Simple Distillation
Summary
Boiling a coloured salt solution and condensing the vapour gives back crystal-clear water, separating1 it from everything dissolved in it by their vast difference in boiling point. You’ll be able to explain why the salt stays behind, what the still-head thermometer really reads, and why separating two volatile liquids is harder.
History
Distillation is one of the oldest chemical techniques. Ancient Egyptians and Mesopotamians distilled aromatics and perfumes around 3500 BCE. The Arab alchemist Al-Kindi described systematic distillation methods in the 9th century CE, and by the medieval period distillation had become central to European pharmacy and alchemy.
The word “distillation” comes from the Latin destillare — to drip down — which perfectly describes the process: vapor rises, cools in a condenser, and drips down as purified liquid. Industrial distillation remains one of the most energy-intensive chemical processes in the world, used to purify drinking water, refine petroleum, produce spirits, and separate gases from air.
This experiment uses the simplest form: a mixture of water, salt, and food coloring is heated, and the water vapor — carrying none of the dissolved solids — is collected as crystal-clear distillate.
Hazards & preparation
PPE: safety glasses; keep hands clear of hot glass and steam.
- Never heat a sealed system — always leave a path for vapour to escape through the condenser, or pressure can build dangerously.
- Add boiling chips to prevent violent bumping; start the cooling water before heating; let glassware cool before disassembling.
Disposal: the salt/food-colour residue and the distilled water are harmless — rinse down the drain. See the Safety page.
Materials
Procedure
- Dissolve 15 g salt in 150 mL water and add food coloring — a brightly coloured solution.
- Add 2–3 boiling chips to prevent bumping.
- Pour into the distillation flask (no more than one-third full).
- Assemble the glassware: flask on the heat source, condenser angled down, receiver at the lower end.
- Start cooling water flowing through the condenser jacket.
- Heat gently until boiling — the thermometer should read close to 100°C.
- Collect distillate in the receiver; stop after 50–75 mL.
- Compare the flasks: coloured salt residue in the boiling flask, clear pure water in the receiver.
What you should see
The boiling flask holds a vivid coloured broth, yet what drips from the condenser is perfectly clear, colourless water — the dye and salt left completely behind. The still-head thermometer parks near 100°C and holds there, a flat plateau that signals pure water vapour coming over.
| Symptom | Likely cause | Fix |
|---|---|---|
| Distillate is tinted | Bumping carried droplets over | Add boiling chips; heat more gently; don’t overfill the flask |
| No distillate collecting | Cooling water off, or heat too low | Run condenser water; bring to a gentle boil |
| Thermometer reads above 100°C | Heat too high, or solution concentrating | Lower the heat; expect a slight rise as it concentrates |
| Violent boiling/bumping | No boiling chips | Cool, add chips, resume |
The Science
Distillation separates mixtures by exploiting differences in boiling point. Water boils at 100°C (at sea level); sodium chloride, being an ionic solid, has a boiling point above 1400°C. When the solution is heated, only water molecules have enough energy to escape the liquid as vapor.
The vapor travels through the still-head and enters the condenser, where cool water removes heat and causes the vapor to re-condense into liquid. The food coloring molecules and dissolved ions are too involatile to evaporate — they remain in the flask.
The thermometer placed at the still-head junction reads the boiling point of the vapor, not the liquid. A flat line near 100°C indicates pure water vapor is being collected. If the temperature climbs above 100°C, it often means the solution is becoming more concentrated (boiling point elevation) or the heat is too high.
Boiling point elevation is the phenomenon whereby dissolving a solute slightly raises the boiling point of the solvent. For a 15 g NaCl solution in 150 mL water, the elevation is only about 0.5°C — barely noticeable — but it illustrates the colligative properties of solutions.
Questions to Explore
Why doesn’t the salt evaporate? Water boils at 100°C, NaCl above 1400°C. Why is the difference so extreme?
Hint / answer
Water molecules are held together only by weak hydrogen bonds, easily broken by heat, so water boils readily. Salt is a lattice of ions locked by strong electrostatic attractions in every direction; freeing them into vapour needs enormous energy, so it stays put while the water boils off.
Why does the thermometer read 100°C, not higher? It’s at the vapour exit. What would make it rise?
Hint / answer
The vapour leaving is essentially pure water, which condenses at 100°C, so the still-head reads 100°C as long as that’s what’s coming over. It rises only if the remaining solution gets concentrated enough to elevate the boiling point, or if a more volatile impurity starts distilling.
Could you separate two liquids that both evaporate? Like water and ethanol — why is that harder?
Hint / answer
When both components are volatile, the vapour is a mixture of the two (richer in the lower-boiling one but not pure), so one pass only partly separates them. You need repeated condensation-and-reboiling (fractional distillation) — and even then water/ethanol hits an azeotrope you can’t fully separate by boiling alone.
What is boiling point elevation? Why does a non-volatile solute raise the boiling point, and does it change as water distills off?
Hint / answer
Dissolved particles get in the way of water molecules escaping, so you must heat a little hotter to boil — that’s the elevation. As water distills off, the solution grows more concentrated, so the effect increases and the flask’s boiling point creeps upward.
How does distillation refine petroleum? Crude oil’s components all evaporate. What lets them be separated, and what if two boil similarly?
Hint / answer
Each fraction (gasoline, kerosene, diesel…) has a different boiling range, so a tall fractionating column collects them at different heights/temperatures. Components with nearly equal boiling points come off together and are hard to separate — needing more stages or other methods.
Going further
- Test its purity. Measure the salt residue left behind, or check the distillate’s conductivity — it should be far lower than the starting solution.
- Push further. Try distilling a water–food-colour mix without salt, or attempt a water/rubbing-alcohol mix to feel why two volatile liquids need fractional distillation.
- Another separation by a different property: split two salts by solubility in Fractional Crystallization.
Footnotes
Solubility — The maximum amount of a substance that will dissolve in a given amount of solvent at a given temperature.↩︎
Robert Bunsen — German chemist (1811–1899) who, with Kirchhoff, developed flame spectroscopy and the Bunsen burner.↩︎