Reaction of acetate salt with dil. H₂SO₄:
2CH₃COO⁻ + H₂SO₄ → 2CH₃COOH + SO₄²⁻
Acetic acid (CH₃COOH) is liberated — volatile liquid (bp 118°C)
Why the clues match:
✅ Colourless vapour — CH₃COOH vapour is colourless
✅ Smell of vinegar — vinegar IS dilute acetic acid! The characteristic pungent sour smell
✅ Turns blue litmus red — CH₃COOH is acidic (weak acid, but still acid)
Why other options are wrong:
CO₃²⁻: gives CO₂ (odourless gas, turns lime water milky — not vinegar smell)
SO₄²⁻: gives H₂SO₄ (no distinctive smell + not volatile from dilute H₂SO₄)
S²⁻: gives H₂S (rotten egg smell — NOT vinegar smell)
CH₃COO⁻ + H⁺ → CH₃COOH (acetic acid)
Acetic acid = vinegar smell + colourless + acidic (turns litmus red)
Answer: Acetate anion (CH₃COO⁻)
Adding dilute H₂SO₄ to a salt liberates the corresponding acid, which can be identified by its properties. This is used in the preliminary tests for anion identification. The reactions are essentially double displacement/acid-base reactions where a stronger acid (H₂SO₄) displaces weaker acids from their salts. The gas or vapour evolved has characteristic properties that help identify the anion.
📌 Acetate (CH₃COO⁻): Vinegar smell (acetic acid vapours), colourless, acidic. Brown ring test also positive.
📌 Carbonate (CO₃²⁻): Brisk effervescence of CO₂, odourless, turns lime water milky (Ca(OH)₂ + CO₂ → CaCO₃↓), turns moist red litmus blue then white (acidic then bleaches)... wait — CO₂ turns moist blue litmus red (acidic). Lime water test is the specific test.
📌 Sulphide (S²⁻): H₂S gas — rotten egg smell, turns lead acetate paper black (PbS). Colourless but distinctly foul-smelling.
📌 Nitrite (NO₂⁻): Brown fumes of NO₂, or colourless NO which turns brown in air.
📌 Sulphite (SO₃²⁻): SO₂ gas — pungent smell (burning matchstick), turns potassium dichromate paper green.
📌 Chloride (Cl⁻): HCl gas — pungent, fumes with NH₃, white fumes with NH₃ aq. (NH₄Cl)
📌 Bromide (Br⁻): HBr + Br₂ (brown fumes) — H₂SO₄ oxidises HBr to Br₂
📌 Iodide (I⁻): HI + I₂ (violet vapour) + H₂S + SO₂ (further reduction of H₂SO₄)
📌 Nitrate (NO₃⁻): Brown fumes of NO₂ with conc. H₂SO₄ + copper. Ring test with FeSO₄ (brown ring = iron-NO complex).
📌 Phosphate (PO₄³⁻): Ammonium molybdate test — yellow precipitate (ammonium phosphomolybdate)
Acetic acid (ethanoic acid, CH₃COOH) is a weak organic acid (Ka = 1·8×10⁻⁵). Pure acetic acid is called glacial acetic acid (freezes at 16·6°C). Vinegar is 5–8% aqueous acetic acid. Produced industrially by: (1) Oxidation of ethanol. (2) Carbonylation of methanol (Monsanto/Cativa process). Properties: colourless liquid, pungent characteristic smell, boiling point 118°C, miscible with water. Uses: food preservative, vinegar, synthesis of esters (ethyl acetate — solvent, nail polish remover), synthesis of aspirin (acetylsalicylic acid), manufacture of plastics, pharmaceuticals.
Specific test for NO₃⁻: Add freshly prepared FeSO₄ solution to the salt solution. Slowly add conc. H₂SO₄ along the side of the test tube. A brown ring appears at the interface of two layers = presence of NO₃⁻. Reaction: NO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O; then NO + FeSO₄ → [Fe(H₂O)₅NO]²⁺ (brown ring complex = pentaaquanitrosyliron(II) complex). Important: NO₂⁻ (nitrite) also gives positive brown ring but the ring disappears on shaking — this distinguishes nitrite from nitrate.
Lime water = Ca(OH)₂ solution. On passing CO₂: Ca(OH)₂ + CO₂ → CaCO₃↓ (white precipitate) + H₂O — turns lime water milky. Excess CO₂: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble) — milky solution clears again. This dual behaviour (milky then clear) is a unique and confirmatory test for CO₂. Used to distinguish CO₂ from SO₂ — SO₂ also turns lime water milky (CaSO₃↓) but CaSO₃ dissolves in excess SO₂ forming Ca(HSO₃)₂, similar to CO₂.
Organic compound is fused with sodium metal to convert covalent elements into ionic salts: N → NaCN, S → Na₂S, halogens → NaX (X=Cl, Br, I). The fused mass is dissolved in water (sodium fusion extract = SFE). Tests on SFE: N: add FeSO₄ + FeCl₃ + dil. HCl → Prussian blue (Fe₄[Fe(CN)₆]₃) confirms N. S: add sodium nitroprusside → purple colour (sodium nitroprusside test) OR lead acetate → black PbS. Cl: add AgNO₃ → white AgCl ppt (soluble in NH₃). Br: add AgNO₃ → pale yellow AgBr ppt (slightly soluble in NH₃). I: add AgNO₃ → yellow AgI ppt (insoluble in NH₃).
The vinegar smell with dilute H₂SO₄ is a strong indicator. Confirmatory test: Add neutral FeCl₃ solution to acetate salt solution → blood red/reddish brown coloration (ferric acetate complex [Fe(CH₃COO)₃]). On boiling: red colour deepens and brown precipitate forms (basic ferric acetate). This is the standard confirmatory test for acetate in qualitative analysis. Acetate is also confirmed by the ester test: add alcohol + conc. H₂SO₄ and warm → fruity smell (ester formation).