Chemistry · General Principles Of Organic Chemistry · NEET
Glycerol has a very high boiling point (about 563 K) and it decomposes when heated near that temperature. If you tried simple distillation, the glycerol would break down before it boiled, so you would lose the product. Under reduced pressure the boiling point drops well below the decomposition temperature, so the glycerol boils and is collected pure. This exact example (glycerol from spent-lye in the soap industry) is straight from NCERT and is tested in NEET.
A liquid boils at the temperature where its vapour pressure equals the external (surface) pressure. If you use a water pump or vacuum pump to reduce the external pressure, the liquid needs a much smaller vapour pressure to boil, and it reaches that smaller vapour pressure at a lower temperature. So less external pressure means a lower boiling point. This is the one line you must remember to answer any NEET question on this method.
Simple distillation separates a volatile liquid from a non-volatile impurity, or two liquids with a large difference in boiling points, at normal (atmospheric) pressure. Vacuum distillation is used only when the liquid has a very high boiling point or decomposes on heating; here you lower the pressure so it boils at a safe, lower temperature. Same basic setup, but vacuum distillation adds a pump to reduce pressure.
Spent-lye is the leftover liquid from the soap-making process. During soap manufacture, fats are treated with alkali (lye), and the leftover mixture (spent-lye) contains glycerol as a valuable by-product. Glycerol is recovered from this mixture by distillation under reduced pressure. You do not need the chemistry of soap for NEET; you only need to link glycerol from spent-lye to vacuum distillation.
Match List I (Mixture) with List II (Method of Separation). List I: 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.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a way to boil a liquid at a lower temperature by lowering the pressure above it, so high-boiling or heat-sensitive liquids can be purified without decomposing.
Glycerol is separated from spent-lye in the soap industry using distillation under reduced pressure. Remember this one example for NEET.
A water pump or a vacuum pump reduces the pressure on the surface of the liquid, according to NCERT.
No. Steam distillation is for water-immiscible, steam-volatile liquids like aniline. Vacuum distillation is for high-boiling or heat-decomposing liquids like glycerol.
