Distillation
High-Yield Summary
- Distillation separates liquids by differences in boiling point (not solubility), via evaporation then condensation. The lowest-boiling component vaporizes first, travels to a condenser, and collects as the distillate.
- Simple distillation: boiling points under 150°C AND at least 25°C apart between the two liquids.
- Vacuum distillation: target boiling point over 150°C — lowering system pressure lowers the boiling point, avoiding thermal degradation of heat-sensitive compounds.
- Fractional distillation: boiling points less than 25°C apart — a packed fractionating column forces repeated vaporization-condensation cycles, progressively enriching the vapor in the lower-boiling component.
- Choice of method is a two-variable decision: absolute boiling point (for vacuum vs. the other two) and the boiling-point gap between components (for simple vs. fractional).
Choosing a Distillation Method
| Condition | Method |
|---|---|
| Both b.p. < 150°C, gap ≥ 25°C | Simple distillation |
| Target b.p. > 150°C | Vacuum distillation (lower pressure → lower b.p., avoids degradation) |
| Gap between b.p.s < 25°C | Fractional distillation (packed column, repeated vaporization-condensation) |
Fractional Distillation Setup
- 1Round-bottom flask holds the liquid mixture, heated from below.
- 2Fractionating column (packed with glass beads/steel wool) sits atop the flask, increasing surface area.
- 3Vapor rises, condenses and re-vaporizes repeatedly in the column, enriching in the lower-boiling component each cycle.
- 4Enriched vapor exits the column top into a water-cooled condenser, turning back to liquid.
- 5Liquid drips into a separate receiving flask as the distillate.
Key Terms
- Distillate
- The liquid collected after vapor condenses back down during distillation.
- Condenser
- Water-cooled apparatus that cools rising vapor back into liquid.
- Fractionating column
- Column packed with high-surface-area material (glass beads, steel wool) that forces repeated condensation/re-vaporization to resolve close boiling points.
Common MCAT Trap
- Vacuum distillation lowers boiling point by lowering pressure — don't think the vacuum is just for removing air; the physics is pressure-boiling point relationship, not solvent removal.
- The 150°C and 25°C thresholds are two separate, independent decision axes (absolute b.p. vs. b.p. gap) — a compound could have a high boiling point AND a small gap from its partner, requiring judgment about which constraint dominates (vacuum for the heat sensitivity).
- Don't assume distillation separates by solubility — that's the previous family of methods (extraction/filtration/recrystallization). Distillation is purely boiling-point-based.
Quick Recall
Two liquids boil at 80°C and 200°C. Which distillation method, and why?
Two liquids both boil under 150°C but only 10°C apart. Which method?
Why does lowering pressure lower a liquid's boiling point?