Question
How are cations identified systematically using group reagents? What colours do different cations show in flame tests and borax bead tests?
(JEE Main and CBSE 12 — qualitative analysis is a direct scoring topic)
Solution — Step by Step
Cations are separated into six groups using group reagents that selectively precipitate specific ions:
| Group | Reagent | Cations | Precipitate |
|---|---|---|---|
| 0 | Dilute HCl | NH4+ | No precipitate (detected by NaOH + heat) |
| I | Dilute HCl | Pb2+, Ag+, Hg22+ | Chlorides (PbCl2, AgCl) |
| II | H2S in acidic medium | Cu2+, Pb2+, Bi3+, As3+, Sb3+, Sn2+ | Sulphides (CuS black, PbS black) |
| III | NH4Cl + NH4OH | Fe3+, Al3+, Cr3+ | Hydroxides (Fe(OH)3 brown, Al(OH)3 white) |
| IV | H2S in alkaline medium | Co2+, Ni2+, Mn2+, Zn2+ | Sulphides (CoS black, ZnS white) |
| V | (NH4)2CO3 | Ba2+, Sr2+, Ca2+ | Carbonates (BaCO3, SrCO3, CaCO3) |
| VI | No group reagent | Mg2+, Na+, K+ | Identified by individual tests |
| Cation | Flame Colour |
|---|---|
| Na+ | Golden yellow |
| K+ | Violet (lilac) |
| Ca2+ | Brick red |
| Sr2+ | Crimson red |
| Ba2+ | Apple green |
| Cu2+ | Blue-green |
| Li+ | Crimson red |
The borax bead test identifies coloured transition metal ions:
| Cation | Oxidising flame | Reducing flame |
|---|---|---|
| Cu2+ | Green (hot), Blue (cold) | Red (opaque) |
| Co2+ | Blue | Blue |
| Cr3+ | Green | Green |
| Mn2+ | Violet | Colourless |
| Fe3+ | Yellow/brown | Green |
| Ni2+ | Brown | Grey |
graph TD
A["Unknown Salt"] --> B{"Add dil. HCl"}
B -->|Precipitate| C["Group I: Pb, Ag"]
B -->|No ppt| D{"Pass H2S in acid"}
D -->|Precipitate| E["Group II: Cu, Pb, Bi"]
D -->|No ppt| F{"Add NH4OH"}
F -->|Precipitate| G["Group III: Fe, Al, Cr"]
F -->|No ppt| H{"Pass H2S in base"}
H -->|Precipitate| I["Group IV: Co, Ni, Mn, Zn"]
H -->|No ppt| J{"Add NH42CO3"}
J -->|Precipitate| K["Group V: Ba, Sr, Ca"]
J -->|No ppt| L["Group VI: Mg, Na, K"]Why This Works
The systematic approach exploits differences in solubility products. Group I chlorides are insoluble, so they precipitate first with HCl. Group II sulphides are insoluble even in acidic solution (very low Ksp), while Group IV sulphides need alkaline conditions to precipitate (higher Ksp). This gradient of solubility allows sequential separation.
Flame tests work because metal ions absorb energy from the flame and electrons jump to higher energy levels. When they fall back, they emit light of characteristic wavelengths — each metal has a unique emission spectrum.
Alternative Method
For JEE Main, memorise flame colours with this association: Na = yellow (sodium street lamps), K = violet (potassium permanganate is purple), Ba = green (barium in fireworks), Cu = green-blue (copper patina on old roofs), Ca = brick red (think of red bricks).
Common Mistake
The biggest confusion: K+ flame colour appears violet, but Na+ contamination masks it with intense yellow. In the lab, you must view the K+ flame through a cobalt blue glass, which filters out the yellow Na emission and lets the violet K+ colour pass through. If a question mentions “cobalt blue glass,” it is testing K+ identification.
Also, Pb2+ appears in both Group I and Group II — it can precipitate as PbCl2 (Group I) or PbS (Group II). If Group I analysis is incomplete, lead may show up in Group II.