify it.
Real-world uses:
- Forensics — comparing ink from a crime scene pen to a suspect's pen. Run chromatography on both; if the spots appear at the same Rf values, the inks are the same.
- Food science — checking whether food dyes in a product are the approved colours or contain banned additives.
- Drug testing — identifying unknown substances in urine or blood samples.
Example: if a red dye spot travels 6 cm and the solvent front travels 10 cm, the Rf value = 6 ÷ 10 = 0.6. Compare this to a reference chart of known dyes — if the official red dye also has Rf = 0.6, it's a match.
A student runs paper chromatography on a black food dye. The solvent travels 12 cm. The student sees three spots: one at 3 cm, one at 7.2 cm, and one at 10.8 cm from the start line. Predict: (a) how many pigments are in the black dye? (b) Calculate the Rf value of the spot at 7.2 cm. (c) Which spot is the most soluble in the solvent?
(a) Three separate spots appear, so the black dye is actually a mixture of at least three different coloured pigments. This is a classic demonstration — "black" ink is nearly always a mix of several pigments. Chromatography reveals this hidden complexity.
(b) Rf = distance spot moved ÷ distance solvent moved = 7.2 ÷ 12 = 0.6. Simple division.
(c) The spot at 10.8 cm has Rf = 10.8 ÷ 12 = 0.9 — it travelled 90% of the way up the paper, nearly as far as the solvent. This means it is the most soluble in the mobile phase: the solvent "likes" carrying it along, so it gets swept further. The spot at 3 cm (Rf = 0.25) is least soluble — the paper held it back more than the solvent could pull it forward.
How close was your prediction?
Well done — you connected solubility to distance travelled and applied the Rf formula correctly.
Key rule: Rf = spot distance ÷ solvent distance. Higher Rf = more soluble in the solvent.
A student has a mixture of ethanol (boiling point 78 °C) and water (boiling point 100 °C). They want to collect the ethanol in a separate flask.
- List the equipment they would need.
- Describe the steps in order (at least 5 steps).
- At what thermometer reading should they start collecting the distillate into Flask B?
- What would remain in Flask A at the end?
A student tests three food dyes (A, B, C) by paper chromatography. The solvent front travels 9.0 cm. Their results are below:
| Dye | Distance spot moved (cm) | Rf value (calculate this) |
|---|---|---|
| A | 2.7 | |
| B | 6.3 | |
| C | 8.1 |
- Calculate the Rf value for each dye. Show your working.
- Which dye is the most soluble in the solvent? How do you know?
- The legal limit for Rf of an approved food dye is between 0.65 and 0.75. Is any dye in this range?
Q1. Explain how you would use distillation to separate a mixture of ethanol and water (boiling points 78 °C and 100 °C). Name the equipment needed. (3 marks)
Q2. A student performs paper chromatography on a food dye. The spot travels 4.5 cm and the solvent front travels 9.0 cm. Calculate the Rf value and explain what it means. (3 marks)
Q3. Compare distillation and chromatography as separation techniques. When would you choose each method? (4 marks)
Answers
▾MCQ 1
B — Distillation separates liquids (or dissolved solids from liquids) using differences in boiling point. Colour is used in no separation technique. Mass is used in density separation or centrifugation. Solubility differences are used in chromatography, not distillation.
MCQ 2
C — The condenser's only job is to cool vapour back into liquid. It does this by allowing cold water to flow around the outside of the inner tube, removing the heat energy from the vapour. It does not heat, dissolve, or filter anything.
MCQ 3
C — The more soluble a substance is in the mobile phase (solvent), the more the solvent "carries" it along, so it travels further. The least soluble substance is held back more by the paper (stationary phase) and barely moves. Mass and gas state are not relevant here.
MCQ 4
B — Rf = distance spot moved ÷ distance solvent moved. The spot is always in the numerator (top) and the solvent is always in the denominator (bottom). Since the solvent always travels at least as far as any spot, Rf will always be a number between 0 and 1. If you flip it (A), you'd get a number greater than 1, which doesn't make sense.
MCQ 5
B — Petroleum refining uses fractional distillation to separate crude oil into fuels (petrol, diesel, kerosene) and other products. Australian refineries include sites at Botany Bay, NSW, and Kwinana, WA. The other industries do not rely on distillation as a core process.
Short Answer 1
Model answer: Equipment needed: round-bottomed flask (Flask A), Bunsen burner / heat source, Liebig condenser, thermometer, collection flask (Flask B), clamp stand (1 mark for naming at least 3 items). Steps: Pour ethanol-water mixture into Flask A and heat gently. Ethanol (boiling point 78 °C) vaporises first. The vapour travels through the condenser where it is cooled by cold water flowing around the outside. The vapour condenses to liquid ethanol and drips into Flask B. The water (boiling point 100 °C) stays in Flask A (2 marks for correct stepwise description including which substance comes over first and why).
Short Answer 2
Model answer: Rf = 4.5 ÷ 9.0 = 0.5 (1 mark for correct formula and calculation). This means the spot travelled exactly half the distance the solvent travelled (1 mark for explanation of what 0.5 means in context). Every substance has a unique Rf value in a given solvent, so this number can be compared to a reference table to identify which dye it is (1 mark for identification/fingerprinting use).
Short Answer 3
Model answer: Distillation separates mixtures where the components are liquids (or a dissolved solid in a liquid) with different boiling points. You would choose distillation when you need to separate two miscible liquids (e.g., ethanol and water) or when you need to collect a pure liquid from a solution (e.g., pure water from seawater) (2 marks). Chromatography separates mixtures where multiple substances are dissolved in the same solvent and have different solubilities. You would choose chromatography when you need to identify the components of a mixture (e.g., the dyes in a food colour, the pigments in ink) or when the substances are present in very small amounts (e.g., forensic samples, drug testing). The key difference: distillation collects a pure substance in a separate flask, while chromatography spreads components across a surface for identification (2 marks).
At the start of this lesson you were asked: A forensic scientist has a suspicious blue pen at a crime scene and five other blue pens in the office — how could they figure out which one wrote the note, without seeing anyone write? Now you know the answer!
Explain exactly how chromatography could solve the forensic scientist's problem. Use the words mobile phase, stationary phase, solubility and Rf value in your answer.
- Distillation separates mixtures using different boiling points — the lowest boiling point component vaporises, travels through a condenser, and is collected as a pure liquid.
- Chromatography separates dissolved substances by how far they travel in a solvent — more soluble = further = higher Rf value.
- Rf = distance spot moved ÷ distance solvent moved. Each substance has a unique Rf, used as an identification fingerprint.