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Crystallization, sublimation, distillation and differential extraction: NEET MCQs with answers

90 questions on this topic of Purification and Characterisation of Organic Compounds (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Distillation under reduced pressure is most appropriately selected for a substance that:

  1. Forms a constant-boiling azeotropic mixture with water
  2. Is completely immiscible with water and steam-volatile
  3. Has a boiling point lower than 40 °C at 1 atm
  4. Decomposes at or below its normal boiling point
Show answer and explanation

Answer: (D) Decomposes at or below its normal boiling point

Distillation under reduced pressure (vacuum distillation) lowers the boiling point of a compound below its normal boiling point. It is used specifically to purify substances that decompose or undergo thermal degradation at or below their normal boiling points.

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2. Given below are two statements: Statement I: In differential extraction, extracting an aqueous layer multiple times with smaller volumes of an organic solvent is more efficient than extracting once with the total volume of the solvent. Statement II: The distribution law dictates that the amount of solute remaining unextracted decreases exponentially with the number of extraction cycles. In the light of the above statements, choose the most appropriate answer:

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct.

According to the distribution law, W_n = W [V / (V + K·v)]ⁿ. Since the fraction inside the bracket is strictly less than 1, increasing the number of extractions n with smaller volumes v results in a much smaller residual unextracted mass W_n compared to a single extraction with total volume n·v.

hard

3. Given below are two statements: Statement I: During fractional distillation, a fractionating column provides multiple condensing surfaces where rising vapor repeatedly condenses and falls back as reflux. Statement II: The liquid dripping back down the fractionating column becomes richer in the more volatile component compared to the ascending vapors. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are true.
  2. Both Statement I and Statement II are false.
  3. Statement I is true but Statement II is false.
  4. Statement I is false but Statement II is true.
Show answer and explanation

Answer: (C) Statement I is true but Statement II is false.

Statement I is true because packings/trays provide surfaces for repeated condensation and vaporization cycles. Statement II is false because the ascending vapor becomes richer in the more volatile (lower b.p.) component, while the descending liquid reflux becomes enriched in the less volatile (higher b.p.) component.

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4. During fractional distillation, various types of fractionating columns are used. What is the fundamental physical function of the obstructions (such as glass beads or indentations) inside the column?

  1. They react chemically with the higher boiling component to suppress its volatility
  2. They provide a large surface area for heat exchange between upward vapors and downward condensing liquid
  3. They prevent vapors of the lower boiling component from reaching the condenser
  4. They decrease the total vapor pressure of the mixture below atmospheric pressure
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Answer: (B) They provide a large surface area for heat exchange between upward vapors and downward condensing liquid

The packings or indentations in a fractionating column provide a large surface area for repeated condensations and vaporizations (heat exchange) between the rising vapor and the descending liquid.

easy

5. A mixture of chloroform (b.p. 61 °C) and aniline (b.p. 184 °C) can be best separated by which of the following techniques?

  1. Fractional crystallization
  2. Steam distillation
  3. Simple distillation
  4. Vacuum distillation
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Answer: (C) Simple distillation

Chloroform and aniline are two miscible liquids with a large difference in boiling points (ΔT = 184 °C - 61 °C = 123 °C > 25 °C). Hence, they can be easily separated by simple distillation.

hard

6. In a steam distillation, an immiscible organic liquid with molar mass 160 g/mol distills at 99 °C along with water (molar mass 18 g/mol). At 99 °C, the vapor pressure of water is 730 mm Hg and total atmospheric pressure is 760 mm Hg. What is the mass ratio of the organic substance to water (m_organic / m_water) in the distillate?

  1. 1.23
  2. 0.37
  3. 0.18
  4. 2.67
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Answer: (B) 0.37

The partial pressure of the organic compound is P_org = 760 - 730 = 30 mm Hg. The ratio of masses in the distillate is m_org / m_water = (P_org × M_org) / (P_water × M_water) = (30 × 160) / (730 × 18) = 4800 / 13140 ≈ 0.365 ≈ 0.37.

easy

7. Which of the following organic compounds does NOT undergo sublimation and therefore cannot be purified directly by this method?

  1. Naphthalene
  2. Anthracene
  3. Sucrose
  4. Benzoic acid
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Answer: (C) Sucrose

Naphthalene, anthracene, and benzoic acid are typical organic solids with high vapor pressures below their melting points that readily sublime. Sucrose chars and decomposes upon heating rather than subliming.

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8. A water-insoluble organic substance is purified by steam distillation at 98 °C under a barometric pressure of 760 mm Hg. At 98 °C, the vapor pressure of pure water is 707 mm Hg. What is the partial vapor pressure exerted by the organic substance in the vapor phase?

  1. 813 mm Hg
  2. 707 mm Hg
  3. 760 mm Hg
  4. 53 mm Hg
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Answer: (D) 53 mm Hg

According to Dalton's law of partial pressures for an immiscible mixture, P_total = P_water + P_organic. Substituting the values gives 760 mm Hg = 707 mm Hg + P_organic, so P_organic = 760 - 707 = 53 mm Hg.

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9. In steam distillation, the mixture of water and an immiscible organic liquid boils at 95 °C under 760 mm Hg. If the vapor pressure of pure water at 95 °C is 634 mm Hg, what is the partial vapor pressure of the organic compound at this temperature?

  1. 126 mm Hg
  2. 634 mm Hg
  3. 760 mm Hg
  4. 886 mm Hg
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Answer: (A) 126 mm Hg

According to Dalton's law for immiscible liquids, the total pressure is P = P(water) + P(organic). Therefore, P(organic) = P - P(water) = 760 mm Hg - 634 mm Hg = 126 mm Hg.

easy

10. An organic compound present in an aqueous medium is separated by shaking with an immiscible organic solvent in a separating funnel. This is:

  1. Differential extraction
  2. Sublimation
  3. Steam distillation
  4. Crystallisation
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Answer: (A) Differential extraction

In differential extraction the compound is more soluble in the organic solvent; the two layers separate and the solvent is then removed.

NCERT Class 11

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11. Match the separation mixtures in Column-I with the most suitable purification technique in Column-II: Column-I: (A) Chloroform (b.p. 60 °C) and Aniline (b.p. 189 °C) (B) Crude oil fractions (C) o-Nitrophenol and p-Nitrophenol (D) Benzoic acid and Sodium chloride Column-II: (1) Fractional distillation (2) Steam distillation (3) Simple distillation (4) Crystallization from water

  1. (A)-(2), (B)-(1), (C)-(3), (D)-(4)
  2. (A)-(1), (B)-(3), (C)-(2), (D)-(4)
  3. (A)-(3), (B)-(2), (C)-(1), (D)-(4)
  4. (A)-(3), (B)-(1), (C)-(2), (D)-(4)
Show answer and explanation

Answer: (D) (A)-(3), (B)-(1), (C)-(2), (D)-(4)

Chloroform and aniline have a large boiling point difference (ΔT > 100 °C), so simple distillation is used. Crude oil fractions have close boiling points and are separated by fractional distillation. o-Nitrophenol is steam volatile due to intramolecular H-bonding, while p-nitrophenol is not, so steam distillation separates them. Benzoic acid dissolves in hot water and crystallizes out upon cooling, while NaCl remains in solution.

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12. Consider the following statements regarding fractional distillation: Statement I: A fractionating column provides multiple theoretical plates for repeated condensations and vaporizations. Statement II: Vapors rising up the column become richer in the higher boiling component. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct as the packing or trays provide surface area for successive cycles of condensation and re-evaporation. Statement II is incorrect because ascending vapors become progressively richer in the more volatile (lower boiling) component, not the higher boiling one.

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