Distillation and Chromatography

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 uses fractional distillation to separate crude oil into petrol, diesel, kerosene and bitumen. Each fraction has a different boiling point range. Kurnell on Botany Bay is a historical Australian example: it ceased refining in 2014 and now operates as a fuel import terminal. Source: Ampol’s Kurnell site history.
• 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.

Here is a standard simple distillation setup:

Step-by-step sequence:

1. Flask A holds the mixture (e.g., ethanol + water).
2. The mixture is heated gently. The vapour and distillate become enriched in the more volatile component (ethanol in this example; pure ethanol cannot be obtained by ordinary distillation of the ethanol-water mixture).
3. Vapour travels along the glass tube into the condenser. Cold water flowing around the outside removes the heat.
4. The vapour condenses back into liquid and drips into Flask B.
5. The liquid remaining in Flask A becomes enriched in the less volatile component, water.
6. Result: distillate is ethanol-enriched and residue is water-enriched; they are not pure components.

A thermometer monitors the vapour temperature. The simple boiling-point comparison is useful for predicting which component is enriched in the vapour, but ethanol and water form an azeotrope: ordinary distillation cannot yield pure ethanol and pure water from this mixture.

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.

In Lesson 6 you built a chain of questions for choosing a separation method: is anything magnetic, is anything an undissolved solid, is anything dissolved. That chain ran out of answers at exactly the point this lesson picks up, because it could recover a dissolved solid and nothing else. Two more questions finish it.

1. Which part do you actually want to keep? This is the question students skip, and it is the one that separates evaporation from distillation.
2. Do you need to collect the parts, or only find out what they are? Collecting is distillation. Identifying is chromatography.

Take the first question with a real case. Evaporation and distillation are both used on salt water, both work on boiling point, and both leave the salt behind. They are not interchangeable. Evaporation boils the water off and lets the steam escape into the room, so you end up holding the salt and the water is gone. Distillation catches that same steam in a condenser and cools it back to liquid, so you end up holding both, with pure water in the second flask. Same mixture, same property, opposite thing kept, and which one is right depends entirely on whether you wanted the salt or the drink.

That is why a desalination plant distils and a salt pan evaporates. The Coorong salt works in South Australia lets seawater dry in the sun because salt is the product and the water is of no interest. A desalination plant does the reverse, and it costs far more, because the condenser and the energy to run it exist entirely to save the part the salt pan throws away.

Now the second question. Chromatography does not really separate a mixture in the sense the other techniques do, because at the end of it the dyes are still on the paper and you cannot pour any of them into a bottle. What it gives you is a measurement, an Rf value for each part, and a measurement is enough when the question is "which pen wrote this?" rather than "give me the ink". Choose chromatography when identification is the goal, when the amounts are far too small to collect anyway, or when you need to know how many different substances are present before you do anything else at all.