Repeated vaporization-condensation using a column to separate close-boiling liquids.
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Fractional distillation separates liquids whose boiling points are close together by using a fractionating column that provides many repeated vaporization and condensation cycles. Each cycle on the packing or plates of the column enriches the vapour in the more volatile component, and the cumulative effect is equivalent to many simple distillation steps. The number of these equilibrium stages is called theoretical plates, and it determines how sharp the separation is. The temperature at the top of the column stays near the boiling point of the more volatile component as it distils over. Industrially, the process separates crude oil into fractions such as petrol, kerosene and diesel, and separates liquid air into oxygen and nitrogen.
Key ideas: Separation quality is measured in theoretical plates. Packing the column with glass beads or helices adds surface area, and a high reflux ratio improves purity at the cost of speed. Keep the column insulated and the heating steady, so the temperature gradient is stable. Raoult's law explains why the vapour above an ideal mixture is richer in the more volatile component.
Common mistakes and connections: Students think that distillation gives 100 percent pure components for any mixture, but azeotropes such as ethanol and water (95.6 percent ethanol) cannot be separated beyond the azeotropic composition. Distilling too quickly floods the column. Related to simple distillation, Raoult's law and phase diagrams, and to petroleum refining.
The ethanol-water boiling diagram: each vaporization-condensation step in the column enriches ethanol, but separation stops at the azeotrope near 95 percent. The interactive lesson shows this as a live animation or chart.
Separating ethanol (boiling point 78 C) from water enriches ethanol to about 95 percent, the azeotrope limit.
Repeated vaporization-condensation using a column to separate close-boiling liquids.
Fractional Distillation is a intermediate topic in Analytical, Lab & Spectroscopy. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Analytical, Lab & Spectroscopy first: Volumetric Glassware, Filtration, Distillation.
The next topics in Analytical, Lab & Spectroscopy are Steam Distillation and Crystallization.
Yes. The interactive lesson includes a self-check question on Fractional Distillation, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Fractional Distillation is topic 23 of 99 in Analytical, Lab & Spectroscopy.