Purification - Crystallization, sublimation, distillation, differential extraction, and chromatography - principles and their applications.: NEET MCQs with Answers

15 free questions on Purification - Crystallization, sublimation, distillation, differential extraction, and chromatography - principles and their applications. for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

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Question 1

Previous-year question
An organic compound that is volatile in steam and immiscible with water boils at 250°C. During its steam distillation, the mixture of this compound and water will boil: [Re-NEET 2024]
  1. above 100°C but below 250°C
  2. above 250°C
  3. at 250°C
  4. close to but below 100°C
Show answer
Answer: (D) close to but below 100°C
Explanation: In steam distillation the mixture boils when p(organic) + p(water) = atmospheric pressure. Since the organic compound contributes some vapour pressure, water need supply less than 760 mm, so the mixture boils below 100°C; as the high-boiling compound has a small vapour pressure, the temperature is only slightly below 100°C. It can never be above 100°C or near 250°C.

Question 2

Previous-year question
Which technique is used to purify liquids that have high boiling points and decompose at or below their boiling points? [NEET 2024 (Sawaimadhopur)]
  1. steam distillation
  2. simple distillation
  3. distillation under reduced pressure
  4. fractional distillation
Show answer
Answer: (C) distillation under reduced pressure
Explanation: Lowering the external pressure lowers the boiling point of a liquid, so liquids that decompose at or below their normal boiling points are distilled under reduced pressure, e.g. glycerol from spent lye. Simple and fractional distillation would require heating to the normal boiling point, causing decomposition.

Question 3

Previous-year question
Match the mixtures/samples in List-I with the purification techniques in List-II and choose the correct option. [NEET 2023 (Manipur)]
List-I (Mixture/Sample):
(A) Glycerol from spent lye
(B) Chloroform + Aniline
(C) Fractions of crude oil
(D) Aniline + Water
List-II (Technique used for purification):
(I) Steam distillation
(II) Fractional distillation
(III) Distillation under reduced pressure
(IV) Distillation
  1. (A)-(III), (B)-(IV), (C)-(II), (D)-(I)
  2. (A)-(IV), (B)-(II), (C)-(I), (D)-(III)
  3. (A)-(I), (B)-(II), (C)-(III), (D)-(IV)
  4. (A)-(I), (B)-(III), (C)-(II), (D)-(IV)
Show answer
Answer: (A) (A)-(III), (B)-(IV), (C)-(II), (D)-(I)
Explanation: Glycerol is obtained from spent lye by distillation under reduced pressure (A-III); chloroform and aniline differ widely in boiling point and are separated by simple distillation (B-IV); crude oil fractions are separated by fractional distillation (C-II); aniline is separated from water by steam distillation (D-I).

Question 4

Previous-year question
Match the mixtures in List-I with the separation techniques in List-II and choose the correct option. [NEET 2022 (Overseas)]
List-I:
(a) Separation of aniline–water mixture
(b) Separation of aniline–chloroform mixture
(c) Separation of glycerol from spent-lye
(d) Separation of different fractions of crude oil
List-II:
(i) Fractional distillation
(ii) Distillation under reduced pressure
(iii) Distillation
(iv) Steam distillation
  1. (a)-(iv), (b)-(i), (c)-(iii), (d)-(ii)
  2. (a)-(iv), (b)-(ii), (c)-(iii), (d)-(i)
  3. (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)
  4. (a)-(i), (b)-(iii), (c)-(ii), (d)-(iv)
Show answer
Answer: (C) (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)
Explanation: Aniline–water is separated by steam distillation (a-iv); aniline–chloroform by simple distillation, as their boiling points differ widely (b-iii); glycerol is recovered from spent-lye by distillation under reduced pressure since it decomposes at its boiling point (c-ii); fractions of crude oil are separated by fractional distillation (d-i).

Question 5

Previous-year question
A liquid compound (x) can be purified by steam distillation only when it is: [NEET 2020]
  1. Steam volatile, immiscible with water
  2. Not steam volatile, miscible with water
  3. Steam volatile, miscible with water
  4. Not steam volatile, immiscible with water
Show answer
Answer: (A) Steam volatile, immiscible with water
Explanation: Steam distillation is used for substances that are volatile in steam and immiscible with water (e.g. aniline). The mixture boils when the sum of vapour pressures of the compound and water equals atmospheric pressure, so the compound distils below its boiling point and separates from the water in the distillate.

Question 6

Previous-year question
Paper chromatography is an example of [NEET 2020]
  1. partition chromatography
  2. thin layer chromatography
  3. column chromatography
  4. adsorption chromatography
Show answer
Answer: (A) partition chromatography
Explanation: The stationary phase in paper chromatography is the water trapped in the cellulose fibres and the mobile phase is a liquid solvent, so separation depends on partition of the solute between two liquids.

Question 7

Two samples of the same crystalline organic compound are tested after purification. Sample P melts sharply at one temperature, while sample Q begins melting at a lower temperature and melts gradually over several degrees. What is the most reasonable conclusion?
  1. Both are equally pure, because melting point gives no information about purity
  2. Q is likely to be pure, because a wide melting range shows a single compound
  3. P must be a mixture, because only mixtures melt at one sharp temperature
  4. P is likely to be pure, while Q probably still contains impurity
Show answer
Answer: (D) P is likely to be pure, while Q probably still contains impurity
Explanation: The purity of an organic compound is ascertained by determining its melting or boiling point. Most pure compounds have sharp melting and boiling points. A gradual melting over a range therefore points to remaining impurity. Chromatographic and spectroscopic methods are newer ways of checking purity.
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Question 8

A mixture of colourless organic compounds has been run on a thin layer chromatography plate and the plate has been dried. Which of the following would NOT help to locate the separated spots?
  1. Keeping the plate in a closed jar containing iodine crystals
  2. Viewing the plate under an ultraviolet lamp, if the compounds fluoresce
  3. Spraying the plate with ninhydrin, if the compounds are amino acids
  4. Measuring the height of the solvent front above the base line
Show answer
Answer: (D) Measuring the height of the solvent front above the base line
Explanation: Colourless spots must be made visible. Fluorescent compounds show up under an ultraviolet lamp, compounds that adsorb iodine appear as brown spots in an iodine jar, and amino acids develop colour with ninhydrin. The distance moved by the solvent front is needed only to calculate the retardation factor, and it says nothing about where the spots lie.

Question 9

A mixture of liquids A (b.p. 340 K) and B (b.p. 370 K) is distilled through a fractionating column. Which statement correctly describes what happens inside the column?
  1. No condensation occurs in the column, and separation takes place only in the condenser
  2. Both vapours condense at the same height, because they rise together
  3. The vapour of A condenses in the lower part of the column, while B passes out at the top
  4. The vapour of B condenses more readily lower in the column, so the vapour reaching the top becomes richer in A
Show answer
Answer: (D) The vapour of B condenses more readily lower in the column, so the vapour reaching the top becomes richer in A
Explanation: As the mixed vapour rises it cools and meets descending liquid. The higher-boiling component condenses preferentially in the lower, hotter part, and repeated condensation and vaporisation on the column surfaces enrich the rising vapour in the lower-boiling component. The vapour leaving the top is therefore rich in A.

Question 10

A solid mixture contains camphor and benzoic acid, and both of them sublime on heating. Which is the best way to separate them?
  1. Filter a solution of the mixture in ether, since benzoic acid does not dissolve in ether
  2. Dissolve the mixture in ether, shake with dilute HCl, separate the layers and evaporate the aqueous layer
  3. Heat the mixture gently and collect the first sublimate as pure camphor
  4. Dissolve the mixture in ether, shake with aqueous NaHCO₃, separate the layers and acidify the aqueous layer
Show answer
Answer: (D) Dissolve the mixture in ether, shake with aqueous NaHCO₃, separate the layers and acidify the aqueous layer
Explanation: Sublimation fails because both components sublime. Benzoic acid reacts with sodium hydrogen carbonate to form water-soluble sodium benzoate, which moves into the aqueous layer, while neutral camphor stays in the ether. Acidifying the aqueous layer precipitates benzoic acid, and evaporating the ether gives camphor.

Question 11

Consider two statements about purification by crystallisation. Statement I: Coloured impurities can be removed by adding activated charcoal to the hot solution and filtering. Statement II: A good solvent dissolves the compound readily at room temperature but only sparingly on heating.
  1. Statement I is true, Statement II is false
  2. Both statements are true
  3. Both statements are false
  4. Statement I is false, Statement II is true
Show answer
Answer: (A) Statement I is true, Statement II is false
Explanation: Activated charcoal adsorbs coloured impurities, which are then removed by filtration, so Statement I is true. Statement II has the requirement reversed: the compound should be sparingly soluble at room temperature and appreciably soluble when hot. Only then does it separate as crystals when the hot concentrated solution cools.

Question 12

An organic compound is to be recovered from its aqueous solution using a separating funnel. Which condition must the added solvent satisfy?
  1. It must be denser than water and dissolve in it completely
  2. It must be miscible with water and have a higher boiling point than water
  3. It must react with the organic compound to form a water-soluble salt
  4. It must be immiscible with water and dissolve the organic compound better than water does
Show answer
Answer: (D) It must be immiscible with water and dissolve the organic compound better than water does
Explanation: Differential extraction depends on the compound distributing itself between two immiscible liquids according to its solubility in each. The solvent must form a separate layer so the layers can be run off, and it must dissolve the compound more readily than water does. Evaporating the solvent layer then gives the compound.

Question 13

A mixture of X, which is weakly adsorbed on silica gel, and Y, which is strongly adsorbed on it, is loaded at the top of a silica gel column and eluted with a solvent. What is observed?
  1. X stays at the top of the column, because the silica gel acts as the mobile phase
  2. Y leaves the column first, because stronger adsorption pushes it down faster
  3. X and Y leave together, because the mobile phase carries all solutes at one speed
  4. X leaves the column first, and a more polar solvent may be needed to bring Y down
Show answer
Answer: (D) X leaves the column first, and a more polar solvent may be needed to bring Y down
Explanation: In column chromatography the solid adsorbent is the stationary phase and the liquid eluent is the mobile phase. A weakly adsorbed component spends less time held on the solid, so it travels down faster and is eluted first. A strongly adsorbed component moves slowly and is often removed by changing to a more polar eluent.

Question 14

Components X and Y of a mixture are spotted on the same base line of a chromatography paper. Their Rf values in the chosen solvent are 0.60 and 0.25 respectively. When the solvent front has risen 12.0 cm above the base line, the distance between the two spots is
  1. 4.2 cm
  2. 7.2 cm
  3. 10.2 cm
  4. 3.0 cm
Show answer
Answer: (A) 4.2 cm
Explanation: Rf is the distance moved by a substance divided by the distance moved by the solvent front from the same base line. X moves 0.60 × 12.0 = 7.2 cm and Y moves 0.25 × 12.0 = 3.0 cm. The gap between the spots is 7.2 – 3.0 = 4.2 cm.

Question 15

A crude organic solid containing a small amount of soluble impurity is to be purified by crystallisation. Which statement about this technique is incorrect?
  1. Coloured impurities can be adsorbed by boiling the hot solution with activated charcoal and then filtering
  2. A suitable solvent dissolves the compound readily at room temperature but only sparingly on heating
  3. The impurity stays in the mother liquor because its concentration remains below saturation on cooling
  4. Repeated crystallisation may be needed if the impurity has a solubility comparable to that of the compound
Show answer
Answer: (B) A suitable solvent dissolves the compound readily at room temperature but only sparingly on heating
Explanation: Crystallisation relies on the compound being sparingly soluble at room temperature but appreciably soluble at higher temperature, which is the reverse of the incorrect statement. On cooling, the solution becomes saturated with the main compound and it crystallises. The small quantity of impurity does not reach saturation and is left in solution. Charcoal and repeated crystallisation are standard refinements.

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