Purification Methods

Q: What is the principle of steam distillation?

Two immiscible liquids each exert their full vapour pressure, so the mixture boils when the sum equals atmospheric pressure. Each partial pressure is below 760 mmHg, so a steam-volatile, water-immiscible compound such as aniline distils with water below 373 K, well under its own boiling point, and without decomposing.

Q: Why is o-nitrophenol steam volatile but p-nitrophenol is not?

In o-nitrophenol the OH forms a hydrogen bond with the neighbouring nitro oxygen inside the same molecule, so the molecules do not associate and the compound is volatile. p-Nitrophenol molecules are linked to each other and to water by intermolecular hydrogen bonds, so p-nitrophenol stays in the flask.

Q: What is the Rf value and can it be greater than 1?

The retardation factor is the distance moved by a spot divided by the distance moved by the solvent, both measured from the baseline. A spot cannot overtake the solvent front, so Rf is always less than 1. It has no units and is constant for a compound with a given plate, solvent and temperature.

Q: What is the difference between adsorption and partition chromatography?

Adsorption chromatography uses a solid stationary phase such as silica gel or alumina, and components separate by how strongly they stick to its surface (column chromatography, TLC). Partition chromatography uses a liquid stationary phase, the water held in paper, and components separate by distribution between two liquids.

Q: Why is glycerol purified by distillation under reduced pressure?

Glycerol boils at 563 K and decomposes at about that temperature. Lowering the pressure above it lowers the temperature at which its vapour pressure equals the external pressure, so it distils well below 563 K without decomposing. This is how glycerol is recovered from spent lye in the soap industry.

Q: When is fractional distillation used instead of simple distillation?

When the boiling points of the liquids are close, their vapours form over the same temperature range and simple distillation gives a mixture. A fractionating column provides many vaporisation-condensation steps (theoretical plates), so the vapour at the top becomes nearly pure lower-boiling liquid, as in refining crude oil.

Q: Which purification questions are common in NEET?

NEET usually asks you to match a mixture with its method: camphor with sublimation, glycerol with reduced-pressure distillation, aniline or o-nitrophenol with steam distillation, crude oil with fractional distillation, plus Rf calculations, the stationary phase in paper chromatography and the order of elution from a column.

Q: How is purification tested in JEE Main and JEE Advanced?

JEE adds reasoning and numbers: why o-nitrophenol is steam volatile, the composition of a steam distillate from partial pressures and molar masses, single versus multiple extractions using the distribution law, acid-base extraction sequences for mixtures, azeotropes, and ranking Rf values by polarity on silica gel.

ChemistryPurification and Characterisation of Organic CompoundsFor JEE aspirants

Purification of organic compounds means separating a compound from its impurities by exploiting one difference in a physical property: volatility (sublimation, distillation), solubility (crystallisation, extraction) or adsorption (chromatography). Picking the right technique is the whole game: the purification of organic compounds is tested in NEET and JEE Main mostly as "which method separates X from Y". This page covers each method's principle, apparatus and standard examples, the $R_{f}$ value, steam distillation of o-nitrophenol, acid-base extraction and how purity is confirmed.

On this page1Choosing a method2Sublimation3Crystallisation4Distillation (4 types)5Differential extraction6Chromatography7Checking purity

Key Formulas - Quick Reference

★ Must learn $R_{f} = \frac{distance moved by the substance}{distance moved by the solvent}$ (both from the baseline); $0 < R_{f} < 1$ , no units
★ Must learn A liquid boils when its vapour pressure equals the external pressure: lower the pressure, lower the boiling point (reduced-pressure distillation)
★ Must learn Steam distillation: $p_{water} + p_{organic} = p_{atm}$ , so the mixture boils below $373 K$
Steam distillate: $\frac{m _{org}}{m _{water}} = \frac{p _{org} M _{org}}{p _{water} M _{water}}$
★ Must learn Distribution law: $K_{D} = \frac{C _{org}}{C _{aq}}$ ; fraction left in $V$ of water after $n$ extractions with $v$ of solvent each $= (\frac{V}{V + K _{D} v})^{n}$
Simple distillation: boiling points far apart (rough rule $Δ T_{b} \geq 25 K$ ); fractional distillation: boiling points close
★ Must learn On silica or alumina (polar), the least polar, least adsorbed component moves fastest: it elutes first and has the highest $R_{f}$
A pure solid melts sharply; an impurity lowers and broadens the melting range (mixed melting point test)

1. Choosing a Purification Method

Every method works on one difference between the compound and its impurity. Ask two questions: is the compound a solid or a liquid, and which property differs most (volatility, solubility at different temperatures, boiling point, solubility in two solvents, or strength of adsorption)?

Figure 1: Choosing a purification method. Ask two questions first: is it a solid or a liquid, and which physical property differs most between the compound and its impurity?

Method	Difference exploited	Standard example
Sublimation	solid turns directly to vapour; impurity does not	camphor from $CaS O_{4}$ or NaCl
Crystallisation	solubility rises steeply with temperature	benzoic acid from hot water; sugar
Simple distillation	boiling points far apart	chloroform (334 K) from aniline (457 K)
Fractional distillation	boiling points close	crude oil fractions; benzene from toluene
Distillation under reduced pressure	liquid decomposes at or below its b.p.	glycerol from spent lye
Steam distillation	steam volatile and immiscible with water	aniline, nitrobenzene, o-nitrophenol, essential oils
Differential extraction	more soluble in an organic solvent than in water	organic compound from water with ether
Chromatography	different adsorption or partition	plant pigments, amino acids, drug mixtures

Key idea

Name the property first, then the method: volatility of a solid means sublimation, a hot-cold solubility gap means crystallisation, boiling points mean distillation, two solvents mean extraction, adsorption means chromatography.

2. Sublimation

Sublimation purifies a solid that changes directly from solid to vapour on heating and back to solid on cooling, from an impurity that does not sublime.

The impure solid is warmed gently in a china dish covered by an inverted funnel with a cotton plug in its stem. The vapour condenses as pure crystals on the cool funnel wall; the non-volatile impurity stays in the dish.

Figure 2: Sublimation. The solid passes straight to vapour and back to solid on the cool funnel, while a non-sublimable impurity (e.g.

CaS O_{4}

with camphor) stays behind in the dish.

Sublimable organic solids: camphor, naphthalene, anthracene, benzoic acid (inorganic: $I_{2}$ , $N H_{4} Cl$ ).
Camphor + $CaS O_{4}$ or camphor + common salt: sublimation works because only camphor sublimes.
Naphthalene + benzoic acid: both sublime, so sublimation fails. Use acid-base extraction (Section 5.2).

3. Crystallisation

Crystallisation purifies a solid using a solvent in which it is sparingly soluble when cold but highly soluble when hot; the impurities either stay dissolved (mother liquor) or do not dissolve at all (removed by hot filtration).

Dissolve the impure solid in the minimum volume of hot solvent to get a nearly saturated solution.
Add a little activated charcoal if the solution is coloured; it adsorbs the coloured impurities.
Filter the hot solution to remove insoluble impurities and charcoal.
Cool slowly: pure crystals separate. Filter, wash with a little cold solvent and dry.
The filtrate (mother liquor) keeps the soluble impurities and a small amount of the compound.

Figure 3: Crystallisation needs a solvent in which the compound is far more soluble hot than cold. Benzoic acid in water:

56.3 g/L

100^{\circ} C

but

3.4 g/L

25^{\circ} C

, so about

94%

crystallises on cooling.

Mixed solvent: if the compound is very soluble in one solvent and nearly insoluble in another, crystallise it from a mixture of the two (e.g. ethanol and water).
Fractional (repeated) crystallisation: needed when the impurity has a solubility close to that of the compound; each crystallisation enriches the less soluble component.
Common solvents: water, ethanol, methanol, acetone, ether, chloroform, benzene, petroleum ether.

Key idea

Crystallisation depends on the solubility gap between hot and cold solvent, not on the absolute solubility.

Quick Recall: tap to check

How would you separate camphor from calcium sulphate?

Sublimation: camphor sublimes,

CaS O_{4}

does not.

Two compounds have different solubilities in a solvent S. How can they be separated?

Fractional crystallisation from S: the less soluble compound crystallises first on cooling; repeat to purify.

Why is activated charcoal added during crystallisation?

It adsorbs coloured impurities, giving colourless crystals.

4. Distillation

Distillation separates (i) a volatile liquid from non-volatile impurities and (ii) liquids with different boiling points. The liquid is vaporised and the vapour is condensed and collected. Four versions exist; which one you use depends on the boiling points and on the stability of the liquid.

4.1 Simple Distillation

Used when the boiling points differ widely. The lower-boiling liquid distils first, then the higher-boiling one. Chloroform (334 K) and aniline (457 K) are separated this way.

Figure 4: Simple distillation. The lower-boiling liquid distils first; the thermometer bulb sits at the side arm so it reads the temperature of the vapour that is actually distilling. Cold water enters the condenser at the lower end.

The thermometer bulb sits level with the side arm, so it reads the temperature of the vapour actually distilling.
Boiling chips (porous pot) give smooth boiling and prevent bumping.
Cooling water enters the condenser at the lower end and leaves at the upper end, so the jacket stays full.

4.2 Fractional Distillation

If the boiling points are close, both liquids vaporise in the same temperature range and simple distillation cannot separate them. A fractionating column (packed with glass beads, or a bubble-plate column) is fitted between the flask and the condenser. It offers many surfaces where rising vapour meets descending liquid.

Figure 5: Fractional distillation. Rising vapour (red) and falling liquid (blue) exchange heat on every bead; each vaporise-condense unit is one theoretical plate, so a tall column does many simple distillations in one go.

At each surface the higher-boiling component condenses and the lower-boiling component re-vaporises, so the vapour becomes richer in the more volatile liquid as it climbs. Each vaporisation-condensation unit is a theoretical plate. The boiling point diagram shows why several plates are needed:

Figure 6: Why a column works. A liquid with 30 mol% benzene boils at

98.5^{\circ} C

and gives vapour with 51 mol% benzene; 4 vaporise-condense steps (plates) raise it to about 94 mol%.

Uses: separating crude oil into fractions (petroleum refining), benzene from toluene, liquid air into $N_{2}$ and $O_{2}$ .
Limitation: an azeotrope boils at a constant temperature with a constant composition, so no column can separate it. Ethanol-water gives at best 95.6% ethanol (by mass), boiling at 351 K.

Simple distillation

Boiling points far apart. No column. One vaporisation and one condensation. Example: $CHC l_{3}$ + aniline.

Fractional distillation

Boiling points close. Fractionating column gives many plates. Example: crude oil, benzene + toluene.

4.3 Distillation under Reduced Pressure

Some liquids have very high boiling points or decompose at or below their normal boiling point. A liquid boils when its vapour pressure equals the pressure above it, so lowering the pressure (with a water pump or vacuum pump) makes it boil at a lower temperature, where it does not decompose. A fine capillary lets in a stream of air bubbles to prevent bumping, and a manometer reads the pressure.

Glycerol (b.p. 563 K, decomposes) is recovered from spent lye in the soap industry this way.
Also used to concentrate sugar-cane juice in sugar factories and for other heat-sensitive liquids.

4.4 Steam Distillation

Steam distillation purifies a compound that is steam volatile and immiscible with water. Steam is passed through the heated mixture; the compound distils along with water below 373 K and is separated from the water layer with a separating funnel.

Because the two liquids do not mix, each exerts its own full vapour pressure. The mixture boils when

p_{water} + p_{organic} = p_{atm}

so each vapour pressure is below 760 mmHg and the mixture boils below the boiling point of either liquid. The graph shows it for aniline (b.p. 457 K), which distils with steam at about 371.5 K (98.4 °C).

Figure 7: A liquid boils when its vapour pressure equals the external pressure. (a) Lowering the pressure to

150 mmHg

drops the b.p. of water from

100

60^{\circ} C

. (b) With immiscible liquids the pressures add:

p_{water} + p_{aniline} = 760 mmHg

98.4^{\circ} C

, below the b.p. of both.

Figure 8: Steam distillation. Steam carries a water-immiscible, steam-volatile compound over below

373 K

; the distillate separates into a water layer and an organic layer (aniline is slightly denser than water, so it forms the lower layer).

Conditions: the compound must be immiscible with water, stable at about 373 K and have a reasonable vapour pressure near 373 K.
Examples: aniline, nitrobenzene, bromobenzene, turpentine and other essential oils from plant material.

The most asked example is the separation of o-nitrophenol from p-nitrophenol:

Figure 9: The o/p-nitrophenol pair is the classic steam distillation question. The ortho isomer hides its OH in an intramolecular H-bond and distils with steam; the para isomer is held by intermolecular H-bonds and stays behind.

Exam Trick

"Ortho goes over." An OH (or NH) locked in an intramolecular H-bond with a neighbouring C=O or $N O_{2}$ makes the ortho isomer steam volatile: o-nitrophenol, o-hydroxybenzaldehyde (salicylaldehyde), o-hydroxyacetophenone. The para and meta isomers form intermolecular H-bonds and stay in the flask.

JEE Advanced

How much compound comes over with the steam? In the vapour, moles are in the ratio of the partial pressures, $\frac{n _{org}}{n _{w}} = \frac{p _{org}}{p _{w}}$ , so the mass ratio in the distillate is

\frac{m _{org}}{m _{w}} = \frac{p _{org} M _{org}}{p _{w} M _{w}}

For aniline at 371.5 K: $p_{w} \approx 717$ mmHg and $p_{aniline} \approx 43$ mmHg, so

\frac{m _{aniline}}{m _{w}} = \frac{43 \times 93}{717 \times 18} = 0.31

Aniline is only 5.7% of the vapour by moles but about 24% of the distillate by mass, because its molar mass is five times that of water. A high molar mass makes up for a low vapour pressure; this is why steam distillation is practical.

Reduced-pressure distillation

Lowers the external pressure so the liquid boils below its normal b.p. Used for liquids that decompose (glycerol). Needs a vacuum pump.

Steam distillation

Keeps 1 atm but adds water vapour: $p_{w} + p_{org} = 760$ mmHg, so boiling occurs below 373 K. Only for water-immiscible, steam-volatile compounds.

Key idea

All four distillations obey one rule: boiling happens when the total vapour pressure equals the pressure above the liquid. Simple and fractional distillation use differences in boiling point; reduced pressure lowers the target pressure; steam distillation adds water's vapour pressure.

Quick Recall: tap to check

Why does an organic liquid vaporise below its boiling point in steam distillation?

The mixture boils when

p_{water} + p_{organic} = p_{atm}

; since

p_{organic}

is less than

p_{atm}

, the organic liquid distils at a temperature below its own boiling point (below 373 K).

Why is the thermometer bulb kept level with the side arm?

It must measure the temperature of the vapour entering the condenser, which tells you which component is distilling.

Why can fractional distillation not give 100% ethanol from aqueous ethanol?

Ethanol and water form an azeotrope (95.6% ethanol) that boils at constant composition, 351 K.

5. Differential Extraction

5.1 Extraction with an Immiscible Solvent

An organic compound dissolved in water is shaken with an organic solvent that does not mix with water and in which the compound is more soluble (ether, chloroform, benzene). The compound distributes itself between the two layers, which are then separated in a separating funnel. Evaporating or distilling off the solvent gives the compound.

Figure 10: Differential extraction. Shaking lets the compound share itself between the two immiscible layers in the fixed ratio

K_{D} = C_{org} / C_{aq}

; the organic layer is then evaporated to recover the compound.

At a fixed temperature the ratio of concentrations is constant (distribution or partition law):

K_{D} = \frac{C _{org}}{C _{aq}}

If a volume $V$ of water holds the compound and it is extracted $n$ times with a volume $v$ of solvent each time, the fraction left in the water is

f_{n} = (\frac{V}{V + K _{D} v})^{n}

Several small portions beat one large portion of the same total volume (Solved Example 3).
If the compound is only slightly more soluble in the solvent, a huge volume would be needed; instead the same solvent is recycled over the sample repeatedly: continuous extraction (Soxhlet extractor).

5.2 Chemically Active (Acid-Base) Extraction

A reagent converts one compound into a water-soluble salt, which moves into the aqueous layer; the other compounds stay in the organic layer. Adding acid or base to the aqueous layer regenerates the compound.

Figure 11: Chemically active extraction. Each reagent turns one compound into a water-soluble salt, which moves into the aqueous layer; acidifying or basifying the layer gives the compound back.

Exam Trick

"Bicarb, Base, Acid." $NaHC O_{3}$ pulls out only carboxylic acids (stronger than $H_{2} C O_{3}$ ); NaOH then pulls out phenols; dilute HCl pulls out amines. Whatever remains in the ether is neutral (hydrocarbons, ethers, ketones).

6. Chromatography

Chromatography (Greek chroma, colour: first used for plant pigments) separates, purifies and identifies the components of a mixture, even in milligram amounts. The mixture is placed on a stationary phase (solid or liquid) and a mobile phase (a liquid or a gas) moves over it. Components that are held more strongly by the stationary phase move more slowly, so they separate.

Type	Principle	Stationary phase	Examples
Adsorption	different degrees of adsorption on a solid surface	silica gel or alumina	column chromatography, TLC
Partition	continuous distribution between two liquid phases	water held in paper (cellulose)	paper chromatography

6.1 Column Chromatography

The adsorbent is packed in a glass column with a stopcock at the bottom. The mixture is loaded at the top and an eluant (a solvent or solvent mixture) is run slowly down the column. Each component moves at its own speed.

Figure 12: Column chromatography (adsorption). The eluant carries each component down at its own speed: the least adsorbed (a) elutes first, the most strongly adsorbed (c) is retained near the top.

6.2 Thin Layer Chromatography (TLC)

A thin layer (about 0.2 mm) of silica gel or alumina is spread on a glass plate (the chromaplate). The mixture is spotted about 2 cm above the lower edge and the plate stands in a closed jar with the eluant below the spot. As the solvent rises, the components move up to different heights. Each is described by its retardation factor:

R_{f} = \frac{distance moved by the substance from the baseline ( x )}{distance moved by the solvent from the baseline ( y )}

Figure 13:

R_{f} = \frac{distance moved by spot}{distance moved by solvent}

, both measured from the baseline: here

R_{f} (A) = 0.70

and

R_{f} (B) = 0.30

. B is held more strongly by the stationary phase. TLC separates by adsorption, paper by partition.

Coloured compounds are seen directly.
Colourless compounds that fluoresce are seen under ultraviolet light.
Iodine vapour (a few crystals in a closed jar) shows compounds that adsorb iodine as brown spots.
Spraying a reagent: ninhydrin shows amino acids as purple (violet) spots.

6.3 Paper Chromatography

Paper chromatography is partition chromatography. Chromatography paper holds water in its cellulose fibres: this water is the stationary phase. The spotted strip hangs in a jar with its lower edge in the solvent (the mobile phase), which rises by capillary action. Components that dissolve better in the moving solvent travel further. The developed strip is a chromatogram; colourless spots are shown with UV light or a spray reagent, as in TLC.

Adsorption chromatography

Solid stationary phase (silica gel, alumina). Components are held on the surface to different extents. Column chromatography and TLC.

Partition chromatography

Liquid stationary phase (water in paper). Components share themselves between two liquids. Paper chromatography.

Exam Trick

$R_{f}$ rules: always less than 1, has no units, measured from the baseline (not the plate edge). On polar silica the less polar compound runs further (higher $R_{f}$ ). The same compound gives the same $R_{f}$ with the same plate, solvent and temperature, so a matching $R_{f}$ helps identify a spot.

Key idea

Whatever is held more strongly by the stationary phase moves less: last out of the column, lowest

R_{f}

on the plate.

Quick Recall: tap to check

Can an

R_{f}

value be greater than 1?

No. A spot cannot travel further than the solvent front, so

R_{f} \leq 1

(in practice below 1).

What is the stationary phase in paper chromatography?

Water trapped in the cellulose fibres of the paper, not the paper itself.

In column chromatography on alumina, which component comes out first?

The least strongly adsorbed (usually the least polar) component.

How are colourless amino acids located on a chromatogram?

By spraying ninhydrin, which gives purple spots.

7. Checking Purity

Melting point: a pure solid melts sharply over 1 K or less. Impurities lower the melting point and widen the range.
Mixed melting point: mix the sample with an authentic pure specimen. No depression means the two are the same compound; a lower, broader melting range means they are different.
Boiling point: a pure liquid boils at a constant temperature (azeotropes are the exception).
Chromatography: a single spot on TLC in more than one solvent system.
Spectroscopy (IR, NMR, mass spectra) confirms both purity and identity in modern laboratories.

Figure 14: Purification methods at a glance. Each method exploits one difference in a physical property: volatility, solubility, boiling point, distribution between solvents, or adsorption.

8. Solved Examples

Solved Example 1

In a TLC run on silica gel the solvent front moved 8.0 cm from the baseline. Spots of compounds P and Q were at 6.0 cm and 2.4 cm from the baseline. Find their

R_{f}

values and say which compound is more polar.

Solution:

$R_{f} (P) = \frac{6.0}{8.0} = 0.75$ and $R_{f} (Q) = \frac{2.4}{8.0} = 0.30$ .

Q moves less, so it is held more strongly by the polar silica gel: Q is more polar.

Solved Example 2

Bromobenzene (M = 157) is steam distilled at 1 atm. The mixture boils at about 368 K, where the vapour pressure of water is 634 mmHg. What is the mass percentage of bromobenzene in the distillate?

Solution:

$p_{org} = 760 - 634 = 126$ mmHg.

\frac{m _{org}}{m _{w}} = \frac{126 \times 157}{634 \times 18} = 1.73

Mass % of bromobenzene $= \frac{1.73}{1 + 1.73} \times 100 = 63.4%$ . Even though water supplies most of the pressure, the heavier bromobenzene makes up about two-thirds of the distillate by mass.

Solved Example 3

100 mL of water contains 1.0 g of compound X.

K_{D}

(ether/water) = 4. How much X is extracted by (i) one 100 mL portion of ether, (ii) two 50 mL portions?

Solution:

(i) Fraction left $= \frac{100}{100 + 4 \times 100} = 0.20$ , so $0.80$ g is extracted.

(ii) Fraction left $= (\frac{100}{100 + 4 \times 50})^{2} = (\frac{1}{3})^{2} = 0.111$ , so $0.889$ g is extracted.

The same 100 mL of ether removes more X when used in two portions (four 25 mL portions would remove 0.94 g).

Solved Example 4

Which pair of compounds can be separated by steam distillation?
(A) o-nitrophenol and p-nitrophenol
(B) benzene and toluene
(C) naphthalene and benzoic acid
(D) glycerol and water

Solution:

Answer: (A). o-Nitrophenol has an intramolecular H-bond, so it is steam volatile; p-nitrophenol is associated by intermolecular H-bonds and stays behind. (B) needs fractional distillation, (C) acid-base extraction, and (D) distillation under reduced pressure.

Solved Example 5

Glycerol is recovered from spent lye in the soap industry by
(A) steam distillation
(B) simple distillation
(C) distillation under reduced pressure
(D) sublimation

Solution:

Answer: (C). Glycerol boils at 563 K and decomposes near that temperature. Under reduced pressure it boils well below 563 K without decomposing. It is miscible with water, so steam distillation cannot be used.

Solved Example 6

A solid mixture contains naphthalene and benzoic acid. Why does sublimation fail, and how can they be separated?

Solution:

Both compounds sublime, so heating carries both to the funnel. Instead dissolve the mixture in ether and shake with aqueous $NaHC O_{3}$ . Benzoic acid becomes sodium benzoate and passes into the water layer; naphthalene stays in the ether.

C_{6} H_{5} COOH + NaHC O_{3} C_{6} H_{5} COONa + H_{2} O + C O_{2} ↑

Acidifying the aqueous layer precipitates benzoic acid; evaporating the ether leaves naphthalene.

Solved Example 7

15.0 g of impure benzoic acid is dissolved in the minimum volume of boiling water (solubility 56.3 g/L at 100 °C) and the solution is cooled to 25 °C (solubility 3.44 g/L). Ignoring the impurity, what mass of benzoic acid crystallises?

Solution:

Volume of water needed $= \frac{15.0}{56.3} = 0.266$ L (266 mL).

Benzoic acid still dissolved at 25 °C $= 0.266 \times 3.44 = 0.92$ g.

Mass crystallised $= 15.0 - 0.92 = 14.1$ g, a recovery of about 94% (Figure 3). Using more water than the minimum would leave more acid in the mother liquor.

Solved Example 8

A mixture is run on a silica gel TLC plate with hexane as the solvent. Which component has the highest

R_{f}

?
(A) benzoic acid
(B) benzyl alcohol
(C) naphthalene
(D) benzaldehyde

Solution:

Answer: (C). Naphthalene is a non-polar hydrocarbon, so it is held least by the polar silica gel and travels furthest with the non-polar solvent. The acid, with the strongest hydrogen bonding to silica, has the lowest $R_{f}$ .

Practice Questions

State the principle of crystallisation, distillation and chromatography with one example each.Answer: Solubility difference hot vs cold (benzoic acid from water); boiling point difference (chloroform from aniline); different adsorption or partition (separating plant pigments on a column).
What is the difference between simple distillation, distillation under reduced pressure and steam distillation?Answer: Simple: liquids with widely different b.p. at 1 atm. Reduced pressure: lowers the external pressure for liquids that decompose (glycerol). Steam: water-immiscible, steam-volatile compounds distil below 373 K (aniline).
Explain the principle of paper chromatography.Answer: Partition: components distribute between water held in the paper (stationary) and the rising solvent (mobile) and travel different distances.
The best and latest technique for isolation, purification and separation of organic compounds is (A) crystallisation (B) distillation (C) sublimation (D) chromatography.Answer: (D) chromatography.
A spot moved 3.0 cm while the solvent front moved 7.5 cm. Find $R_{f}$ .Answer: $R_{f} = 3.0/7.5 = 0.40$ .
50 mL of water contains 0.50 g of X; $K_{D}$ (chloroform/water) = 3. How much X does one 50 mL portion of chloroform extract?Answer: Fraction left $= 50/ (50 + 150) = 0.25$ , so 0.375 g is extracted.
Can pure (100%) ethanol be obtained by fractional distillation of dilute aqueous ethanol?Answer: No. The azeotrope (95.6% ethanol by mass, b.p. 351 K) distils unchanged.

Common Mistakes to Avoid

Watch out

Saying a steam-distilled liquid boils at 373 K or above: the mixture always boils below 373 K.
Using simple distillation for liquids with close boiling points (benzene and toluene need a fractionating column).
Thinking the paper itself is the stationary phase in paper chromatography: it is the water held in the paper.
Measuring $R_{f}$ from the bottom edge of the plate: both distances are measured from the baseline. An $R_{f}$ above 1 is impossible.
Assuming the first band out of a column is the most strongly adsorbed: it is the least adsorbed.
Writing p-nitrophenol as steam volatile: only the ortho isomer (intramolecular H-bond) distils with steam.
Dipping the spot into the solvent: the baseline must stay above the solvent level or the spot dissolves away.
Using sublimation when both components sublime (naphthalene and benzoic acid) or expecting fractional distillation to break an azeotrope.

Frequently Asked Questions

What is the principle of steam distillation?

Two immiscible liquids each exert their full vapour pressure, so the mixture boils when the sum equals atmospheric pressure. Each partial pressure is below 760 mmHg, so a steam-volatile, water-immiscible compound such as aniline distils with water below 373 K, well under its own boiling point, and without decomposing.

Why is o-nitrophenol steam volatile but p-nitrophenol is not?

In o-nitrophenol the OH forms a hydrogen bond with the neighbouring nitro oxygen inside the same molecule, so the molecules do not associate and the compound is volatile. p-Nitrophenol molecules are linked to each other and to water by intermolecular hydrogen bonds, so p-nitrophenol stays in the flask.

What is the Rf value and can it be greater than 1?

The retardation factor is the distance moved by a spot divided by the distance moved by the solvent, both measured from the baseline. A spot cannot overtake the solvent front, so Rf is always less than 1. It has no units and is constant for a compound with a given plate, solvent and temperature.

What is the difference between adsorption and partition chromatography?

Adsorption chromatography uses a solid stationary phase such as silica gel or alumina, and components separate by how strongly they stick to its surface (column chromatography, TLC). Partition chromatography uses a liquid stationary phase, the water held in paper, and components separate by distribution between two liquids.

Why is glycerol purified by distillation under reduced pressure?

Glycerol boils at 563 K and decomposes at about that temperature. Lowering the pressure above it lowers the temperature at which its vapour pressure equals the external pressure, so it distils well below 563 K without decomposing. This is how glycerol is recovered from spent lye in the soap industry.

When is fractional distillation used instead of simple distillation?

When the boiling points of the liquids are close, their vapours form over the same temperature range and simple distillation gives a mixture. A fractionating column provides many vaporisation-condensation steps (theoretical plates), so the vapour at the top becomes nearly pure lower-boiling liquid, as in refining crude oil.

Which purification questions are common in NEET?

NEET usually asks you to match a mixture with its method: camphor with sublimation, glycerol with reduced-pressure distillation, aniline or o-nitrophenol with steam distillation, crude oil with fractional distillation, plus Rf calculations, the stationary phase in paper chromatography and the order of elution from a column.

How is purification tested in JEE Main and JEE Advanced?

JEE adds reasoning and numbers: why o-nitrophenol is steam volatile, the composition of a steam distillate from partial pressures and molar masses, single versus multiple extractions using the distribution law, acid-base extraction sequences for mixtures, azeotropes, and ranking Rf values by polarity on silica gel.

Previous year questions on Purification Methods

13 questions from past papers, each with a step-by-step solution.

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