Once an organic compound is extracted from a natural source or synthesised in the laboratory, it is essential to purify it. Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it. The common techniques used for purification are as follows : (i) Sublimation (ii) Crystallisation (iii) Distillation (iv) Differential extraction and (v) Chromatography Finally, the purity of a compound is ascertained by determining its melting or boiling point. Most of the pure compounds have sharp melting points and boiling points. New methods of checking the purity of an organic compound are based on different types of chromatographic and spectroscopic techniques.
-- NCERT Class 11 Chemistry, Ch. 8, p. 278Distillation
8 MCQs9-step worked example
Source: NCERT Purification and Characterisation of Organic CompoundsPYQ coverage: NEET 2025Official key: NTA-verifiedLast updated: 27 Sep 2026
Distillation, explained for NEET
The costly move is reaching for simple distillation when the two boiling points sit close together. Simple distillation only works when the difference is large — roughly 20 K or more — because the vapour must be almost pure in one component for a single condensation to separate anything. Close-boiling pairs give vapour that carries both liquids, and you recover a mixture in your receiver.
NCERT Class 11 Chemistry, Chapter 8, pages 279–280 set the base case: distillation separates liquids that boil without decomposing and differ appreciably in boiling point. The liquid vaporises, the vapour condenses in a water-cooled condenser, and the lower-boiling component distils over first.
Fractional distillation (page 280) handles the close-boiling case. A fractionating column packed with glass beads gives the vapour many surfaces to condense on and re-vaporise from. Each cycle enriches the rising vapour in the more volatile component, so the column performs many simple distillations in series.
Two variants exist for compounds that cannot survive a normal boil.
Reduced-pressure distillation (page 280) lowers the external pressure, so the liquid boils at a lower temperature — a liquid boils when its vapour pressure equals the external pressure, and less external pressure needs less vapour pressure. Glycerol decomposes at its 1 atm boiling point but distils cleanly under reduced pressure. Students frequently invert this and say reduced pressure raises the boiling point.
Steam distillation (page 281) applies to compounds that are steam volatile and immiscible with water. Here the mixture boils when the combined vapour pressure of water and the organic compound equals atmospheric pressure — so boiling happens below 373 K, not above it. Aniline is purified this way.
Watch-out: "insoluble in water" is a requirement of steam distillation, not an obstacle to it. A water-soluble compound is the wrong candidate.
Can you answer these Distillation MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Distillation as a purification technique is applicable to liquids that:
Show answer and why every option is right or wrong
Answer: D. D is correct. NCERT Class 11 Chemistry, Chapter 8, pages 279–280 state that distillation is used to separate volatile liquids from non-volatile impurities, and liquids whose boiling points differ appreciably — the liquids must boil without decomposing.
Why A is wrong: A is wrong because a liquid that decomposes at its boiling point cannot be distilled at ordinary pressure at all; that case calls for distillation under reduced pressure. Density difference is irrelevant to distillation.
Why B is wrong: B is wrong because a substance that passes directly from solid to vapour is purified by sublimation, which is a separate technique from distillation.
Why C is wrong: C is wrong because identical vapour pressures mean the vapour has the same composition as the liquid, so no separation occurs at any stage.
MCQ 2Direct ApplicationPractice
A mixture of chloroform (b.p. 334 K) and aniline (b.p. 457 K) is to be separated. Neither liquid decomposes below its boiling point. The appropriate technique is:
Show answer and why every option is right or wrong
Answer: A. A is correct. The two boiling points are 123 K apart, so the lower-boiling chloroform vaporises first and is condensed and collected before aniline follows. NCERT Class 11 Chemistry, Chapter 8, page 279 names this exact pair as the case simple distillation handles.
Why B is wrong: B is wrong because steam distillation requires the compound to be immiscible with water, and it is used to bring a compound over below 373 K — neither condition is what this separation needs. It would also leave chloroform unaddressed.
Why C is wrong: C is wrong because a fractionating column is needed only when the boiling points are close, so that the vapours form over the same temperature range and condense together. With a 123 K gap one condensation already separates the pair.
Why D is wrong: D is wrong because reduced pressure is reserved for liquids that decompose at or below their boiling points. The stem states that neither liquid decomposes, so there is nothing to avoid.
MCQ 3Easy RecallPractice
In steam distillation, the mixture of the organic compound and water boils:
Show answer and why every option is right or wrong
Answer: C. C is correct. NCERT Class 11 Chemistry, Chapter 8, page 281 explains that the liquid mixture boils when the sum of the vapour pressures of water and the organic compound equals atmospheric pressure; because each contributes, that sum is reached before either pure liquid would boil, so the temperature stays below 373 K.
Why A is wrong: A is wrong because it draws the opposite conclusion from the correct premise. Two contributing vapour pressures reach atmospheric pressure sooner, which lowers the boiling temperature rather than raising it.
Why B is wrong: B is wrong because it ignores the organic compound's own vapour pressure. If water alone had to supply the full atmospheric pressure the temperature would indeed be 373 K, but the compound contributes as well.
Why D is wrong: D is wrong because the mixture boils below the boiling point of either pure component, not at the boiling point of one of them. That is the reason the technique protects heat-sensitive compounds.
MCQ 4Direct ApplicationPractice
Glycerol decomposes when heated to its boiling point at atmospheric pressure. It is purified by distillation under reduced pressure because lowering the external pressure:
Show answer and why every option is right or wrong
Answer: C. C is correct. A liquid boils when its vapour pressure equals the external pressure; reduce the external pressure and a lower vapour pressure suffices, which is reached at a lower temperature. NCERT Class 11 Chemistry, Chapter 8, page 280 gives glycerol as the worked case.
Why A is wrong: A is wrong because it inverts the pressure–boiling point relationship. Raising the boiling point would make decomposition more likely, not less — the technique would defeat its own purpose.
Why B is wrong: B is wrong because decomposition here is thermal, caused by the temperature the liquid must reach, not by dissolved air. Evacuating the apparatus lowers the boiling temperature; it does not chemically protect the compound.
Why D is wrong: D is wrong because vapour pressure at a given temperature is a property of the liquid itself and is not changed by the external pressure above it. What changes is the pressure the vapour pressure has to match.
MCQ 5Easy RecallPractice
A fractionating column packed with glass beads improves the separation of a close-boiling pair because the beads:
Show answer and why every option is right or wrong
Answer: B. B is correct. NCERT Class 11 Chemistry, Chapter 8, page 280 describes the column as providing a large surface area for successive condensation and vaporisation, so the column performs the equivalent of many simple distillations in series.
Why A is wrong: A is wrong because it describes adsorption, which is the basis of chromatography, not fractional distillation. The beads are inert packing; nothing is permanently retained on them.
Why C is wrong: C is wrong because the column is open to the condenser and receiver, so it is at essentially atmospheric pressure. External pressure affects both liquids' boiling points together and does not widen the gap between them.
Why D is wrong: D is wrong because the column has a temperature gradient rather than one fixed temperature, and separation comes from repeated equilibration along that gradient, not from a single thermal cut-off.
MCQ 6Direct ApplicationPractice
Aniline is separated from a mixture with non-volatile impurities by steam distillation. The properties of aniline that make this possible are that it is:
Show answer and why every option is right or wrong
Answer: D. D is correct. NCERT Class 11 Chemistry, Chapter 8, page 281 states that steam distillation applies to substances that are steam volatile and immiscible with water — aniline meets both conditions, which is why it is the textbook example.
Why A is wrong: A is wrong on both counts. A water-soluble compound cannot be recovered as a separate layer from the distillate, and a compound that is not steam volatile will not travel over with the steam at all.
Why B is wrong: B is wrong because it gets the immiscibility right but the volatility wrong. Both conditions are needed together; a compound with negligible vapour pressure at the boiling temperature will simply stay behind.
Why C is wrong: C is wrong because miscibility with water defeats the technique: the whole point is that the two liquids exert independent vapour pressures and the compound separates as a distinct layer on condensing.
MCQ 7CalculationPractice
An organic liquid boils at 478 K at atmospheric pressure but begins to char at about 440 K. A chemist must recover it in pure form from a non-volatile residue. The correct procedure and its reason are:
Show answer and why every option is right or wrong
Answer: A. A is correct. The two facts must be combined: the liquid decomposes at 440 K, which is below its 478 K boiling point, so it can never be boiled intact at atmospheric pressure; reducing the external pressure lowers the boiling point past 440 K, and the compound then distils without decomposing. NCERT Class 11 Chemistry, Chapter 8, page 280 covers this case.
Why B is wrong: B is wrong because a fractionating column does not lower the temperature the liquid must reach to boil; if anything the liquid is held hot in the flask for longer. The column addresses close boiling points, not thermal instability.
Why C is wrong: C is wrong because steam distillation is not available to every high-boiling liquid. It requires the compound to be steam volatile and immiscible with water, neither of which is stated here.
Why D is wrong: D is wrong because it stops at the residue being non-volatile and ignores the charring temperature. Heating this liquid to 478 K destroys it before any of it reaches the condenser.
MCQ 8CalculationPractice
Two miscible liquids X and Y boil at 329 K and 334 K respectively. They are distilled through a fractionating column. Which statement about the distillate collected first is correct?
Show answer and why every option is right or wrong
Answer: B. B is correct. The 5 K gap is too close for simple distillation, which is why the column is needed; within the column each condensation–vaporisation cycle enriches the ascending vapour in the more volatile liquid, so the 329 K component X reaches the top and distils first. NCERT Class 11 Chemistry, Chapter 8, page 280 describes this stagewise enrichment.
Why A is wrong: A is wrong because it swaps the two components. The column does not retain the volatile liquid; it preferentially returns the less volatile liquid to the flask as condensate, sending the volatile one upward.
Why C is wrong: C is wrong because a 5 K gap is precisely the situation a fractionating column is built for. Constant-proportion distillate describes an azeotrope, which is a different phenomenon and is not stated here.
Why D is wrong: D is wrong because the relationship is inverted: the liquid with the higher boiling point has the lower vapour pressure at any given temperature, which is why it boils only when heated further.
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How do you solve a Distillation question? A worked example
- 1
Given
A mixture contains two miscible organic liquids. Liquid P boils at 3.53 × 10² K and liquid Q boils at 3.56 × 10² K, both at atmospheric pressure. Neither liquid decomposes on heating to its boiling point. A fractionating column and a simple distillation set-up are both available.
- 2
Required
Which technique separates P from Q, and which liquid is collected first?
- 3
Concept
A liquid boils when its vapour pressure equals the external pressure. In a boiling mixture the vapour is richer in the more volatile (lower-boiling) component, but not pure — the degree of enrichment from one vaporisation depends on how far apart the two boiling points are. Simple distillation gives the vapour one chance to separate; fractional distillation gives it many.
- 4
Formula
No algebraic formula applies. The governing rule is the qualitative selection criterion from NCERT Class 11 Chemistry, Chapter 8, pages 279–280: simple distillation for an appreciable boiling-point difference, fractional distillation when the boiling points are close.
- 5
Substitution
Boiling-point difference: ΔT = 3.56 × 10² K − 3.53 × 10² K.
- 6
Calculation
ΔT = 3 K.
Both boiling points are given to three significant figures; their difference is fixed by the decimal places carried, giving 3 K. This is a close-boiling pair by any reading — far below the roughly 20 K gap at which a single vaporisation would deliver near-pure vapour.
Neither liquid decomposes, so reduced-pressure distillation is unnecessary. Both are miscible with each other and no immiscibility with water is stated, so steam distillation does not apply. - 7
Final answer
Fractional distillation. Liquid P, boiling at 3.53 × 10² K, is the more volatile component and is collected first; Q remains in the flask and distils later, at a higher head temperature.
- 8
Common trap
Two traps sit in this question. The first is choosing simple distillation because "the boiling points are different" — they are, but 3 K of difference is not enough for a single condensation, and the receiver would collect a mixture of P and Q. The second is swapping which component leaves the column first. The liquid with the lower boiling point has the higher vapour pressure at any temperature, so it is the one enriched upward through the column and the one that reaches the condenser first.
- 9
Similar NEET-style question
A mixture of two liquids boiling at 3.41 × 10² K and 3.44 × 10² K is to be separated, and a second mixture contains a liquid that chars 30 K below its atmospheric boiling point. Which pair of techniques is appropriate, in that order?
(a) Simple distillation; steam distillation
(b) Fractional distillation; distillation under reduced pressure
(c) Fractional distillation; steam distillation
(d) Distillation under reduced pressure; fractional distillation
Answer: (b).
What to remember before solving Distillation questions
Key Fact
Distillation
This important method is used to separate (i) volatile liquids from nonvolatile impurities and (ii) the liquids having sufficient difference in their boiling points. Liquids having different boiling points vaporise at different temperatures. The vapours are cooled and the liquids so formed are collected separately. Chloroform (b.p 334 K) and aniline (b.p. 457 K) are easily separated by the technique of distillation (Fig 8.5). The liquid mixture is taken in a round bottom flask and heated carefully. On boiling, the vapours of lower boiling component are formed first. The vapours are condensed by using a condenser and the liquid is collected in a receiver. The vapours of higher boiling component form later and the liquid can be collected separately.
-- NCERT Class 11 Chemistry, Ch. 8, p. 279Diagram
Fig.8.5 Simple distillation
Fig.8.5 Simple distillation. The vapours of a substance formed are condensed and the liquid is collected in conical flask. Apparatus labelled in the figure: R.B. flask (holding the liquid substance) heated over a burner on a wire gauze; adaptor and thermometer at the flask mouth; condenser with water inlet and outlet ("to sink"); a second adaptor delivering into a conical flask that collects the liquid substance.
-- NCERT Class 11 Chemistry, Ch. 8, p. 279Key Fact
Fractional distillation
Fractional Distillation: If the difference in boiling points of two liquids is not much, simple distillation cannot be used to separate them. The vapours of such liquids are formed within the same temperature range and are condensed simultaneously. The technique of fractional distillation is used in such cases. In this technique, vapours of a liquid mixture are passed through a fractionating column before condensation. The fractionating column is fitted over the mouth of the round bottom flask (Fig.8.6, page 280). Vapours of the liquid with higher boiling point condense before the vapours of the liquid with lower boiling point. The vapours rising up in the fractionating column become richer in more volatile component. By the time the vapours reach to the top of the fractionating column, these are rich in the more volatile component. Fractionating columns are available in various sizes and designs as shown in Fig.8.7. A fractionating column provides many surfaces for heat exchange between the ascending vapours and the descending condensed liquid. Some of the condensing liquid in the fractionating column obtains heat from the ascending vapours and revaporises. The vapours thus become richer in low boiling component. The vapours of low boiling component ascend to the top of the column. On reaching the top, the vapours become pure in low boiling component and pass through the condenser and the pure liquid is collected in a receiver. After a series of successive distillations, the remaining liquid in the distillation flask gets enriched in high boiling component. Each successive condensation and vaporisation unit in the fractionating column is called a theoretical plate. Commercially, columns with hundreds of plates are available. One of the technological applications of fractional distillation is to separate different fractions of crude oil in petroleum industry.
-- NCERT Class 11 Chemistry, Ch. 8, p. 279Fig.8.6 Fractional distillation. The vapours of lower boiling fraction reach the top of the column first followed by vapours of higher boiling fractions. Apparatus labelled in the figure: thermometer; fractionating column with packing seated on the flask holding the liquid to be distilled; oil bath over a burner; condenser with water inlet and "to sink" outlet; adaptor delivering distilled liquid into a conical flask.
-- NCERT Class 11 Chemistry, Ch. 8, p. 280Fig.8.7 Different types of fractionating columns. Two designs are shown: a simple packed column (filled with glass beads) and a bubble plate column.
-- NCERT Class 11 Chemistry, Ch. 8, p. 281Distillation under reduced pressure: This method is used to purify liquids having very high boiling points and those, which decompose at or below their boiling points. Such liquids are made to boil at a temperature lower than their normal boiling points by reducing the pressure on their surface. A liquid boils at a temperature at which its vapour pressure is equal to the external pressure. The pressure is reduced with the help of a water pump or vacuum pump (Fig.8.8). Glycerol can be separated from spent-lye in soap industry by using this technique.
-- NCERT Class 11 Chemistry, Ch. 8, p. 281Fig.8.8 Distillation under reduced pressure. A liquid boils at a temperature below its vapour pressure by reducing the pressure. Apparatus labelled in the figure: stopcock and capillary to regulate the air entering the distillation flask; heated flask; condenser with water inlet and "to sink" outlet; receiver collecting the distilled liquid; manometer and vacuum pump on the reduced-pressure side.
-- NCERT Class 11 Chemistry, Ch. 8, p. 281Key Fact
Steam distillation
Steam Distillation: This technique is applied to separate substances which are steam volatile and are immiscible with water. In steam distillation, steam from a steam generator is passed through a heated flask containing the liquid to be distilled. The mixture of steam and the volatile organic compound is condensed and collected. The compound is later separated from water using a separating funnel. In steam distillation, the liquid boils when the sum of vapour pressures due to the organic liquid (p1) and that due to water (p2) becomes equal to the atmospheric pressure (p), i.e. p = p1 + p2. Since p1 is lower than p, the organic liquid vaporises at lower temperature than its boiling point. Thus, if one of the substances in the mixture is water and the other, a water insoluble substance, then the mixture will boil close to but below, 373K. A mixture of water and the substance is obtained which can be separated by using a separating funnel. Aniline is separated by this technique from aniline - water mixture (Fig.8.9, Page 282).
-- NCERT Class 11 Chemistry, Ch. 8, p. 282Diagram
Fig.8.9 Steam distillation
Fig.8.9 Steam distillation. Steam volatile component volatilizes, the vapours condense in the condenser and the liquid collects in conical flask. Apparatus labelled in the figure: steam generator flask of water fitted with a safety tube; condensed water vapours passing into the flask holding the compound to be distilled; condenser with water inlet and "to sink" outlet; conical flask collecting water and distilled liquid.
-- NCERT Class 11 Chemistry, Ch. 8, p. 282Distillation questions from past NEET papers
1 question from NEET 2025. Answers verified against NTA official keys.
NEET 2025
1A-III, B-IV, C-II, D-I
2A-IV, B-III, C-I, D-II
3A-IV, B-III, C-II, D-I
4A-III, B-IV, C-I, D-II
NTA Answer: Option 2(final)
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Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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