Purification and Characterisation of Organic Compounds — NEET Chemistry MCQs with answers

Purification and Characterisation of Organic Compounds is a Class 11 Chemistry chapter in the NEET (UG) syllabus. NEET720 has 204 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Purification techniques · Qualitative detection of elements · Quantitative estimation · Quantitative analysis · Distillation techniques · Steam distillation · Crystallisation and extraction · Qualitative elemental analysis · Distillation · Chromatography · Sublimation · Crystallisation · Extraction · Sublimation vs Crystallisation · Qualitative analysis · Qualitative and quantitative nitrogen analysis · Crystallisation and purity verification · Combined quantitative estimation · Crystallisation solvent choice · Solvent extraction · Fractional distillation · Vacuum distillation

8 free Purification and Characterisation of Organic Compounds practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · easy · Purification techniques

   Crystallisation is an effective purification method mainly because impurities present in a small amount:

   • A.decompose at the crystallisation temperature and escape as gas
   • B.remain dissolved in the mother liquor while the pure compound separates as crystals on cooling a saturated solution
   • C.are adsorbed selectively on the surface of the crystals and removed by washing with the same solvent used for dissolution
   • D.react chemically with the solvent to form a soluble complex that precipitates out first
   Show answer and explanation

   Answer: B. remain dissolved in the mother liquor while the pure compound separates as crystals on cooling a saturated solution

   Crystallisation exploits the difference in solubility of the compound and its impurities in a chosen solvent: on cooling a hot saturated solution, the less-soluble major component crystallises out while impurities present in small amounts stay dissolved in the mother liquor.

   In crystallisation, the impure solid is dissolved in a solvent in which it is sparingly soluble in the cold but freely soluble on heating. The hot saturated solution is filtered (if needed) and cooled slowly. As the solution cools, it becomes supersaturated with respect to the major compound, which crystallises out in pure form because impurities, being present in much smaller quantity, do not reach their own saturation point and remain dissolved in the mother liquor. The pure crystals are then separated by filtration. This purely physical solubility-difference principle is why option B is correct; no decomposition, adsorption, or chemical reaction with solvent is involved.

   Common mistake: Believing crystallisation removes impurities by decomposing or chemically reacting with them rather than by differential solubility

2. Question 2 · easy · Purification techniques

   A solid organic compound X sublimes readily on heating, while a non-volatile inorganic salt is mixed with it as an impurity. The best method to purify X is:

   • A.Sublimation
   • B.Simple crystallisation from water
   • C.Steam distillation
   • D.Fractional distillation
   Show answer and explanation

   Answer: A. Sublimation

   Sublimation directly converts the sublimable solid into vapour, leaving the non-volatile impurity behind, then the vapour is re-condensed as pure solid — the most direct and efficient technique here.

   Sublimation is the technique of choice when the desired compound can pass directly from solid to vapour state on heating while the impurity cannot. Here X sublimes but the inorganic salt does not, so heating the mixture converts X to vapour, which is then collected on a cold surface as pure solid, leaving the salt in the residue. Crystallisation would work but is slower and requires solvent handling; steam distillation and fractional distillation are techniques for liquids/volatile substances and are not applicable to a solid-solid separation of this type.

   Common mistake: Defaulting to crystallisation as the 'general purpose' purification method even when sublimation is clearly more suitable

3. Question 3 · medium · Purification techniques

   Glycerol decomposes on heating to about 290°C at atmospheric pressure, below which temperature it does not boil under normal pressure. It is not steam-volatile enough for practical steam distillation. Which technique is used industrially to purify glycerol without decomposition, and on what principle?

   • A.Steam distillation, because glycerol is miscible with water and co-distils efficiently with steam at low temperature
   • B.Simple distillation at atmospheric pressure with a longer condenser to prevent decomposition
   • C.Vacuum (reduced-pressure) distillation, because lowering the external pressure lowers the boiling point of glycerol below its decomposition temperature
   • D.Fractional distillation using a highly efficient column to separate glycerol from decomposition products formed during heating
   Show answer and explanation

   Answer: C. Vacuum (reduced-pressure) distillation, because lowering the external pressure lowers the boiling point of glycerol below its decomposition temperature

   A liquid boils when its vapour pressure equals the external pressure; reducing the external pressure (vacuum distillation) lets glycerol boil at a much lower temperature than 290°C, avoiding thermal decomposition.

   Boiling point depends on external pressure: a liquid boils when its vapour pressure equals the surrounding pressure. Under vacuum, the external pressure is reduced, so a liquid needs a much lower vapour pressure (i.e., a much lower temperature) to boil. Glycerol, which decomposes near 290°C at 1 atm, can be distilled safely at a much lower temperature under reduced pressure, well below its decomposition point. This is why vacuum distillation is the industrial method for purifying high-boiling, heat-sensitive liquids such as glycerol. Steam distillation requires the compound to have appreciable volatility in the presence of steam and be virtually immiscible with water, conditions glycerol does not meet since it is fully miscible with water and has very low vapour pressure. Simple distillation at atmospheric pressure would still require heating close to 290°C, causing decomposition; condenser length is irrelevant to boiling temperature.

   Common mistake: Assuming steam distillation applies to any water-miscible liquid, ignoring the volatility requirement

4. Question 4 · medium · Purification techniques

   Nitrobenzene, contaminated with a small amount of a non-volatile coloured impurity, needs separation. Nitrobenzene is immiscible with water but reacts with steam only negligibly and remains chemically stable up to 210°C, its normal boiling point. Given this thermal stability, which distillation technique would still be preferred in practice for its purification, and why is it advantageous over simple distillation at atmospheric pressure?

   • A.Simple distillation at atmospheric pressure, since nitrobenzene is stable at 210°C, so there is no benefit to any alternative technique
   • B.Fractional distillation, because nitrobenzene and its coloured impurity have close boiling points requiring a fractionating column
   • C.Vacuum distillation, because it is the only method capable of separating a non-volatile coloured impurity from a volatile liquid
   • D.Steam distillation, because nitrobenzene is immiscible with water and sufficiently volatile in steam, allowing it to distil over below 100°C, saving energy and reducing the chance of any thermal side reactions even though outright decomposition is not expected
   Show answer and explanation

   Answer: D. Steam distillation, because nitrobenzene is immiscible with water and sufficiently volatile in steam, allowing it to distil over below 100°C, saving energy and reducing the chance of any thermal side reactions even though outright decomposition is not expected

   Nitrobenzene is immiscible with water and steam-volatile, so steam distillation lets it distil over below 100°C — a lower-energy, gentler process than heating to its 210°C boiling point, even though outright decomposition is not the deciding factor here.

   This tests the practical/laboratory reasoning: steam distillation is preferred whenever a compound is (i) immiscible with water and (ii) has appreciable vapour pressure in the presence of steam, because the mixture boils below 100°C by Dalton's law of partial pressures, avoiding the need to heat to the compound's full boiling point (210°C for nitrobenzene). This saves energy, is gentler even in the absence of decomposition risk, and non-volatile coloured impurities are left behind in the distillation flask exactly as with simple distillation, but at far lower operating temperature. Vacuum distillation is not uniquely required — it is generally reserved for high-boiling, heat-sensitive liquids that are not steam-volatile or are miscible with water. Fractional distillation is unnecessary since the impurity is non-volatile, so no column-based enrichment is needed; the correct historical/practical choice remains steam distillation, matching standard laboratory practice for nitrobenzene purification.

   Common mistake: Assuming steam distillation is chosen only to prevent decomposition, missing the energy/practicality advantage

5. Question 5 · medium · Purification techniques

   In column chromatography using alumina as the stationary phase, components of a mixture separate mainly because of differences in:

   • A.the rate at which each component is adsorbed on the stationary phase and eluted by the moving solvent (mobile phase), i.e., differential adsorption/partition between the two phases
   • B.the molecular weight of each component, with heavier molecules always eluting first regardless of polarity
   • C.the boiling points of the components, since chromatography is essentially a form of fractional distillation performed on a solid support
   • D.the colour of each component, since only coloured compounds can be separated by this technique
   Show answer and explanation

   Answer: A. the rate at which each component is adsorbed on the stationary phase and eluted by the moving solvent (mobile phase), i.e., differential adsorption/partition between the two phases

   Chromatographic separation relies on differences in how strongly each component is adsorbed onto the stationary phase relative to how readily it is carried along by the mobile phase — components with weaker adsorption move faster and elute earlier.

   Chromatography separates mixture components based on their differential distribution between a stationary phase (here, alumina) and a mobile phase (eluting solvent). Components that adsorb more strongly onto the stationary phase move slowly down the column, while those adsorbed weakly are carried along faster by the mobile phase and elute first. This differential adsorption/partition behaviour, not molecular weight, boiling point, or colour per se, is the basis of separation. Colour is often used merely as a convenient visual indicator for coloured mixtures (as originally used by Tswett), but colourless mixtures are equally separable and can be detected by UV light or chemical spot tests.

   Common mistake: Assuming chromatography works like distillation (by boiling point) or requires coloured compounds

6. Question 6 · medium · Purification techniques

   Consider the following statements about crystallisation and sublimation as purification techniques: (I) A good solvent for crystallisation should dissolve the compound readily on heating but only sparingly in the cold. (II) If crystals separate too rapidly on cooling, impurities may get trapped within the crystal lattice, reducing purity. (III) Sublimation can be used to purify a solid even if a non-sublimable impurity is also somewhat volatile, provided that impurity does not condense at the temperature of the cold surface used. (IV) Repeated (fractional) crystallisation from the same solvent can never improve purity beyond a single crystallisation, since the equilibrium solubility is fixed. Which of the above statements are correct?

   • A.I and IV only
   • B.II and III only
   • C.I, III and IV only
   • D.I, II and III only
   Show answer and explanation

   Answer: D. I, II and III only

   Statements I, II and III correctly describe crystallisation solvent choice, slow-cooling practice, and the selectivity condition for sublimation; statement IV is false because repeated fractional crystallisation genuinely improves purity by successively removing residual impurity in the mother liquor.

   Statement I is correct: an ideal crystallisation solvent shows large solubility difference between hot and cold conditions, maximising recovery of pure crystals on cooling. Statement II is correct: slow, controlled cooling allows an orderly crystal lattice to form, excluding impurities into the mother liquor, whereas rapid cooling can trap impurities within or between crystals (occlusion), lowering purity. Statement III is correct: sublimation purification only requires that the target compound sublimes and that other volatile-but-non-sublimable materials do not re-condense as solid on the cold surface used to collect the product; a partially volatile impurity that stays as vapour or escapes the system does not contaminate the collected sublimate. Statement IV is false: performing repeated (fractional) crystallisations, especially with fresh batches of solvent, progressively removes residual impurity because each crystallisation event further partitions impurity into the mother liquor, and this is a standard technique to raise purity beyond what one crystallisation achieves. Hence I, II and III are correct, matching option D.

   Common mistake: Believing that a single crystallisation is equivalent to repeated fractional crystallisation in purifying ability

7. Question 7 · medium · Purification techniques

   A student claims: "Since vacuum distillation lowers the boiling point of a liquid, it must also lower the temperature needed to purify any heat-sensitive solid by recrystallisation." Which statement correctly evaluates this claim?

   • A.The claim is incorrect; vacuum distillation and recrystallisation are unrelated techniques for liquids and solids respectively, and reducing pressure does not meaningfully lower the dissolution/crystallisation temperatures used in recrystallisation, which depend on solvent solubility behaviour, not vapour-liquid equilibrium under reduced pressure
   • B.The claim is correct; lowering pressure lowers all phase-transition temperatures equally, so recrystallisation performed under vacuum always proceeds at a safely reduced temperature
   • C.The claim is correct only for solids that sublime, since sublimation pressure is also reduced under vacuum, aiding recrystallisation
   • D.The claim is incorrect because vacuum distillation actually raises boiling points, not lowers them, so no analogy to recrystallisation applies
   Show answer and explanation

   Answer: A. The claim is incorrect; vacuum distillation and recrystallisation are unrelated techniques for liquids and solids respectively, and reducing pressure does not meaningfully lower the dissolution/crystallisation temperatures used in recrystallisation, which depend on solvent solubility behaviour, not vapour-liquid equilibrium under reduced pressure

   Vacuum distillation's boiling-point reduction is a liquid-vapour phenomenon governed by vapour pressure versus external pressure; recrystallisation is a solid-liquid solubility phenomenon and is not affected by reducing atmospheric pressure in the same way, so the analogy is invalid.

   This question probes a common student overgeneralisation: because reducing external pressure lowers a liquid's boiling point (since boiling occurs when vapour pressure equals external pressure), students sometimes assume any reduction in operating temperature for heat-sensitive materials can be achieved similarly by 'applying vacuum' to any purification process. However, recrystallisation purifies solids based on differential solubility in a solvent at different temperatures — a solid-liquid equilibrium governed by solubility curves, not by vapour pressure versus atmospheric pressure. Reducing the pressure above a recrystallisation solution does not significantly change the solid's solubility or the temperature needed to dissolve/crystallise it (unless the solvent's own boiling point becomes limiting, which is a separate, secondary consideration). Therefore, the claim conflates two unrelated physical principles, and the correct evaluation is that it is incorrect, as in option A. Heat-sensitive solids are instead purified by choosing a suitable solvent (often at a lower dissolution temperature) or in the extreme, by sublimation, rather than by 'vacuum recrystallisation' analogous to vacuum distillation.

   Common mistake: Assuming any pressure-lowering trick used in distillation transfers directly to solid-liquid purification techniques like recrystallisation

8. Question 8 · medium · Purification techniques

   A mixture contains two solids, P (highly soluble in ethanol at all temperatures) and Q (sparingly soluble in cold ethanol but freely soluble in hot ethanol). Both are non-volatile and thermally stable. Which single technique, applied once, best isolates pure Q from this mixture, and why?

   • A.Sublimation, because non-volatile solids can still be separated by careful heating under vacuum
   • B.Steam distillation, since passing steam through the ethanol mixture will selectively volatilise Q away from P
   • C.Simple filtration of the solid mixture without any solvent, since P and Q have different particle sizes
   • D.Crystallisation from hot ethanol followed by cooling: both P and Q dissolve in hot ethanol, but on cooling, Q (sparingly soluble in cold ethanol) crystallises out while P (soluble at all temperatures) stays dissolved in the mother liquor
   Show answer and explanation

   Answer: D. Crystallisation from hot ethanol followed by cooling: both P and Q dissolve in hot ethanol, but on cooling, Q (sparingly soluble in cold ethanol) crystallises out while P (soluble at all temperatures) stays dissolved in the mother liquor

   Since Q's solubility drops sharply on cooling while P remains soluble in cold ethanol, dissolving the mixture in hot ethanol and cooling selectively crystallises out pure Q, leaving P in solution.

   The distinguishing property between P and Q is their solubility behaviour in cold ethanol: P remains highly soluble even when cold, whereas Q is only freely soluble when hot and becomes sparingly soluble on cooling. Dissolving the mixture in a minimum quantity of hot ethanol brings both P and Q into solution. On cooling, Q's solubility falls below the amount present, so it crystallises out of solution, while P, being soluble at all temperatures, remains entirely in the mother liquor. Filtering then isolates pure crystalline Q. Sublimation and steam distillation are inapplicable since both solids are stated to be non-volatile; plain filtration without solvent does nothing to separate two solids mixed together with no stated size difference.

   Common mistake: Applying sublimation or steam distillation reflexively without checking the stated volatility of the compounds

Questions about Purification and Characterisation of Organic Compounds for NEET

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Questions are original NEET720 compositions reviewed for correctness, syllabus fit and option quality. Counts update as the bank grows (204 active practice questions in this chapter today).