Chemistry · General Principles Of Organic Chemistry · NEET
Because the compound and water do not mix, they behave like two separate liquids sitting side by side. Each pushes out its own vapour pressure independently. The mixture boils when the SUM of both vapour pressures equals atmospheric pressure. Since we are adding two pressures, that sum is reached at a lower temperature than either liquid alone needs. So the mixture boils below 100 C and below the compound's normal boiling point. This gentle heating protects heat-sensitive compounds from decomposing.
Two conditions only: (1) the compound must be immiscible (insoluble) in water, and (2) it must be volatile in steam (it must have an appreciable vapour pressure at ~100 C so it travels with the steam). If either fails, steam distillation will not work. A compound that dissolves in water cannot be separated this way because it will not form a separate layer.
Aniline is immiscible with water and is volatile in steam, so it meets both conditions. Steam is passed through impure aniline; aniline vapour travels out with the steam, and on cooling both condense together. Because they do not mix, the condensate forms two layers and aniline is simply separated in a separating funnel. Non-volatile impurities stay behind in the flask. This is the exact example NCERT and NEET use.
Simple distillation separates two MISCIBLE liquids whose boiling points are far apart (roughly >25 C), like chloroform and aniline. Steam distillation separates a water-IMMISCIBLE, steam-volatile compound from non-volatile impurities. In simple distillation you heat the liquid directly; in steam distillation you pass steam into the mixture and it distils over below 100 C.
o-Nitrophenol has intramolecular hydrogen bonding (chelation) between its own -OH and -NO2 groups, so its molecules stay separate and it easily turns to vapour in steam. p-Nitrophenol has intermolecular hydrogen bonding, so molecules bind to each other, making it non-volatile in steam. That is why a 1:1 mixture of the two is separated by steam distillation - the ortho isomer distils over and the para stays behind (NEET 2017).
The most suitable method of separation of a 1:1 mixture of ortho- and para-nitrophenols is:
Match List I (Mixture) with List II (Method of Separation). A. CHCl3 + C6H5NH2 B. Crude oil in petroleum industry C. Glycerol from spent-lye D. Aniline-water | I. Distillation under reduced pressure II. Steam distillation III. Fractional distillation IV. Simple distillation
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An immiscible, steam-volatile liquid mixed with water boils when the sum of the two separate vapour pressures equals atmospheric pressure, so it distils over below 100 C.
Because the mixture boils below 100 C, the compound never gets very hot, so compounds that would decompose at their normal boiling point are protected.
Purification of aniline from an aniline-water mixture. Aniline is immiscible with water and volatile in steam, so it distils over and is then separated as a lower/upper layer.
No. The compound must be immiscible with water. If it dissolves, it will not form a separate layer and cannot be recovered by this method.
No. It distils over at a temperature below 100 C, which is also below its own boiling point, because the two vapour pressures add together.
