Chemistry · General Principles Of Organic Chemistry · NEET
Look only at the boiling point gap. If the two liquids have a LARGE difference in boiling point, use simple distillation. NCERT's example is chloroform (b.p. 334 K) and aniline (b.p. 457 K) — a gap of over 100 K, so simple distillation works. If the boiling points are CLOSE, the vapours form in the same temperature range and condense together, so simple distillation cannot separate them. Then you must use fractional distillation, which passes the vapours through a fractionating column first.
The column gives many surfaces for heat exchange between the rising vapours and the liquid dripping back down. As vapours rise, some condense, and this liquid takes heat from new rising vapours and re-vaporises. Each such step makes the rising vapour richer in the lower boiling (more volatile) component. By the time the vapours reach the top, they are almost pure in the low boiling liquid, which then passes to the condenser. Think of the column as doing many tiny distillations stacked on top of each other.
Crude oil is a mixture of many hydrocarbons whose boiling points are close and graded (petrol, kerosene, diesel etc.). Because the gaps are small, simple distillation would give a mixed vapour. Fractional distillation uses a tall column so each fraction can be collected at its own height and temperature. This is why petrol, diesel and kerosene are obtained by the fractional distillation of petroleum.
No. Distillation needs at least one component to be a volatile liquid (or a liquid mixture). To purify a SOLID that vaporises directly, you use sublimation. To purify a solid from soluble impurities, you use crystallisation. A NEET trap gives you a solid mixture and offers 'distillation' as a wrong option — reject it because there is no volatile liquid to distil.
Fractional distillation is chosen when two liquids have CLOSE boiling points. Distillation under reduced pressure (vacuum distillation) is chosen when a liquid has a VERY HIGH boiling point or decomposes at or below its boiling point — lowering the pressure lets it boil at a safer, lower temperature. NCERT's example is separating glycerol from spent-lye in the soap industry. Do not mix these two up in match-the-column questions.
Match List I (Mixture) with List II (Method of Separation). A. CHCl3 + C6H5NH2, B. Crude oil in the petroleum industry, C. Glycerol from spent-lye, D. Aniline-water. List II: I. Distillation under reduced pressure, II. Steam distillation, III. Fractional distillation, IV. Simple distillation.
The most suitable method of separation of a 1:1 mixture of ortho- and para-nitrophenols is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Different liquids boil (vaporise) at different temperatures. The mixture is heated, the vapours are cooled in a condenser, and the pure liquid is collected. It separates a volatile liquid from non-volatile impurities, or two liquids with different boiling points.
To separate (i) a volatile liquid from non-volatile impurities, and (ii) two liquids that have a sufficient difference in their boiling points, such as chloroform and aniline.
The lower boiling (more volatile) component. The fractionating column enriches the rising vapours in the low boiling liquid, so it reaches the top and condenses first.
An azeotrope is a mixture whose liquid and vapour have the same composition and boils at a constant temperature. Since the vapour composition never changes, further separation stops — for example, ethanol-water fractionates only up to about 95% ethanol.
Yes. Petrol, diesel and kerosene are obtained by the fractional distillation of petroleum, because these fractions have close boiling points and need a fractionating column to separate.
