Class 12 Chemistry Notes Chapter 7 (Systematic Qualitative Analysis) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 7: Systematic Qualitative Analysis from your Lab Manual. This is a cornerstone of practical chemistry and frequently tested in various examinations, including government exams, because it assesses your understanding of ionic equilibrium, solubility, and reaction chemistry.

Systematic Qualitative Analysis: Detailed Notes

1. Introduction & Objective:

• Qualitative Analysis: The process of identifying the constituent chemical species (ions) present in a given unknown inorganic salt or mixture. It answers "What is present?".
• Systematic Approach: It involves a step-by-step procedure, performing tests in a specific sequence to avoid interference between ions and ensure reliable identification.
• Scope (Class 12): Primarily focuses on identifying one common cation (basic radical) and one common anion (acid radical) in a given simple inorganic salt.

2. Basic Principles:

• Solubility Product (Ksp): The equilibrium constant for the dissolution of a sparingly soluble salt. Precipitation occurs when the Ionic Product (IP) of the ions exceeds the Solubility Product (Ksp).
  • IP > Ksp: Precipitation occurs.
  • IP < Ksp: Solution is unsaturated, no precipitation.
  • IP = Ksp: Solution is saturated, equilibrium exists.
• Common Ion Effect: The suppression of the dissociation of a weak electrolyte (or solubility of a sparingly soluble salt) by the addition of a strong electrolyte containing a common ion. This principle is crucial for selective precipitation in cation analysis (e.g., using HCl for Group I, H₂S in presence of HCl for Group II, NH₄OH in presence of NH₄Cl for Group III).

3. Steps in Systematic Qualitative Analysis:

Step I: Preliminary Examination
These tests provide preliminary clues but are not conclusive.

• (a) Physical State: Crystalline or amorphous.
• (b) Colour: Provides hints about certain cations (e.g., Cu²⁺ - blue/green, Fe³⁺ - brown/yellow, Ni²⁺ - green, Co²⁺ - pink/blue, Mn²⁺ - light pink/buff).
• (c) Solubility: Test solubility in cold water, hot water, dilute HCl. This helps in preparing the Original Solution (OS).
• (d) Dry Heating Test: Heat a small amount of salt in a dry test tube. Observe changes like colour change, gas evolution, sublimation, decrepitation (crackling sound).
  • Examples: Ammonium salts sublime; hydrates release water vapour; carbonates release CO₂ (turns lime water milky); nitrates release NO₂ (brown gas); acetates give acetone smell.
• (e) Flame Test: Make a paste of the salt with conc. HCl on a clean platinum wire loop and introduce it to a non-luminous Bunsen flame. Observe the characteristic colour imparted to the flame.
  • Examples: Ca²⁺ - Brick Red, Sr²⁺ - Crimson Red, Ba²⁺ - Apple Green, Cu²⁺ - Greenish-blue, Na⁺ - Golden Yellow, K⁺ - Lilac (viewed through blue glass).
• (f) Borax Bead Test (for coloured salts): Heat borax on a platinum loop to get a transparent bead (Na₂B₄O₇). Touch the hot bead with a tiny amount of salt and heat again in oxidizing and reducing flames. Observe the colour of the bead.
  • Examples (Oxidizing Flame): Cu²⁺ - Blue, Fe³⁺ - Yellow/Brownish-yellow, Co²⁺ - Deep Blue, Ni²⁺ - Brown, Mn²⁺ - Violet/Amethyst, Cr³⁺ - Green.
• (g) Charcoal Cavity Test (less common now, primarily for metal confirmation): Mix salt with sodium carbonate (Na₂CO₃) and heat strongly in a charcoal cavity using a blowpipe (reducing flame). Observe any metallic bead or coloured incrustation.

Step II: Analysis of Anions (Acid Radicals)
Performed sequentially using dilute and concentrated acids, followed by specific tests.

• (A) Dilute H₂SO₄ / Dilute HCl Test: Add dilute H₂SO₄ to a small amount of salt. Observe any reaction (gas evolution) in the cold or on gentle warming.

  • CO₃²⁻ (Carbonate): Brisk effervescence, CO₂ gas evolved (colourless, odourless), turns lime water [Ca(OH)₂] milky. Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ (milky) + H₂O(l)
  • S²⁻ (Sulphide): H₂S gas evolved (colourless, rotten egg smell), turns lead acetate paper black. (CH₃COO)₂Pb(aq) + H₂S(g) → PbS(s)↓ (black) + 2CH₃COOH(aq)
  • SO₃²⁻ (Sulphite): SO₂ gas evolved (colourless, pungent smell of burning sulphur), turns acidified K₂Cr₂O₇ paper green. K₂Cr₂O₇ + H₂SO₄ + 3SO₂ → K₂SO₄ + Cr₂(SO₄)₃(green) + H₂O
  • NO₂⁻ (Nitrite): Brown fumes of NO₂ evolved (pungent smell). 2NO₂⁻ + 2H⁺ → 2HNO₂ → H₂O + NO + NO₂↑ (brown fumes may intensify on adding copper turnings)
  • CH₃COO⁻ (Acetate): Vinegar-like smell (acetic acid) on warming. Confirmatory: Neutral FeCl₃ test gives blood-red colouration. Ester test (warm with ethanol and conc. H₂SO₄) gives fruity smell.

• (B) Concentrated H₂SO₄ Test: If no reaction with dilute acid, take fresh salt, add conc. H₂SO₄ and warm gently.

  • Cl⁻ (Chloride): HCl gas evolved (colourless, pungent), gives dense white fumes with a glass rod dipped in NH₄OH. NH₃(g) + HCl(g) → NH₄Cl(s)↓ (white fumes). Confirmatory: Chromyl chloride test (heat salt with K₂Cr₂O₇ and conc. H₂SO₄, pass orange-red vapours of CrO₂Cl₂ into NaOH solution, then acidify with acetic acid and add lead acetate - yellow ppt of PbCrO₄ confirms Cl⁻). AgNO₃ test (OS + dil HNO₃ + AgNO₃ → white ppt of AgCl, soluble in NH₄OH).
  • Br⁻ (Bromide): Reddish-brown fumes of Br₂ evolved (pungent). Confirmatory: AgNO₃ test (OS + dil HNO₃ + AgNO₃ → pale yellow ppt of AgBr, sparingly soluble in NH₄OH). Layer test (OS + dil HCl + CCl₄/CS₂ + chlorine water → shake → organic layer turns orange/brown).
  • I⁻ (Iodide): Violet vapours of I₂ evolved (pungent), turns starch paper blue. Confirmatory: AgNO₃ test (OS + dil HNO₃ + AgNO₃ → yellow ppt of AgI, insoluble in NH₄OH). Layer test (OS + dil HCl + CCl₄/CS₂ + chlorine water → shake → organic layer turns violet).
  • NO₃⁻ (Nitrate): Light brown fumes of NO₂ (intensify on adding copper turnings). Confirmatory: Brown Ring Test (OS + freshly prepared FeSO₄ solution + add conc. H₂SO₄ carefully along the sides of the test tube → a brown ring of [Fe(H₂O)₅NO]SO₄ forms at the junction of the two layers).
  • C₂O₄²⁻ (Oxalate): Colourless gas mixture (CO + CO₂) evolved. CO burns with a blue flame, CO₂ turns lime water milky. Confirmatory: Warm OS with dil H₂SO₄ and add KMnO₄ solution → pink colour is discharged. CaCl₂ test (Neutral OS + CaCl₂ → white ppt of CaC₂O₄, insoluble in acetic acid but soluble in dil mineral acids).

• (C) Independent Group / Specific Tests (for anions not detected above):

  • SO₄²⁻ (Sulphate): Add dilute HCl to OS (to remove interfering carbonate/sulphite), then add BaCl₂ solution → thick white precipitate of BaSO₄, insoluble in conc. HCl or conc. HNO₃.
  • PO₄³⁻ (Phosphate): Acidify OS with conc. HNO₃, boil, cool, then add ammonium molybdate solution [(NH₄)₂MoO₄] and warm gently → canary yellow precipitate of ammonium phosphomolybdate confirms PO₄³⁻.

Step III: Preparation of Original Solution (OS)

• Dissolve the salt in a suitable solvent in the following order of preference:
  1. Cold distilled water
  2. Hot distilled water
  3. Dilute HCl
  4. Concentrated HCl (Boil off excess HCl before proceeding to Group II)
• The clear solution obtained is called the Original Solution (OS) and is used for cation analysis (except Group 0).

Step IV: Analysis of Cations (Basic Radicals)
Performed systematically by adding group reagents in a specific order to precipitate cations group by group.

• Group 0:

  • Test: Warm a small amount of the original salt (or OS if prepared in water) with NaOH solution.
  • Observation: Smell of ammonia (NH₃ gas evolved), turns moist red litmus paper blue, gives white fumes with a glass rod dipped in conc. HCl.
  • Cation: NH₄⁺ (Ammonium)
  • Confirmatory: Pass the gas through Nessler's reagent (K₂[HgI₄] in KOH) → Brown precipitate or colouration confirms NH₄⁺.

• Group I:

  • Group Reagent: Dilute HCl
  • Procedure: Add dil. HCl to the OS.
  • Observation: White precipitate.
  • Cation Precipitated: Pb²⁺ (as PbCl₂)
  • Confirmatory (for Pb²⁺): Ppt dissolves in hot water. Divide hot solution:
    • Add KI solution → Yellow ppt of PbI₂ (reappears on cooling).
    • Add K₂CrO₄ solution → Yellow ppt of PbCrO₄.

• Group II:

  • Group Reagent: H₂S gas in presence of dilute HCl (acidic medium, low S²⁻ concentration).
  • Procedure: If no ppt in Group I, or after filtering Group I ppt, pass H₂S gas through the acidic filtrate.
  • Observation: Black or Yellow precipitate.
  • Cations Precipitated: Pb²⁺ (if not completely removed in Gp I - Black ppt, PbS), Cu²⁺ (Black ppt, CuS), As³⁺ (Yellow ppt, As₂S₃ - less common in syllabus).
  • Confirmatory (for Cu²⁺): Dissolve black ppt in dil. HNO₃, boil off H₂S, add excess NH₄OH → Deep blue solution ([Cu(NH₃)₄]²⁺). Acidify with acetic acid and add K₄[Fe(CN)₆] → Chocolate brown ppt.

• Group III:

  • Group Reagent: NH₄OH in presence of NH₄Cl (alkaline medium, common ion effect suppresses OH⁻ concentration to prevent precipitation of higher group hydroxides).
  • Procedure: Take the filtrate from Group II. Boil off H₂S completely (test with lead acetate paper). Add solid NH₄Cl, warm, then add NH₄OH solution in excess.
  • Observation: Gelatinous White or Reddish-Brown precipitate.
  • Cations Precipitated: Al³⁺ (Gelatinous white ppt, Al(OH)₃), Fe³⁺ (Reddish-brown ppt, Fe(OH)₃), Cr³⁺ (Greenish ppt, Cr(OH)₃ - less common).
  • Confirmatory (for Fe³⁺): Dissolve ppt in dil. HCl. Add K₄[Fe(CN)₆] (Potassium ferrocyanide) solution → Prussian blue colour/precipitate. Add KCNS (Potassium thiocyanate) solution → Blood red colouration.
  • Confirmatory (for Al³⁺): Dissolve ppt in NaOH (forms soluble sodium aluminate). Acidify with dil HCl, add NH₄OH → White ppt reappears. Lake test: Suspend ppt in water, add blue litmus solution and NH₄OH dropwise → Blue floating mass (lake).

• Group IV:

  • Group Reagent: H₂S gas in presence of NH₄OH and NH₄Cl (alkaline medium, high S²⁻ concentration).
  • Procedure: Pass H₂S gas through the filtrate from Group III.
  • Observation: White, Black, or Buff/Pink precipitate.
  • Cations Precipitated: Zn²⁺ (White ppt, ZnS), Ni²⁺ (Black ppt, NiS), Co²⁺ (Black ppt, CoS), Mn²⁺ (Buff/Pink ppt, MnS).
  • Confirmatory (for Zn²⁺): Dissolve ppt in dil HCl. Add NaOH solution → White ppt, soluble in excess NaOH. Pass H₂S through alkaline solution → White ppt reappears.
  • Confirmatory (for Ni²⁺): Dissolve ppt in aqua regia. Evaporate to dryness, dissolve residue in water. Add NH₄OH (excess) and Dimethylglyoxime (DMG) solution → Bright red precipitate.
  • Confirmatory (for Co²⁺): Dissolve ppt as for Ni²⁺. Acidify with acetic acid, add KNO₂ solution and warm → Yellow precipitate.
  • Confirmatory (for Mn²⁺): Dissolve ppt in dil HCl. Add NaOH → White ppt, turns brown/black on exposure to air (oxidation). Fuse ppt with Na₂CO₃ and KNO₃ (oxidizing mixture) → Green mass (Na₂MnO₄).

• Group V:

  • Group Reagent: (NH₄)₂CO₃ (Ammonium carbonate) in presence of NH₄OH and NH₄Cl (alkaline medium).
  • Procedure: Take the filtrate from Group IV. Boil off H₂S. Add NH₄OH and then (NH₄)₂CO₃ solution. Warm.
  • Observation: White precipitate.
  • Cations Precipitated: Ba²⁺ (as BaCO₃), Sr²⁺ (as SrCO₃), Ca²⁺ (as CaCO₃).
  • Separation & Confirmation: Dissolve ppt in dilute acetic acid.
    • Add K₂CrO₄ solution → Yellow ppt confirms Ba²⁺ (BaCrO₄). Filter.
    • To filtrate, add (NH₄)₂SO₄ solution and boil → White ppt confirms Sr²⁺ (SrSO₄). Filter.
    • To filtrate, add ammonium oxalate [(NH₄)₂C₂O₄] solution → White ppt confirms Ca²⁺ (CaC₂O₄).
  • Flame Tests: Can be performed on the individual precipitates (dissolved in conc. HCl) for confirmation: Ba²⁺ (Apple Green), Sr²⁺ (Crimson Red), Ca²⁺ (Brick Red).

• Group VI:

  • Group Reagent: No specific group reagent. Tested in the filtrate from Group V.
  • Procedure: Take the filtrate from Group V. Acidify with acetic acid.
  • Cation: Mg²⁺
  • Confirmatory (for Mg²⁺): Add NH₄OH and then disodium hydrogen phosphate (Na₂HPO₄) or ammonium phosphate solution → White crystalline precipitate of Magnesium ammonium phosphate (Mg(NH₄)PO₄).

4. Important Reagents & Their Roles:

• Dilute HCl: Group I reagent (precipitates Gp I chlorides), provides acidic medium for Gp II.
• H₂S: Group II & IV reagent (precipitates Gp II & IV sulphides).
• NH₄Cl: Added before Gp III reagent (NH₄OH) to suppress OH⁻ concentration via common ion effect, preventing precipitation of Gp IV, V, VI hydroxides. Also used before Gp V reagent.
• NH₄OH: Group III reagent (precipitates Gp III hydroxides), provides alkaline medium for Gp IV & V.
• (NH₄)₂CO₃: Group V reagent (precipitates Gp V carbonates).
• Dilute H₂SO₄: Anion test (Gp A), acidification.
• Conc. H₂SO₄: Anion test (Gp B), dehydrating agent.
• AgNO₃: Confirmatory test for Cl⁻, Br⁻, I⁻.
• BaCl₂: Confirmatory test for SO₄²⁻.
• Nessler's Reagent (K₂[HgI₄] + KOH): Confirmatory test for NH₄⁺.
• K₄[Fe(CN)₆]: Confirmatory test for Fe³⁺ (Prussian blue), Cu²⁺ (Chocolate brown).
• KCNS: Confirmatory test for Fe³⁺ (Blood red).
• Dimethylglyoxime (DMG): Confirmatory test for Ni²⁺ (Red ppt).
• Ammonium Molybdate: Confirmatory test for PO₄³⁻ (Canary yellow ppt).
• K₂CrO₄: Confirmatory test for Pb²⁺ (Yellow ppt), Ba²⁺ (Yellow ppt).

5. Precautions:

• Use distilled water throughout the analysis.
• Use clean glassware.
• Perform tests in the correct sequence.
• Ensure complete precipitation/removal of a group before proceeding to the next.
• Boil off H₂S completely before adding reagents for Group III and Group V.
• Handle concentrated acids and corrosive reagents with care.
• Dispose of chemical waste properly.

Multiple Choice Questions (MCQs)

1. The group reagent for precipitating Group II cations (like Cu²⁺, Pb²⁺) in qualitative analysis is:
   a) Dilute HCl
   b) H₂S gas in acidic medium (dil. HCl)
   c) NH₄OH in presence of NH₄Cl
   d) H₂S gas in alkaline medium (NH₄OH + NH₄Cl)

2. Addition of NH₄Cl before adding NH₄OH in Group III precipitation is done to:
   a) Increase the concentration of OH⁻ ions
   b) Decrease the concentration of OH⁻ ions by common ion effect
   c) Increase the solubility product of Group III hydroxides
   d) Precipitate Group II cations

3. A salt gives brisk effervescence with dilute H₂SO₄, and the gas evolved turns lime water milky. The anion present is:
   a) SO₃²⁻
   b) S²⁻
   c) CO₃²⁻
   d) NO₂⁻

4. The confirmatory test for NH₄⁺ ion involves the use of:
   a) Dimethylglyoxime (DMG)
   b) Nessler's reagent
   c) Potassium ferrocyanide
   d) Barium chloride

5. A reddish-brown precipitate obtained in Group III of cation analysis indicates the presence of:
   a) Al³⁺
   b) Zn²⁺
   c) Fe³⁺
   d) Cr³⁺

6. The 'Brown Ring Test' is used to confirm the presence of which anion?
   a) Cl⁻
   b) SO₄²⁻
   c) NO₃⁻
   d) Br⁻

7. Which cation gives an apple-green colour in the flame test?
   a) Ca²⁺
   b) Sr²⁺
   c) Ba²⁺
   d) Cu²⁺

8. A white precipitate is formed upon adding BaCl₂ solution to an acidified (with dil. HCl) original solution. This confirms the presence of:
   a) CO₃²⁻
   b) SO₃²⁻
   c) Cl⁻
   d) SO₄²⁻

9. Dimethylglyoxime (DMG) reagent gives a characteristic bright red precipitate with which cation in an ammoniacal medium?
   a) Co²⁺
   b) Ni²⁺
   c) Fe³⁺
   d) Mn²⁺

10. In the systematic analysis of cations, Mg²⁺ is precipitated in:
    a) Group IV using H₂S in alkaline medium
    b) Group V using (NH₄)₂CO₃
    c) Group VI using Na₂HPO₄ in ammoniacal medium
    d) Group III using NH₄OH + NH₄Cl

Answer Key for MCQs:

1. b
2. b
3. c
4. b
5. c
6. c
7. c
8. d
9. b
10. c

Remember to thoroughly understand the principles behind each test, the specific reagents used, and the expected observations. Good luck with your preparation!