Simple Distillation

Separate pure water from a colored salt solution by boiling and condensing the vapor

Difficulty: Medium | Time: 1–2 hours | Visual Impact: Medium

Historical Context

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.

Materials

  • Distillation apparatus (round-bottom flask 250 mL, condenser, receiver flask)
  • Water — 150 mL
  • Salt — 15 g (1 tablespoon)
  • Food coloring — 5–10 drops
  • Hot plate or Bunsen burner
  • Thermometer
  • Boiling chips — 2–3
  • Clamps and stands

Procedure

  1. Dissolve 15 g salt in 150 mL water and add food coloring — brightly colored solution
  2. Add 2–3 boiling chips to prevent bumping
  3. Pour into the distillation flask (no more than one-third full)
  4. Assemble glassware: flask on heat source, condenser at angle, receiver flask at the lower end
  5. Start cooling water flowing through the condenser jacket
  6. Heat gently until boiling begins — thermometer should read close to 100°C
  7. Collect distillate in the receiver flask; stop after 50–75 mL
  8. Compare the two flasks: colored salt residue in the distillation flask, clear pure water in the receiver

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 (or ice) removes heat and causes the vapor to re-condense into liquid. The food coloring molecules and dissolved ions are too large and 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.

Safety

  • Never heat a sealed system — ensure all joints allow vapor to escape through the condenser
  • Use boiling chips to prevent violent bumping
  • Keep cooling water flowing before applying heat
  • Allow glassware to cool before disassembling

Resources