Chemistry Inorganic - How to Solve: Qualitative Analysis (Group I–V Cation Tests, Anion Tests, Borax Bead, Flame Tests) – IIT JEE Guide

How to Solve: Qualitative Analysis (Group I–V Cation Tests, Anion Tests, Borax Bead, Flame Tests) – IIT JEE Guide

Introduction

Mastering qualitative analysis unlocks 5–10 marks in IIT JEE (Main + Advanced) inorganic chemistry—enough to push you into the top 10%. It’s also the key to real-world forensic labs, water testing, and drug analysis—where identifying unknown ions can save lives.

WHAT YOU NEED TO KNOW FIRST

1. Solubility rules – Which salts dissolve in water vs. precipitate.
2. Common ion effect – How adding ions shifts equilibrium (e.g., HCl suppresses AgCl solubility).
3. Redox reactions – Oxidation states of transition metals (e.g., Fe²⁺ vs. Fe³⁺).

KEY TERMS & FORMULAS

1. Group I Cations (Ag⁺, Pb²⁺, Hg₂²⁺)

• Precipitating agent: Dilute HCl
• Key reactions:
• Ag⁺ + Cl⁻ → AgCl (white ppt, soluble in NH₃)
• Pb²⁺ + 2Cl⁻ → PbCl₂ (white ppt, soluble in hot water)
• Hg₂²⁺ + 2Cl⁻ → Hg₂Cl₂ (white ppt, turns black with NH₃)
• MEMORISE THIS: Group I cations precipitate as chlorides in cold dilute HCl.

2. Group II Cations (Cu²⁺, Bi³⁺, Cd²⁺, As³⁺, Sb³⁺, Sn²⁺)

• Precipitating agent: H₂S in acidic medium (dil. HCl)
• Key reactions:
• Cu²⁺ + H₂S → CuS (black ppt, insoluble in yellow ammonium sulfide)
• Bi³⁺ + 3H₂S → Bi₂S₃ (brown ppt)
• As³⁺ + 3H₂S → As₂S₃ (yellow ppt, soluble in NaOH)
• MEMORISE THIS: Group II sulfides precipitate in acidic medium (unlike Group IV, which needs alkaline medium).

3. Group III Cations (Al³⁺, Fe³⁺, Cr³⁺)

• Precipitating agent: NH₄OH in presence of NH₄Cl (to prevent Group IV precipitation)
• Key reactions:
• Al³⁺ + 3NH₄OH → Al(OH)₃ (white gelatinous ppt, soluble in excess NaOH)
• Fe³⁺ + 3NH₄OH → Fe(OH)₃ (reddish-brown ppt, insoluble in excess NH₄OH)
• Cr³⁺ + 3NH₄OH → Cr(OH)₃ (green ppt, soluble in excess NaOH)
• MEMORISE THIS: NH₄Cl is added to suppress OH⁻ concentration (common ion effect).

4. Group IV Cations (Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺)

• Precipitating agent: H₂S in alkaline medium (NH₄OH)
• Key reactions:
• Co²⁺ + H₂S → CoS (black ppt, insoluble in HCl)
• Ni²⁺ + H₂S → NiS (black ppt, insoluble in HCl)
• Zn²⁺ + H₂S → ZnS (white ppt, soluble in HCl)
• MEMORISE THIS: Group IV sulfides precipitate only in alkaline medium.

5. Group V Cations (Ba²⁺, Ca²⁺, Sr²⁺)

• Precipitating agent: (NH₄)₂CO₃ in presence of NH₄Cl
• Key reactions:
• Ba²⁺ + CO₃²⁻ → BaCO₃ (white ppt, soluble in acetic acid)
• Ca²⁺ + CO₃²⁻ → CaCO₃ (white ppt, soluble in acetic acid)
• MEMORISE THIS: NH₄Cl prevents Mg²⁺ from precipitating (common ion effect).

6. Anion Tests (CO₃²⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, PO₄³⁻)

7. Borax Bead Test (For Transition Metals)

• Principle: Borax (Na₂B₄O₇·10H₂O) forms metaborate beads when heated.
• Colors in oxidizing flame (hot & cold):
• Cu²⁺: Blue (hot), Green (cold)
• Co²⁺: Blue (hot & cold)
• Fe³⁺: Yellow (hot), Colorless (cold)
• Cr³⁺: Green (hot & cold)
• MEMORISE THIS: Hot bead = oxidizing flame, Cold bead = reducing flame.

8. Flame Tests (For Alkali & Alkaline Earth Metals)

STEP-BY-STEP METHOD

Step 1: Preliminary Tests (Dry Tests)

1. Flame test – Dip a clean Pt wire in conc. HCl, then in sample, and heat in flame.
2. Note color (use cobalt glass for K⁺).
3. Borax bead test – Heat borax on a loop, dip in sample, and observe color in oxidizing/reducing flame.
4. Charcoal cavity test – Mix sample with Na₂CO₃, heat in charcoal cavity, and observe residue.

Step 2: Group Separation (Wet Tests)

1. Dissolve sample in water (if insoluble, use dil. HCl).
2. Group I (Ag⁺, Pb²⁺, Hg₂²⁺):
3. Add dil. HCl → white ppt.
4. Test ppt with NH₃ (AgCl dissolves, PbCl₂ dissolves in hot water, Hg₂Cl₂ turns black).
5. Group II (Cu²⁺, Bi³⁺, etc.):
6. Pass H₂S gas in acidic medium → colored sulfides.
7. Test solubility in yellow ammonium sulfide (As₂S₃, Sb₂S₃ dissolve).
8. Group III (Al³⁺, Fe³⁺, Cr³⁺):
9. Add NH₄OH + NH₄Cl → gelatinous ppt.
10. Test solubility in excess NaOH (Al(OH)₃ dissolves, Fe(OH)₃ does not).
11. Group IV (Co²⁺, Ni²⁺, Zn²⁺):
12. Pass H₂S in alkaline medium (NH₄OH) → sulfides.
13. Test solubility in HCl (ZnS dissolves, CoS/NiS do not).
14. Group V (Ba²⁺, Ca²⁺):
15. Add (NH₄)₂CO₃ + NH₄Cl → white ppt.
16. Test solubility in acetic acid (BaCO₃/CaCO₃ dissolve).

Step 3: Anion Tests

1. CO₃²⁻: Add dil. HCl → effervescence (CO₂).
2. SO₄²⁻: Add BaCl₂ + HCl → white ppt (BaSO₄).
3. Cl⁻/Br⁻/I⁻: Add AgNO₃ + HNO₃ → ppt (white/yellow), test solubility in NH₃.
4. NO₃⁻: Brown ring test (FeSO₄ + conc. H₂SO₄).
5. PO₄³⁻: Add (NH₄)₂MoO₄ + HNO₃ → yellow ppt.

Step 4: Confirmatory Tests

• For cations: Use specific reagents (e.g., K₄[Fe(CN)₆] for Cu²⁺ → chocolate brown ppt).
• For anions: Use spot tests (e.g., starch test for I⁻ → blue color).

WORKED EXAMPLES

Example 1 – Basic (Identify Cation in Group II)

Problem: A salt gives a black ppt with H₂S in acidic medium, insoluble in yellow ammonium sulfide. Identify the cation. Solution:
1. Group II test: H₂S in acidic medium → black ppt → Cu²⁺, Bi³⁺, or Hg²⁺.
2. Insoluble in yellow ammonium sulfide → Cu²⁺ or Bi³⁺ (HgS dissolves).
3. Confirmatory test: Add K₄[Fe(CN)₆] → chocolate brown ppt → Cu²⁺. What we did and why: We used group separation logic and confirmatory tests to narrow down Cu²⁺.

Example 2 – Medium (Mixed Cations)

Problem: A mixture contains Ag⁺ and Zn²⁺. How will you separate them? Solution:
1. Add dil. HCl → AgCl (white ppt), Zn²⁺ remains in solution.
2. Filter → AgCl ppt separated.
3. To filtrate (Zn²⁺), add NH₄OH + H₂S → ZnS (white ppt). What we did and why: We used group reagents sequentially to separate ions based on solubility.

Example 3 – Exam-Style (Disguised Problem)

Problem: A salt gives a crimson red flame and a white ppt with (NH₄)₂CO₃ soluble in acetic acid. It also gives a yellow ppt with AgNO₃ insoluble in NH₃. Identify the salt. Solution:
1. Flame test: Crimson red → Li⁺ or Sr²⁺.
2. Group V test: White ppt with (NH₄)₂CO₃ → Ba²⁺, Ca²⁺, or Sr²⁺. - Soluble in acetic acid → Sr²⁺ or Ca²⁺ (BaCO₃ is less soluble).
3. Anion test: Yellow ppt with AgNO₃ insoluble in NH₃ → I⁻.
4. Conclusion: Salt is SrI₂. What we did and why: We combined flame test, group separation, and anion tests to deduce the salt.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up—this is your 5-mark cheat sheet for qualitative analysis:
1. Group I: Dil. HCl → white ppt (Ag⁺, Pb²⁺, Hg₂²⁺). Test with NH₃.
2. Group II: H₂S in acidic medium → colored sulfides (CuS black, As₂S₃ yellow).
3. Group III: NH₄OH + NH₄Cl → gelatinous ppt (Al³⁺, Fe³⁺, Cr³⁺). Al(OH)₃ dissolves in NaOH.
4. Group IV: H₂S in alkaline medium → sulfides (ZnS white, CoS/NiS black).
5. Group V: (NH₄)₂CO₃ + NH₄Cl → white ppt (Ba²⁺, Ca²⁺).
6. Anions: CO₃²⁻ effervesces, SO₄²⁻ gives BaSO₄, Cl⁻/Br⁻/I⁻ give AgX ppt (test solubility in NH₃).
7. Flame tests: Na⁺ = golden yellow, K⁺ = lilac (use cobalt glass), Ca²⁺ = brick red.
8. Borax bead: Cu²⁺ = blue (hot), green (cold); Co²⁺ = blue always. Memorise the group reagents, colors, and confirmatory tests—and you’ll ace the 10-mark question."