Fractional Distillation

Fractional distillation is used to separate two or more miscible liquids whose boiling points are close to each other, typically differing by less than about 25 K. A fractionating column packed with glass beads or shaped plates is fitted between the distillation flask and the condenser; it provides a large surface area where ascending vapours and descending condensed liquid repeatedly come into contact. Each contact is effectively a fresh distillation, so the vapour becomes progressively richer in the more volatile (lower-boiling) component as it climbs the column. The liquid at 329 K distils over first and is collected separately from the 351 K component. Simple distillation in one step is insufficient because a single vaporisation-condensation cycle cannot resolve liquids with such a narrow boiling gap. Steam distillation is meant for steam-volatile water-immiscible substances, not closely boiling miscible liquids. Distillation under reduced pressure is reserved for liquids that decompose at or below their boiling points. The repeated equilibration in the column is the NCERT basis for separating petroleum fractions. Plausibility check: closely spaced boiling points demand many vaporisation stages, which only a fractionating column supplies.