1. Chapter at a Glance
This chapter covers the practical side of chemistry that JEE Main includes in its syllabus: purification of organic compounds, qualitative analysis (detecting elements and functional groups), quantitative analysis (estimating C, H, N, halogens, S), volumetric titrations and inorganic salt analysis. Questions here are short and factual — memorising the reagents, colours and formulas gives quick marks.
- Choosing the right purification method (steam distillation, fractional distillation, chromatography).
- Lassaigne's test colours for N, S and halogens.
- Percentage calculations: Kjeldahl (N), Carius (halogen, S).
- Functional group tests and group reagents in salt analysis.
2. Methods of Purification
| Method | Principle | Example |
|---|---|---|
| Sublimation | Solid changes directly to vapour | Camphor, naphthalene, benzoic acid, iodine |
| Crystallisation | Difference in solubility in a hot vs cold solvent | Sugar, benzoic acid from water |
| Simple distillation | Large difference in boiling points | Chloroform (334 K) + aniline (457 K) |
| Fractional distillation | Small difference in boiling points (fractionating column) | Crude oil fractions, acetone + methanol |
| Distillation under reduced pressure | Liquid decomposes below its normal b.p. | Glycerol from spent lye |
| Steam distillation | Steam-volatile liquid, immiscible with water | Aniline, essential oils, o-nitrophenol |
| Differential extraction | Different solubility in two immiscible solvents | Organic compound from water using ether |
| Chromatography | Different adsorption/partition between two phases | Plant pigments, amino acids |
In steam distillation the mixture boils when pwater + porganic = atmospheric pressure, i.e. below 373 K, so the compound distils without decomposing. o-Nitrophenol is steam volatile (intramolecular H-bond) but p-nitrophenol is not (intermolecular H-bonds).
Chromatography
- Adsorption: column chromatography and thin-layer chromatography (TLC) on silica gel or alumina.
- Partition: paper chromatography — water held in the paper is the stationary phase.
- Retardation factor: Rf = (distance moved by substance)/(distance moved by solvent front). Rf is always less than 1; a less strongly adsorbed compound has a higher Rf. Colourless spots are seen under UV light or with iodine vapour (ninhydrin for amino acids).
3. Detection of Elements (Lassaigne's Test)
The organic compound is fused with sodium metal to convert N, S and halogens into ionic NaCN, Na₂S and NaX. The fusion extract (sodium extract) is then tested.
| Element | Test | Result |
|---|---|---|
| Nitrogen | Boil extract with FeSO₄, acidify with conc. H₂SO₄ | Prussian blue, Fe₄[Fe(CN)₆]₃ |
| Sulphur | Sodium nitroprusside | Violet colour |
| Sulphur | Lead acetate + acetic acid | Black PbS precipitate |
| N and S together | FeCl₃ (NaSCN formed) | Blood-red colour |
| Halogens | Boil with HNO₃ (removes CN⁻, S²⁻), then AgNO₃ | White AgCl (soluble in NH₃), pale yellow AgBr (partly soluble), yellow AgI (insoluble) |
| Phosphorus | Oxidise, then ammonium molybdate | Yellow precipitate |
Hydrazine (NH₂NH₂) does not give the Lassaigne nitrogen test because it has no carbon to form CN⁻. Diazonium salts lose N₂ on heating and also often fail.
4. Quantitative Estimation
| Element | Method | Formula (m = mass of compound) |
|---|---|---|
| C and H | Liebig combustion (CO₂ in KOH, H₂O in CaCl₂) | %C = (12/44)(mCO₂/m) × 100; %H = (2/18)(mH₂O/m) × 100 |
| N | Dumas (N₂ gas volume) | %N = (28/22400)(VN₂ at STP/m) × 100 |
| N | Kjeldahl (NH₃ absorbed in acid) | %N = 1.4 × Nacid × Vacid used(mL)/m |
| Halogen | Carius (as AgX) | %X = (at. mass X/MAgX)(mAgX/m) × 100 |
| S | Carius (as BaSO₄) | %S = (32/233)(mBaSO₄/m) × 100 |
| P | As Mg₂P₂O₇ | %P = (62/222)(mMg₂P₂O₇/m) × 100 |
Kjeldahl's method cannot be used for nitro, azo compounds or nitrogen in rings (pyridine) because that nitrogen is not converted to ammonium sulphate.
5. Tests for Functional Groups
| Group | Test | Observation |
|---|---|---|
| Unsaturation (C=C, C≡C) | Br₂ water / Baeyer's reagent | Decolourised |
| Alcohol | Ceric ammonium nitrate | Red colour |
| Phenol | Neutral FeCl₃ | Violet colour |
| Aldehyde, ketone | 2,4-DNP | Orange-yellow precipitate |
| Aldehyde only | Tollens' / Fehling's / Schiff's | Silver mirror / red ppt / pink colour |
| Carboxylic acid | NaHCO₃ | Effervescence (CO₂) |
| Primary amine | Carbylamine; azo-dye test for aromatic amines | Foul smell; orange/red dye |
| Carbohydrate | Molisch's reagent | Violet ring |
| Protein | Biuret / ninhydrin | Violet / blue-violet |
6. Volumetric Analysis and Salt Analysis
Titrations
- Normality equation: N₁V₁ = N₂V₂. Molarity equation with n-factors: n₁M₁V₁ = n₂M₂V₂.
- Strong acid–strong base: phenolphthalein or methyl orange. Weak acid–strong base (oxalic acid vs NaOH): phenolphthalein. Strong acid–weak base (HCl vs Na₂CO₃ or NH₄OH): methyl orange.
- KMnO₄ titrations (with oxalic acid or Mohr's salt) need no indicator — KMnO₄ is a self-indicator (end point: permanent light pink). Oxalic acid titration is done warm (333 K); dilute H₂SO₄ is used, not HCl.
Inorganic salt analysis (cations)
| Group | Group reagent | Cations (precipitate) |
|---|---|---|
| 0 | NaOH, heat (NH₃ gas) | NH₄⁺ |
| I | Dilute HCl | Pb²⁺ (as PbCl₂) |
| II | H₂S in dilute HCl | Pb²⁺, Cu²⁺, As³⁺ (sulphides) |
| III | NH₄OH in presence of NH₄Cl | Fe³⁺, Al³⁺ (hydroxides) |
| IV | H₂S in ammoniacal medium | Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ (sulphides) |
| V | (NH₄)₂CO₃ with NH₄OH | Ba²⁺, Sr²⁺, Ca²⁺ (carbonates) |
| VI | No group reagent | Mg²⁺ |
The group II sulphides have very low Ksp, so they precipitate even at the low [S²⁻] of acidic H₂S (common-ion effect of H⁺). In group III, NH₄Cl suppresses the ionisation of NH₄OH so that only the least soluble hydroxides precipitate. Anion tests: nitrate — brown ring with FeSO₄ + conc. H₂SO₄ ([Fe(H₂O)₅NO]²⁺); sulphate — white BaSO₄; carbonate — CO₂ turns lime water milky; chloride — white AgCl soluble in NH₄OH.
7. Formula Master Table
| Quantity | Formula |
|---|---|
| Retardation factor | Rf = dsubstance/dsolvent |
| Carbon | %C = 12 mCO₂ × 100/(44 m) |
| Hydrogen | %H = 2 mH₂O × 100/(18 m) |
| Nitrogen (Kjeldahl) | %N = 1.4 NV/m |
| Halogen (Carius) | %X = (X/MAgX) × mAgX × 100/m |
| Sulphur (Carius) | %S = 32 mBaSO₄ × 100/(233 m) |
| Titration | n₁M₁V₁ = n₂M₂V₂ |
8. Mind Map
- Distillation types
- Steam distillation
- Chromatography, Rf
- Lassaigne's test
- Functional group tests
- Liebig (C, H)
- Dumas, Kjeldahl (N)
- Carius (X, S)
- Titration indicators
- Cation groups
- Anion tests
9. Tips and Tricks
- Kjeldahl: when the acid is H₂SO₄, remember normality = 2 × molarity.
- Indicator choice: the indicator's colour-change range must fall inside the pH jump at the equivalence point.
- Carius shortcut for chlorine: %Cl = 24.74 × (mAgCl/m).
- Steam distillation: "immiscible + steam-volatile + decomposes at its b.p."
- Using HCl to acidify KMnO₄ in titrations — it gets oxidised to Cl₂.
- Forgetting to boil the sodium extract with HNO₃ before the halogen test (CN⁻ and S²⁻ interfere).
- Applying Kjeldahl's method to nitrobenzene or pyridine.
10. Important One-liners
- Aniline is purified by steam distillation.
- Glycerol is distilled under reduced pressure.
- Prussian blue in Lassaigne's test confirms nitrogen.
- Paper chromatography is a form of partition chromatography.
- KMnO₄ acts as its own indicator.
- The brown ring test detects nitrate ions.
11. PYQ-pattern Questions with Solutions
These questions are original and modelled on concepts frequently tested in JEE Main.
Aniline is best purified by:
- Sublimation
- Steam distillation
- Crystallisation
- Fractional distillation
Show solution
Aniline is steam volatile and immiscible with water; steam distillation lets it distil below 373 K without decomposition.
Answer: (B)
In Lassaigne's test, a violet colour with sodium nitroprusside indicates:
- Nitrogen
- Sulphur
- Chlorine
- Phosphorus
Show solution
Na₂S in the extract forms a violet complex, Na₄[Fe(CN)₅NOS], with sodium nitroprusside. This confirms sulphur.
Answer: (B)
On a TLC plate the solvent front moves 5.0 cm and a compound moves 3.0 cm from the baseline. Find its Rf value.
Show solution
Rf = 3.0/5.0 = 0.6.
Answer: 0.6
In Kjeldahl's method, the ammonia from 0.5 g of an organic compound neutralised 10 mL of 0.5 M H₂SO₄. Find the percentage of nitrogen.
Show solution
0.5 M H₂SO₄ = 1 N. %N = 1.4 × N × V/m = 1.4 × 1 × 10/0.5 = 28%.
Answer: 28
In the Carius method, 0.25 g of an organic compound gave 0.574 g of AgCl (AgCl = 143.5 g mol−1). The percentage of chlorine is about:
- 28.4%
- 35.5%
- 56.8%
- 71.0%
Show solution
Mass of Cl = 0.574 × 35.5/143.5 = 0.142 g. %Cl = 0.142/0.25 × 100 = 56.8%.
Answer: (C)
The suitable indicator for titrating oxalic acid against NaOH is:
- Methyl orange
- Phenolphthalein
- Methyl red
- No indicator needed
Show solution
Weak acid vs strong base: the equivalence point is basic (pH about 8–10), matching phenolphthalein's range (8.3–10). Methyl orange changes colour too early (3.1–4.4).
Answer: (B)
The group reagent for group III cations (Fe³⁺, Al³⁺) is:
- Dilute HCl
- H₂S in dilute HCl
- NH₄OH in presence of NH₄Cl
- (NH₄)₂CO₃
Show solution
NH₄Cl suppresses the ionisation of NH₄OH, giving just enough OH⁻ to precipitate the very insoluble Fe(OH)₃ and Al(OH)₃, but not the hydroxides of later groups.
Answer: (C)
Kjeldahl's method is NOT suitable for estimating nitrogen in:
- Urea
- Acetamide
- Nitrobenzene
- Aniline
Show solution
In nitro compounds (and azo compounds, pyridine), nitrogen is not converted into ammonium sulphate on digestion with conc. H₂SO₄, so Kjeldahl's method fails. Dumas' method must be used.
Answer: (C)
12. Quick Revision Summary
- Steam distillation: aniline, o-nitrophenol; reduced pressure: glycerol.
- Rf = dsubstance/dsolvent < 1.
- Lassaigne: N → Prussian blue; S → violet (nitroprusside); N + S → blood red.
- %N (Kjeldahl) = 1.4NV/m; %X (Carius) = (X/MAgX)(mAgX/m) × 100.
- Weak acid + strong base → phenolphthalein; strong acid + weak base → methyl orange.
- Cation groups: HCl, H₂S/H⁺, NH₄OH/NH₄Cl, H₂S/NH₃, (NH₄)₂CO₃.