1. Chapter at a Glance
The NEET syllabus includes a unit on the chemistry behind laboratory experiments. It covers tests for elements and functional groups in organic compounds, the preparation of some inorganic and organic compounds, titrations with their indicators, qualitative analysis of cations and anions, and a few physical-chemistry experiments. NEET usually asks one question from this unit, and its ideas also appear in organic and inorganic questions.
- Lassaigne's test for N, S and halogens.
- Tests for –OH (alcoholic and phenolic), –CHO, >C=O, –COOH and –NH₂.
- Indicators for acid–base titrations; KMnO₄ titrations with oxalic acid and Mohr's salt.
- Group reagents and characteristic tests of cations.
- Tests for common anions (CO₃²⁻, S²⁻, SO₄²⁻, NO₃⁻, NO₂⁻, Cl⁻, Br⁻, I⁻).
2. Detection of Elements in Organic Compounds
An organic compound is fused with sodium metal. N, S and halogens are converted to NaCN, Na₂S and NaX. The fused mass is extracted with water to give sodium fusion extract (Lassaigne's extract).
| Element | Test | Positive result | NEET Note |
|---|---|---|---|
| Nitrogen | Boil extract with FeSO₄, acidify with conc. H₂SO₄ | Prussian blue, Fe₄[Fe(CN)₆]₃ | Hydrazine has N but no C, so it fails |
| Sulphur | Sodium nitroprusside | Violet colour | Or lead acetate gives black PbS |
| N and S together | FeCl₃ | Blood-red colour (Fe(SCN)²⁺) | NaSCN forms if Na is limited |
| Halogens | Boil with HNO₃, then AgNO₃ | White (AgCl), pale yellow (AgBr), yellow (AgI) | HNO₃ first removes CN⁻ and S²⁻ |
3. Tests for Functional Groups
| Group | Test | Observation |
|---|---|---|
| Alcoholic –OH | Sodium metal; ceric ammonium nitrate | H₂ gas; red colour |
| Phenolic –OH | Neutral FeCl₃ | Violet / blue / green colour |
| Phenol | Bromine water | White precipitate |
| Aldehydes and ketones | 2,4-Dinitrophenylhydrazine | Yellow/orange precipitate |
| Aldehyde only | Tollens' reagent; Fehling's solution; Schiff's reagent | Silver mirror; red precipitate; pink colour |
| Methyl ketone | Iodoform test (I₂ + NaOH) | Yellow crystals of CHI₃ |
| Carboxylic acid | NaHCO₃ solution | Brisk effervescence of CO₂ |
| Primary amine | Carbylamine test (CHCl₃ + alc. KOH) | Foul smell of isocyanide |
| Aromatic primary amine | Diazotisation then β-naphthol | Orange-red dye |
4. Preparation of Compounds
| Compound | Starting materials | Key point |
|---|---|---|
| Mohr's salt, FeSO₄·(NH₄)₂SO₄·6H₂O | Equimolar FeSO₄·7H₂O and (NH₄)₂SO₄ with a little dilute H₂SO₄ | Acid prevents hydrolysis of Fe²⁺; do not heat too long (oxidation to Fe³⁺) |
| Potash alum, K₂SO₄·Al₂(SO₄)₃·24H₂O | K₂SO₄ and Al₂(SO₄)₃ with a little dilute H₂SO₄ | A double salt; crystallises on cooling |
| Acetanilide | Aniline + acetic anhydride (or acetyl chloride) | Acetylation of –NH₂ |
| p-Nitroacetanilide | Acetanilide + conc. HNO₃/H₂SO₄ (cold) | –NHCOCH₃ directs mainly to para |
| Aniline yellow (p-aminoazobenzene) | Benzenediazonium chloride + aniline | Coupling reaction |
| Iodoform | Ethanol or acetone + I₂ + NaOH (or Na₂CO₃) | Yellow crystals, antiseptic smell |
5. Titrimetric Analysis
- At the end point, equivalents of acid = equivalents of base: N₁V₁ = N₂V₂ (or use moles with the balanced equation).
- Indicators: phenolphthalein (colourless in acid, pink in base; range pH 8.2–10) and methyl orange (red in acid, yellow in base; range pH 3.1–4.4).
| Titration type | Suitable indicator | NEET Note |
|---|---|---|
| Strong acid × strong base | Phenolphthalein or methyl orange | Large pH jump around 7 |
| Weak acid × strong base | Phenolphthalein | End point is basic (pH > 7) |
| Strong acid × weak base | Methyl orange | End point is acidic (pH < 7) |
| Weak acid × weak base | No sharp end point | Not done with simple indicators |
| KMnO₄ × oxalic acid / Mohr's salt | KMnO₄ itself (self-indicator) | End point: permanent light pink; titrate in dilute H₂SO₄ (not HCl) |
- KMnO₄ + oxalic acid: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O. The solution is warmed to about 333 K because the reaction is slow at first; Mn²⁺ formed catalyses it (autocatalysis).
- KMnO₄ + Mohr's salt: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, done at room temperature (heating would oxidise Fe²⁺ by air).
- HCl is not used for acidifying KMnO₄ titrations because KMnO₄ oxidises Cl⁻ to Cl₂.
6. Qualitative Analysis of Cations
| Group | Cations (NEET list) | Group reagent | Precipitate |
|---|---|---|---|
| Zero | NH₄⁺ | NaOH, heat | NH₃ gas (turns red litmus blue; brown with Nessler's reagent) |
| I | Pb²⁺ | Dilute HCl | PbCl₂ (white, soluble in hot water) |
| II | Pb²⁺, Cu²⁺ | H₂S in dilute HCl | Sulphides (black PbS, CuS) |
| III | Al³⁺, Fe³⁺ | NH₄OH in presence of NH₄Cl | Hydroxides: Al(OH)₃ white gelatinous; Fe(OH)₃ reddish-brown |
| IV | Zn²⁺, Ni²⁺ | H₂S in NH₄OH | ZnS white; NiS black |
| V | Ca²⁺, Ba²⁺ | (NH₄)₂CO₃ in NH₄OH | White carbonates |
| VI | Mg²⁺ | Na₂HPO₄ | White Mg(NH₄)PO₄ |
- Confirmatory tests: Pb²⁺ gives yellow PbCrO₄ with K₂CrO₄ and yellow PbI₂ with KI; Cu²⁺ gives deep blue [Cu(NH₃)₄]²⁺ and a chocolate-brown precipitate with K₄[Fe(CN)₆]; Fe³⁺ gives Prussian blue with K₄[Fe(CN)₆] and blood-red colour with KSCN; Ni²⁺ gives a red precipitate with dimethylglyoxime; Zn²⁺ gives white ZnS; Ba²⁺ gives an apple-green flame and Ca²⁺ a brick-red flame.
- In group III, NH₄Cl suppresses the ionisation of NH₄OH (common-ion effect) so that only the hydroxides with very low Ksp precipitate. In group II, acid keeps [S²⁻] low so that only the sulphides with very low Ksp precipitate.
7. Tests for Anions
| Anion | Test | Observation |
|---|---|---|
| CO₃²⁻ | Dilute H₂SO₄ | Effervescence; gas turns lime water milky |
| S²⁻ | Dilute H₂SO₄; sodium nitroprusside | H₂S (rotten-egg smell, blackens lead acetate paper); violet colour |
| NO₂⁻ | Dilute H₂SO₄ | Brown fumes of NO₂ |
| Cl⁻ | AgNO₃ (with HNO₃) | White AgCl, soluble in NH₄OH; chromyl chloride test gives yellow PbCrO₄ |
| Br⁻ | AgNO₃; conc. H₂SO₄ + MnO₂ | Pale yellow AgBr, partly soluble in NH₄OH; brown Br₂ vapour |
| I⁻ | AgNO₃; chlorine water + CCl₄ | Yellow AgI, insoluble in NH₄OH; violet CCl₄ layer |
| NO₃⁻ | Brown ring test (FeSO₄ + conc. H₂SO₄) | Brown ring of [Fe(H₂O)₅NO]²⁺ at the junction |
| SO₄²⁻ | BaCl₂ | White BaSO₄, insoluble in conc. HCl |
8. Physical Chemistry Experiments
- Enthalpy of solution of CuSO₄ and enthalpy of neutralisation of a strong acid and strong base are measured with a calorimeter (q = mcΔT).
- Colloids: lyophilic sols (starch, egg albumin, gum) form easily and are stable; lyophobic sols (aluminium hydroxide, ferric hydroxide, arsenious sulphide) need special methods and coagulate easily.
- Kinetics: the rate of reaction of iodide ions with H₂O₂ at room temperature can be followed using starch as an indicator (the blue colour appears after a measurable time).
9. Formula and Reagent Master Table
| Item | Formula / fact |
|---|---|
| Titration equality | N₁V₁ = N₂V₂ |
| KMnO₄ + oxalate | 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O |
| KMnO₄ + Fe²⁺ | MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O |
| Prussian blue | Fe₄[Fe(CN)₆]₃ |
| Brown ring | [Fe(H₂O)₅NO]²⁺ |
| Nessler's reagent | K₂[HgI₄] in KOH |
| Calorimetry | q = mcΔT |
| Phenolphthalein range | pH 8.2–10 |
| Methyl orange range | pH 3.1–4.4 |
10. Master Summary Table
| Test | Detects | NEET Note |
|---|---|---|
| Lassaigne + FeSO₄ | Nitrogen | Prussian blue |
| Neutral FeCl₃ | Phenols | Violet |
| Tollens' / Fehling's | Aldehydes | Benzaldehyde fails Fehling's |
| NaHCO₃ | –COOH | CO₂ effervescence |
| Brown ring | NO₃⁻ | Iron nitrosyl complex |
| Dimethylglyoxime | Ni²⁺ | Red precipitate |
| KSCN | Fe³⁺ | Blood-red |
| Flame test | Ca²⁺ brick red; Ba²⁺ apple green | Cu²⁺ green-blue |
11. Mind Map
- Lassaigne
- Functional groups
- Mohr's salt, alum
- Acetanilide
- Iodoform, dyes
- Indicators
- KMnO₄
- Cation groups
- Anion tests
12. Tips and Tricks
- Group reagent order: "HCl, H₂S (acid), NH₄OH, H₂S (base), carbonate, phosphate".
- Silver halides and ammonia: AgCl dissolves, AgBr partly, AgI does not.
- Colours of sulphides: ZnS white; CdS yellow; MnS buff/pink; most others black.
- Phenolphthalein for weak acids, methyl orange for weak bases.
- Using HCl to acidify KMnO₄ titrations.
- Forgetting NH₄Cl in group III (Mg²⁺, Zn²⁺ would precipitate otherwise).
- Expecting hydrazine to give Lassaigne's test for N.
- Thinking Mohr's salt titration needs heating like oxalic acid.
13. Important One-liners
- KMnO₄ acts as its own indicator.
- Mohr's salt is a double salt of ferrous sulphate and ammonium sulphate.
- The brown ring test detects nitrate ions.
- Nessler's reagent gives a brown precipitate with ammonia.
- Ni²⁺ forms a red complex with dimethylglyoxime.
- Lassaigne's test converts covalent N, S and X into ionic sodium salts.
- Barium salts give an apple-green flame.
14. NEET-pattern Questions with Detailed Answers
These questions are original and based on concepts that NEET UG asks again and again.
The Prussian blue colour in Lassaigne's test indicates the presence of:
- Sulphur
- Nitrogen
- Chlorine
- Phosphorus
Show solution
NaCN from the fusion reacts with FeSO₄ and then Fe³⁺ to form Fe₄[Fe(CN)₆]₃, which is Prussian blue. Sulphur gives a violet colour with nitroprusside, and chlorine gives a white AgCl precipitate.
Answer: (B)
Which indicator is used in the titration of acetic acid with sodium hydroxide?
- Methyl orange
- Phenolphthalein
- Starch
- No indicator
Show solution
A weak acid titrated with a strong base has a basic end point (pH > 7). Phenolphthalein changes colour in that range. Methyl orange changes colour too early, on the acidic side.
Answer: (B)
In the titration of oxalic acid with KMnO₄, the solution is warmed because:
- KMnO₄ decomposes when cold
- The reaction is slow at room temperature at the start
- Oxalic acid is insoluble when cold
- Heating removes Cl⁻
Show solution
The reaction between permanganate and oxalate starts slowly. Warming to about 60 °C speeds it up, and the Mn²⁺ produced then catalyses the rest of the reaction (autocatalysis).
Answer: (B)
The group reagent for group III cations is:
- Dilute HCl
- H₂S in acid
- NH₄OH in the presence of NH₄Cl
- (NH₄)₂CO₃
Show solution
Group III (Al³⁺, Fe³⁺) is precipitated as hydroxides by NH₄OH. NH₄Cl lowers the [OH⁻] through the common-ion effect, so group IV–VI hydroxides stay in solution.
Answer: (C)
A salt solution gives a white precipitate with AgNO₃ that dissolves in ammonia solution. The anion is:
- Cl⁻
- Br⁻
- I⁻
- NO₃⁻
Show solution
AgCl is white and dissolves in NH₃ to form [Ag(NH₃)₂]⁺. AgBr is pale yellow and only partly soluble, AgI is yellow and insoluble, and nitrate gives no precipitate with AgNO₃.
Answer: (A)
A compound gives a violet colour with neutral FeCl₃ solution. It contains:
- An alcoholic –OH group
- A phenolic –OH group
- A –CHO group
- A –COOH group
Show solution
Phenols form coloured complexes with Fe³⁺. Alcohols, aldehydes and carboxylic acids do not give this violet colour.
Answer: (B)
The brown ring in the test for nitrate is due to:
- [Fe(H₂O)₅NO]²⁺
- Fe₂(SO₄)₃
- NO₂ gas
- [Fe(CN)₆]⁴⁻
Show solution
Nitrate is reduced by Fe²⁺ to NO. The NO then binds to [Fe(H₂O)₆]²⁺, forming the brown nitrosyl complex [Fe(H₂O)₅NO]²⁺ at the junction of the two liquids. Brown NO₂ fumes are seen in the nitrite test, not in the brown-ring test.
Answer: (A)
Dilute H₂SO₄, not dilute HCl, is used to acidify KMnO₄ in titrations because:
- H₂SO₄ is a stronger acid
- KMnO₄ oxidises HCl to Cl₂, giving wrong readings
- HCl reduces the rate
- H₂SO₄ acts as an indicator
Show solution
Permanganate oxidises Cl⁻ to Cl₂, which uses up extra KMnO₄ and gives a falsely high reading. Sulphate is not oxidised, so it does not interfere. HNO₃ is also avoided, because it is itself an oxidising agent.
Answer: (B)
15. Quick Revision Summary
- Lassaigne: N (Prussian blue), S (violet), N + S (blood-red), X (AgX).
- FeCl₃ violet (phenol); 2,4-DNP (carbonyl); Tollens/Fehling (aldehyde); NaHCO₃ (acid); carbylamine (1° amine).
- Phenolphthalein for weak acid × strong base; methyl orange for strong acid × weak base; KMnO₄ self-indicator in H₂SO₄.
- Cation groups: HCl → H₂S/acid → NH₄OH/NH₄Cl → H₂S/base → carbonate → phosphate.
- Anions: lime water (CO₃²⁻), brown ring (NO₃⁻), BaSO₄ (SO₄²⁻), AgX solubility in NH₃.