How is fractional distillation different from simple distillation?

Simple distillation separates liquids with large boiling point differences, while fractional distillation separates mixtures of liquids with close boiling points using a fractionating column for better purity. Both methods separate liquids by heating a mixture so that the liquid with the lower boiling point turns into vapor first and then condenses back to liquid. 

The big difference is that fractional distillation adds a tall fractionating column between the flask and the condenser. Inside this column, vapor repeatedly evaporates and condenses (mini-distillations), which cleans the vapor and gives a much purer separation when boiling points are close.

Definitions

• Simple distillation: A method to separate a volatile liquid from a non-volatile substance (like salt water) or from a second liquid that has a very different boiling point (difference usually ≥ 40–50°C).
• Fractional distillation: A method to separate a mixture of two or more liquids with close boiling points (small difference), by using a fractionating column (packed with beads or plates) that creates repeated vapor–liquid contact for high purity.

Process steps (side-by-side)

Why the fractionating column matters

The column (packed with glass beads, steel wool, or trays/plates) provides a large surface area. As hot vapor rises, it meets cooler surfaces, condenses, then revaporizes as heat from lower vapor warms it again. Each cycle favors the more volatile component. By the time the vapor reaches the top, it is richer in the lower-boiling liquid. This repeated enrichment gives well-separated fractions.

Typical uses and examples

• Simple distillation:
  • Purifying water from salt water (desalination demo).
  • Separating acetone from a mixture if the other component has a much higher boiling point.
  • Recovering solvent from a solution where the solute does not evaporate.
• Fractional distillation:
  • Separating ethanol (bp ~78°C) from water (bp 100°C) with better purity than simple distillation.
  • Petroleum refining: crude oil into LPG, gasoline, kerosene, diesel, lubricating oil, etc.
  • Separating close-boiling organic solvents in laboratories.

Common mistakes to avoid

• Heating too fast: Causes poor separation and bumping; always heat gently and steadily.
• Wrong thermometer position: The bulb should be level with the sidearm to read true vapor temperature.
• Poor insulation or wet column packing: Keep the column at the right temperature gradient; avoid flooding the column.
• Collecting fractions too widely or too narrowly: Use stable temperature plateaus to decide cut points.