Principles Related to Practical Chemistry

1. Detection of Extra Elements

• Why a fusion step is needed: nitrogen, sulphur and the halogens in an organic compound are covalently bonded, so they give none of the ionic tests directly. Fusing the compound with a small piece of sodium metal converts them into ionic salts, NaCN, Na₂S and NaX, which dissolve in water. The filtrate is the sodium fusion extract (Lassaigne's extract), and every test below is a test on these simple ions.
• Nitrogen: boil the extract with freshly prepared FeSO₄, then acidify with dilute H₂SO₄. A Prussian blue colour, Fe₄[Fe(CN)₆]₃, confirms nitrogen.
• Sulphur: sodium nitroprusside gives a purple colour (Na₄[Fe(CN)₅NOS]); or, after acidifying with acetic acid, lead acetate gives black PbS.
• Nitrogen and sulphur together: the extract contains NaSCN, which gives a blood-red colour with Fe³⁺, due to [Fe(SCN)]²⁺. No Prussian blue forms in this case.
• Halogens: boil the extract with dilute HNO₃ (this destroys CN⁻ and S²⁻ as HCN and H₂S) and then add AgNO₃. AgCl is white and dissolves in dilute NH₄OH; AgBr is pale yellow and only partly soluble; AgI is yellow and insoluble.
• Phosphorus: the compound is oxidised to phosphate, which gives a canary-yellow precipitate with ammonium molybdate in hot HNO₃.

2. Functional Group Tests

• Unsaturation (C=C, C≡C): Br₂ water or Br₂ in CCl₄ is decolourised; cold, dilute, alkaline KMnO₄ (Baeyer's reagent) is also decolourised.
• Alcohol (–OH): sodium metal gives H₂ gas. Lucas test (conc. HCl + anhydrous ZnCl₂): tertiary alcohols give turbidity at once, secondary after about 5 minutes, primary none at room temperature. The rate follows carbocation stability.
• Phenol: neutral FeCl₃ gives a violet, blue or green colour; bromine water gives a white precipitate of 2,4,6-tribromophenol.
• Aldehydes and ketones (C=O): 2,4-DNP (2,4-dinitrophenylhydrazine) gives a yellow-orange or red precipitate with BOTH.
• Only aldehydes reduce Tollens' reagent (silver mirror) and Fehling's solution (red-brown Cu₂O). Benzaldehyde gives Tollens' but not Fehling's.
• Iodoform test: I₂ + NaOH gives yellow CHI₃ with CH₃CO– compounds (methyl ketones, acetaldehyde) and CH₃CH(OH)– compounds (ethanol, propan-2-ol).
• Carboxylic acids (–COOH): brisk effervescence of CO₂ with NaHCO₃; phenols do not give it.
• Primary amines: the carbylamine test (CHCl₃ + alcoholic KOH, heat) gives a foul-smelling isocyanide. The azo dye test (aromatic primary amines): diazotise with NaNO₂ + HCl at 273–278 K, then couple with alkaline 2-naphthol for an orange-red dye. The Hinsberg test (C₆H₅SO₂Cl) separates primary (product dissolves in alkali), secondary (insoluble) and tertiary (no reaction) amines.

3. Preparations

• Mohr's salt, FeSO₄·(NH₄)₂SO₄·6H₂O: equimolar FeSO₄·7H₂O and (NH₄)₂SO₄ are dissolved in water containing dilute H₂SO₄, which stops hydrolysis of Fe²⁺ (and its oxidation to Fe³⁺). The solution is concentrated, cooled slowly and the pale green crystals are collected. It is a double salt, and it is a good primary standard for redox titrations because it is much less prone to air oxidation than plain FeSO₄.
• Potash alum, K₂SO₄·Al₂(SO₄)₃·24H₂O: aluminium scrap dissolves in hot KOH to give K[Al(OH)₄]; dilute H₂SO₄ is added to dissolve the Al(OH)₃ that first precipitates, and the solution is concentrated and cooled to give colourless octahedral crystals.
• Acetanilide: aniline is acetylated with acetic anhydride (or glacial acetic acid), with zinc dust added to stop aniline oxidising to coloured products. The product is recrystallised from hot water or ethanol.
• p-Nitroacetanilide: acetanilide is nitrated with a nitrating mixture (conc. HNO₃ + conc. H₂SO₄) below about 288 K. The acetyl group protects the –NH₂ from oxidation and from protonation (which would give the meta product); the para isomer is major and the ortho isomer is the minor product, removed on recrystallisation from ethanol.
• Aniline yellow (p-aminoazobenzene): aniline is diazotised (NaNO₂ + HCl, 273–278 K) and the diazonium salt is coupled with more aniline in weakly acidic medium to give a yellow azo dye.
• Iodoform: ethanol or acetone is warmed with I₂ and NaOH (or NaOCl); a yellow precipitate of CHI₃ with an antiseptic smell separates.

4. Titrimetric Exercises

• Principle: at the end point the equivalents of titrant equal the equivalents of analyte. For a reaction in which the n-factors are n₁ and n₂: n₁M₁V₁ = n₂M₂V₂. For acid-base reactions, N₁V₁ = N₂V₂ where N = n × M.
• A standard solution is one of accurately known concentration. A primary standard (oxalic acid dihydrate, anhydrous Na₂CO₃, Mohr's salt) can be weighed directly, because it is pure, stable and non-hygroscopic. NaOH and KMnO₄ are not primary standards and must be standardised against one.
• Indicators for acid-base titrations:
  • Phenolphthalein (pH range about 8.3–10): colourless in acid, pink in base. Use for strong acid vs strong base and weak acid vs strong base (for example oxalic acid vs NaOH).
  • Methyl orange (pH range about 3.1–4.4): red in acid, yellow in base. Use for strong acid vs strong base and strong acid vs weak base (for example HCl vs Na₂CO₃).
  • Weak acid vs weak base: no sharp indicator exists, because there is no steep change of pH near the end point.
• Oxalic acid vs KMnO₄: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O. The oxalic acid is warmed to 60–70 °C (the reaction is slow in the cold) in the presence of dilute H₂SO₄. KMnO₄ is its own indicator: the end point is the first permanent light pink colour. The first few drops decolourise slowly, and then Mn²⁺ formed catalyses the reaction (autocatalysis), so it speeds up.
• Mohr's salt vs KMnO₄: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O. This is done in the cold with dilute H₂SO₄; the n-factor of Mohr's salt is 1 (one Fe²⁺ per formula unit), that of oxalic acid is 2, and that of KMnO₄ in acid is 5.
• Why only dilute H₂SO₄: HCl would be oxidised to Cl₂ by KMnO₄, and HNO₃ is itself an oxidising agent that would oxidise the analyte. Dilute H₂SO₄ is neither oxidised nor oxidising. Without enough acid, KMnO₄ goes only to brown MnO₂.
• Worked example. 20 mL of 0.05 M oxalic acid needs 25 mL of KMnO₄. Oxalic acid n = 2, KMnO₄ n = 5. So 2 × 0.05 × 20 = 5 × M × 25, giving M = 2.0/125 = 0.016 M.

5. Qualitative Salt Analysis

• Preliminary tests: colour, smell and the flame test. A paste of the salt in conc. HCl is held on a platinum wire in the non-luminous flame. For coloured transition-metal ions the borax bead test is used.

Anions: tests with acids

• Dilute H₂SO₄ (or HCl) group: CO₃²⁻ gives CO₂, which turns lime water milky; S²⁻ gives H₂S with a rotten-egg smell, which blackens lead acetate paper; SO₃²⁻ gives SO₂, which turns acidified K₂Cr₂O₇ green; NO₂⁻ gives reddish-brown NO₂ fumes.
• Concentrated H₂SO₄ group: Cl⁻ gives colourless HCl fumes (dense white fumes with NH₃); Br⁻ gives reddish-brown Br₂ vapour; I⁻ gives violet I₂ vapour; NO₃⁻ gives brown fumes when heated with copper turnings.

Anions: confirmatory tests

• Cl⁻, Br⁻, I⁻: AgNO₃ (with dilute HNO₃) gives white (soluble in NH₄OH), pale yellow (partly soluble) and yellow (insoluble) precipitates. Chromyl chloride test (for Cl⁻ only): K₂Cr₂O₇ + conc. H₂SO₄ gives red CrO₂Cl₂ vapours.
• SO₄²⁻: BaCl₂ gives a white precipitate insoluble in dilute HCl.
• S²⁻: sodium nitroprusside gives a purple colour.
• NO₃⁻: brown ring test. Fresh FeSO₄ is added, then conc. H₂SO₄ is run slowly down the side of the test tube. A brown ring of [Fe(H₂O)₅NO]²⁺ forms at the junction of the two layers.
• NO₂⁻: in acetic acid, FeSO₄ gives a dark brown colour, and KI + starch turns blue.

Cations: the group scheme

• Group I (Pb²⁺): dilute HCl gives white PbCl₂, soluble in hot water. Chromate: yellow PbCrO₄. Iodide: yellow PbI₂.

• Group II (Cu²⁺, Pb²⁺): H₂S in presence of dilute HCl gives black CuS. Cu²⁺ gives a deep blue solution with excess NH₄OH, chocolate-brown Cu₂[Fe(CN)₆] with K₄[Fe(CN)₆].

• Group III (Fe³⁺, Al³⁺): NH₄OH in presence of NH₄Cl gives reddish-brown Fe(OH)₃ and white gelatinous Al(OH)₃. Fe³⁺: Prussian blue with K₄[Fe(CN)₆], blood red with KSCN. Al³⁺: Al(OH)₃ dissolves in excess NaOH, and the red lake with aluminon.

• Group IV (Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺): H₂S in ammoniacal medium. ZnS is white (dissolves in HCl), NiS and CoS are black (not soluble in dilute HCl once formed), MnS is buff. Ni²⁺ gives a red precipitate with dimethylglyoxime; Zn²⁺ gives a white or bluish-white precipitate with K₄[Fe(CN)₆]; Co²⁺ with KNO₂ + CH₃COOH gives yellow K₃[Co(NO₂)₆].

• Group V (Ba²⁺, Ca²⁺, Sr²⁺): (NH₄)₂CO₃ in presence of NH₄Cl and NH₄OH gives white carbonates. Ba²⁺ gives yellow BaCrO₄ with K₂CrO₄; the flame colours (apple green, brick red, crimson) distinguish the three.

• Group VI (Mg²⁺): Na₂HPO₄ in NH₄OH gives white crystalline Mg(NH₄)PO₄.

• Group 0 (NH₄⁺): tested on the original salt. Warm with NaOH to liberate NH₃ and detect it with Nessler's reagent (alkaline K₂HgI₄), which gives a brown precipitate or colour.

• Why the scheme works (solubility product): in Group II the HCl keeps [S²⁻] very low (common-ion effect), so only the sulphides with the smallest K_sp precipitate. In Group IV the ammoniacal medium removes H⁺, so [S²⁻] is high enough to precipitate the more soluble sulphides. In Group III, NH₄Cl suppresses ionisation of NH₄OH, lowering [OH⁻] so only the very insoluble hydroxides (K_sp about 10⁻³⁸) precipitate. Before Group III, H₂S is boiled off and a little conc. HNO₃ is added to oxidise Fe²⁺ to Fe³⁺.

6. Enthalpy, Sols and Kinetics Experiments

• Enthalpy of solution of CuSO₄: a known mass of CuSO₄ is dissolved in a known volume of water in a calorimeter (an insulated vessel). From the temperature change, q = m × s × ΔT and Δ_solH = −q / (moles of solute). Anhydrous CuSO₄ dissolves exothermically, while CuSO₄·5H₂O dissolves slightly endothermically.
• Enthalpy of neutralisation: equal volumes of strong acid and strong base of the same concentration are mixed in the calorimeter. For HCl and NaOH, Δ_neutH is about −57 kJ mol⁻¹, because the net reaction is only H⁺ + OH⁻ → H₂O. A weak acid or weak base gives a less negative value, because some energy goes into ionising it.
• Lyophilic and lyophobic sols: a lyophilic (solvent-loving) sol such as starch, gum or egg albumin forms simply by dissolving in water and is stable, reversible, and not easily coagulated. A lyophobic sol such as ferric hydroxide is made by adding FeCl₃ drop by drop to boiling water (hydrolysis gives a reddish-brown Fe(OH)₃ sol); it is unstable, irreversible and coagulated by a small amount of electrolyte. As₂S₃ sol and sulphur sol are other examples.
• Kinetics of I⁻ + H₂O₂: H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O. The rate is found from the time taken for a fixed amount of I₂ to appear, detected by a starch blue colour after a known small amount of Na₂S₂O₃ has been consumed (a clock reaction). The rate is first order in each of H₂O₂ and I⁻; the reaction is therefore overall second order.

7. How NEET Frames These Questions

• Single-fact recall: most NEET questions here are one-line facts: which acid goes into Mohr's salt, which group a cation belongs to, which reaction is NOT part of Lassaigne's test, which colour the indicator shows. Learn the facts in the tables above exactly as written.

• Matching and ordering: the group-number questions come as "arrange in increasing group number" or "match the ion with its group". Memorise the one-line list of groups and read the options for the one that fits every row.

• Statement questions: Lucas test is the classic: the traps are the rates for primary versus tertiary alcohols.

• Calculation: at most one short numerical, such as pH from a solubility-product constant of a hydroxide, or an n-factor titration. Write the equation, plug in, and check the final step (pH = 14 − pOH).

• Quick checklist before the exam:

  1. Colours: Prussian blue (N), purple (S), blood red (N + S), white / pale yellow / yellow AgX, brown ring (NO₃⁻), chocolate brown Cu₂[Fe(CN)₆], buff MnS.
  2. Group order and group reagents, with the cations in each group.
  3. Why dilute H₂SO₄ is used in Mohr's salt preparation and in KMnO₄ titrations, and why HCl and HNO₃ are not.
  4. Indicator for each type of acid-base titration, and the direction of the colour change.
  5. n-factors: KMnO₄ = 5, oxalic acid = 2, Mohr's salt = 1.