Chemistry Inorganic - How to Solve: Qualitative Analysis (Group Cation Tests, Anion Tests, Borax Bead, Flame Tests) – NEET UG Guide

How to Solve: Qualitative Analysis (Group Cation Tests, Anion Tests, Borax Bead, Flame Tests) – NEET UG Guide

Introduction Mastering qualitative analysis unlocks 5-7 direct marks in NEET Chemistry—enough to push you from a 150 to a 160+ score. These tests identify unknown ions in salts, a skill used in forensic labs, water testing, and drug analysis.

WHAT YOU NEED TO KNOW FIRST

1. Basic solubility rules (e.g., chlorides of Ag⁺, Pb²⁺, Hg₂²⁺ are insoluble).
2. Common ion effect (why adding HCl precipitates Group I cations).
3. Flame test principles (excited electrons emit characteristic colors).

KEY TERMS & FORMULAS

Group Cation Tests (Systematic Analysis)

MEMORISE THIS: The group separation table—examiners test it directly.

Anion Tests

MEMORISE THIS: AgNO₃ + HNO₃ is the first test for halides (Cl⁻, Br⁻, I⁻).

Borax Bead Test (For Transition Metals)

• Oxidizing flame (hot, blue): Forms metaborates (colored beads).
• Reducing flame (cool, yellow): Reduces metal ions to lower oxidation states.

MEMORISE THIS: Cu²⁺ → Blue-green (oxidizing), Colorless/Red (reducing).

Flame Tests

MEMORISE THIS: Na⁺ masks K⁺—always use cobalt blue glass for K⁺.

STEP-BY-STEP METHOD

Step 1: Preliminary Tests (Dry Tests)

1. Flame test – Heat a small sample on a platinum wire in a non-luminous flame.
2. Note the color (e.g., golden yellow = Na⁺).
3. Borax bead test – Dip a clean platinum wire in borax, heat to form a transparent bead, touch the sample, and reheat.
4. Observe color in oxidizing and reducing flames.
5. Charcoal cavity test – Mix sample with Na₂CO₃, heat in a charcoal cavity.
6. Observe residue (e.g., yellow when hot, white when cold = Zn²⁺).

Step 2: Group Cation Analysis (Wet Tests)

1. Dissolve the salt in distilled water (or dilute HCl if insoluble).
2. Group I (Ag⁺, Pb²⁺, Hg₂²⁺) – Add dilute HCl.
3. White ppt = Group I present.
4. Filter, test filtrate for other groups.
5. Group II (Cu²⁺, Cd²⁺, etc.) – Pass H₂S gas through the acidic filtrate.
6. Colored ppt = Group II present.
7. Group III (Al³⁺, Fe³⁺, Cr³⁺) – Add NH₄OH + NH₄Cl to the filtrate.
8. Gelatinous ppt = Group III present.
9. Group IV (Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺) – Pass H₂S gas through the basic filtrate.
10. Colored ppt = Group IV present.
11. Group V (Ba²⁺, Ca²⁺, Sr²⁺) – Add (NH₄)₂CO₃.
12. White ppt = Group V present.
13. Group VI (Na⁺, K⁺, Mg²⁺) – Perform flame test or individual tests.

Step 3: Anion Analysis

1. Carbonate (CO₃²⁻) – Add dilute HCl.
2. Effervescence (CO₂) = CO₃²⁻ present.
3. Confirm with lime water (turns milky).
4. Sulphate (SO₄²⁻) – Add BaCl₂ + HCl.
5. White ppt (BaSO₄) = SO₄²⁻ present.
6. Halides (Cl⁻, Br⁻, I⁻) – Add AgNO₃ + HNO₃.
7. White ppt (AgCl) = Cl⁻.
8. Pale yellow ppt (AgBr) = Br⁻.
9. Yellow ppt (AgI) = I⁻.
10. Test solubility in NH₄OH (AgCl dissolves, AgBr partially dissolves, AgI does not).
11. Nitrate (NO₃⁻) – Add FeSO₄ + conc. H₂SO₄.
12. Brown ring (Fe(NO)²⁺) = NO₃⁻ present.
13. Phosphate (PO₄³⁻) – Add (NH₄)₂MoO₄ + HNO₃.
14. Yellow ppt = PO₄³⁻ present.

Step 4: Confirmatory Tests

• For each suspected ion, perform specific confirmatory tests (e.g., K₄[Fe(CN)₆] for Cu²⁺ → chocolate brown ppt).
• Eliminate false positives (e.g., Ba²⁺ interferes with SO₄²⁻ test—remove it first with (NH₄)₂CO₃).

WORKED EXAMPLES

Example 1 – Basic: Identify the cation in a white salt that gives a golden yellow flame and forms a white ppt with HCl.

Step 1: Flame test → Golden yellow = Na⁺. Step 2: Add dilute HCl → White ppt = Group I cation (Ag⁺, Pb²⁺, Hg₂²⁺). Step 3: But Na⁺ is in Group VI (no ppt with HCl). Conclusion: The salt is Pb(NO₃)₂ (Pb²⁺ gives white ppt with HCl, but flame test is misleading—Pb²⁺ does not give a golden yellow flame). Correction: The flame test was contaminated with Na⁺ (common in labs). The actual cation is Pb²⁺. What we did and why: - Used flame test first (quick elimination). - Cross-checked with HCl test (Group I). - Identified contamination (Na⁺ interference).

Example 2 – Medium: A salt gives a blue-green borax bead in oxidizing flame and a white ppt with BaCl₂ + HCl. Identify the anion and cation.

Step 1: Borax bead test → Blue-green (oxidizing) = Cu²⁺. Step 2: Anion test → White ppt with BaCl₂ + HCl = SO₄²⁻. Step 3: Confirm Cu²⁺ with K₄[Fe(CN)₆] → chocolate brown ppt. Conclusion: The salt is CuSO₄. What we did and why: - Borax bead test narrowed cation to Cu²⁺. - BaCl₂ test confirmed SO₄²⁻. - Confirmatory test ensured no false positives.

Example 3 – Exam-Style: A salt is insoluble in water but dissolves in dilute HCl. It gives a crimson red flame and a white ppt with (NH₄)₂CO₃. Identify the salt.

Step 1: Insoluble in water but soluble in HCl = Group V cation (Ba²⁺, Ca²⁺, Sr²⁺). Step 2: Crimson red flame = Sr²⁺. Step 3: White ppt with (NH₄)₂CO₃ = CO₃²⁻ or SO₄²⁻. - But SrCO₃ is insoluble in water, so the salt is SrCO₃. Step 4: Confirm CO₃²⁻ with dilute HCl → effervescence (CO₂). Conclusion: The salt is SrCO₃. What we did and why: - Solubility rules eliminated other groups. - Flame test identified Sr²⁺. - Anion test confirmed CO₃²⁻.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up—this is your 5-mark cheat sheet for qualitative analysis:
1. Flame test first—Na⁺ (yellow), K⁺ (lilac), Ca²⁺ (brick red), Sr²⁺ (crimson), Ba²⁺ (green).
2. Group I = HCl → white ppt (Ag⁺, Pb²⁺, Hg₂²⁺).
3. Group II = H₂S (acidic) → colored ppt (Cu²⁺ = black, Cd²⁺ = yellow).
4. Anions: - CO₃²⁻ → HCl → effervescence (CO₂). - SO₄²⁻ → BaCl₂ + HCl → white ppt. - Cl⁻/Br⁻/I⁻ → AgNO₃ + HNO₃ → white/yellow ppt (check NH₄OH solubility).
5. Borax bead: - Cu²⁺ = blue-green (oxidizing), colorless/red (reducing). - Fe³⁺ = yellow-brown (oxidizing), green (reducing).
6. Confirm every ion with a second test—no shortcuts! Now go crush those 5 marks!