General Chemistry
Distillation
Separating liquids by boiling point — fundamental purification technique
Distillation is a separation technique using differences in boiling points to separate components of a mixture. Heat liquid mixture; vapor (rich in lower-BP component) condensed back to liquid in separate flask. Multiple types: simple distillation (large BP differences, ~25°C+), fractional distillation (closer BPs; column with multiple equilibrium stages), vacuum distillation (lower BPs at reduced pressure; for high-BP or thermally unstable), steam distillation (steam carries volatile compounds; for plant essential oils). Industrial: petroleum refining (crude → gasoline, kerosene, diesel), distilled spirits, water purification, chemical purification.
- PrincipleDifferent BPs of components
- Simple distillationΔBP > 25°C
- Fractional distillationMultiple stages; closer BPs
- Vacuum distillationReduces effective BP
- PetroleumMajor industrial use; separates gasoline, kerosene, etc.
- Drinking waterDistilled = pure water
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Why distillation matters
- Petroleum refining. Crude oil → gasoline, etc.
- Spirits production. Whisky, vodka, etc.
- Water purification. Distilled water.
- Chemical industry. Solvent recovery.
- Pharmaceuticals. Compound purification.
- Essential oils. Steam distillation of plants.
- Industrial processes. Many separation tasks.
Common misconceptions
- Distillation gives 100% pure. Limited by azeotropes.
- Distillation only at high T. Vacuum distillation at low T.
- Simple = inferior. Adequate for far-apart BPs.
- Distillation removes all solutes. Volatile solutes can carry over.
- Distilled water tastes good. Pure water is bland; minerals add taste.
- Distillation is recent. Used since ~3000 BCE.
Frequently asked questions
How does distillation work?
Heat mixture; component with lower BP boils first; vapor rich in that component (Raoult's law deviations). Vapor condensed in separate flask → distillate (pure or enriched). Original flask has higher-BP component (residue). Repeat for multiple cuts. Works because: BP differs based on intermolecular forces.
What's simple vs fractional distillation?
Simple: one stage; works when components have very different BPs (water vs ethanol differ ~20°C; not great). Fractional: vertical column with packing or trays; allows multiple equilibrium stages; can separate close BPs. Each stage = partial vaporization + condensation. Theoretical plate concept: more plates = better separation.
How is petroleum distilled?
Crude oil heated to ~370°C in furnace; vapor rises in fractionating column. As it rises, it cools; different fractions condense at different heights based on BP. Bottom (highest BP): asphalt, heavy oils. Middle: diesel, kerosene. Top (lowest BP): gasoline, naphtha. Continuous process; each fraction processed further.
What's vacuum distillation?
Distillation under reduced pressure. BP decreases when pressure decreases. Used for: high-BP compounds (would decompose at atmospheric BP), heat-sensitive compounds (proteins, vitamins), expensive substances (lower energy cost). Common in: chemical industry, fragrance industry, pharmaceutical.
What's azeotrope?
Mixture with constant boiling point (mixture composition same as vapor composition). Cannot be further separated by simple distillation. Famous: ethanol-water (95% ethanol; max purity by distillation alone). Solutions: drying agents (CaO removes water), pressure swing distillation, alternative solvents. Important: limit on distillation purity.
What's the difference between distillation and evaporation?
Both: liquid → gas. But distillation: vapor recondensed and collected; evaporation: vapor lost to atmosphere. Distillation collects products; evaporation loses them. Same physics, different applications. Distillation good for: separating components. Evaporation good for: concentrating solutions, drying.
How is alcohol distilled?
Spirits production. Fermented mash (~10-15% alcohol) heated. Alcohol (BP 78°C) vaporizes before water (100°C). Vapor cooled, condensed; collected. Multiple distillations for higher proof. Final: 30-50% alcohol typical. Higher (95%+) hits ethanol-water azeotrope; need other methods. Whisky, vodka, rum: differ in source materials and aging.