Qualitative analysis is the systematic process of determining which ions are present in an unknown salt sample. Unlike quantitative analysis (which asks “how much?”), qualitative analysis asks “what is it?” — and it does so through a series of characteristic chemical tests.
Mastering this topic requires understanding both the tests and the underlying chemistry. There are no tricks here — just pattern recognition built on understanding why each reaction happens.
Key Terms & Definitions
Qualitative analysis: Identification of the components of a sample without measuring their amounts.
Preliminary tests: Quick tests (colour, flame test, smell, action with dilute acids) done before wet tests to narrow down possibilities.
Systematic analysis: Organised procedure separating ions into groups (Group I through Group VI in the classical scheme, or Cation Groups 0–V in some textbooks) based on their reactions with common reagents.
Cation: Positively charged ion (e.g., Na⁺, Ca²⁺, Fe³⁺, NH₄⁺).
Anion: Negatively charged ion (e.g., Cl⁻, SO₄²⁻, CO₃²⁻, NO₃⁻).
Characteristic colour: The colour of the solution or precipitate that identifies a specific ion.
Group reagent: A reagent that precipitates an entire group of cations.
Confirmatory test: A specific test that unambiguously identifies one ion.
Preliminary Tests
Before systematic wet analysis, preliminary observations narrow down the field:
Colour of the Salt
| Colour | Likely Cation |
|---|---|
| White/colourless | Na⁺, K⁺, Ca²⁺, Ba²⁺, Al³⁺, Zn²⁺, Mg²⁺ |
| Blue | Cu²⁺ (copper sulphate, copper chloride) |
| Green | Fe²⁺ (ferrous), Ni²⁺, Cr³⁺ |
| Yellow-brown | Fe³⁺ (ferric) |
| Pink/rose | Co²⁺, Mn²⁺ |
| Black | CuO, MnO₂, FeS |
Flame Test — For Cation Identification
Clean a platinum wire with HCl, dip in the salt solution, and hold in a flame. Observe the colour:
| Flame Colour | Cation | Notes |
|---|---|---|
| Golden yellow | Na⁺ (sodium) | Very intense; even trace amounts |
| Violet/lilac | K⁺ (potassium) | View through blue cobalt glass to mask Na |
| Brick red | Ca²⁺ (calcium) | Persistent |
| Crimson red | Sr²⁺ (strontium) | |
| Apple/bluish green | Ba²⁺ (barium) | |
| Blue/green | Cu²⁺ (copper) | Often with white edges |
Action with Dilute HCl / H₂SO₄
| Observation | Anion Present |
|---|---|
| Colourless gas, no smell (CO₂), turns lime water milky | CO₃²⁻ or HCO₃⁻ |
| Colourless, pungent gas (HCl), fumes with NH₃ | Cl⁻ (when acid is conc. H₂SO₄) |
| Yellow gas (Cl₂), bleaches litmus | Cl⁻ (with MnO₂ + HCl) |
| Rotten egg smell (H₂S), turns lead acetate paper black | S²⁻ |
| Brown fumes (NO₂), reddish-brown | NO₃⁻ (brown ring test) |
| Pungent smell (SO₂), turns acidified KMnO₄ colourless | SO₃²⁻ |
Anion (Acid Radical) Identification
Group Tests for Anions
Dilute H₂SO₄ group (CO₃²⁻, S²⁻, SO₃²⁻, NO₂⁻, CH₃COO⁻): All these anions are detected by action with dilute H₂SO₄.
Concentrated H₂SO₄ group (Cl⁻, Br⁻, I⁻, NO₃⁻, C₂O₄²⁻): Detected by reaction with concentrated H₂SO₄, which displaces the volatile acid.
Insoluble group (SO₄²⁻, PO₄³⁻, BO₃³⁻, AsO₃³⁻): Not liberated by acids; detected through precipitation reactions.
Key Confirmatory Tests for Anions
Chloride (Cl⁻):
- Add AgNO₃ solution → white precipitate of AgCl ↓
- White precipitate is soluble in NH₄OH (dilute ammonia) — this distinguishes Cl⁻ from Br⁻ (pale yellow AgBr, slightly soluble in excess NH₄OH) and I⁻ (yellow AgI, insoluble in NH₄OH)
Sulphate (SO₄²⁻):
- Add BaCl₂ solution in dilute HCl → white precipitate of BaSO₄ ↓ (insoluble in HCl)
Carbonate (CO₃²⁻):
- Add dilute HCl → brisk effervescence of colourless CO₂ gas
- Pass gas through Ca(OH)₂ (lime water) → milky white precipitate (CaCO₃)
- Excess CO₂ clears the milkiness (CaCO₃ → Ca(HCO₃)₂, soluble)
Nitrate (NO₃⁻) — Brown Ring Test:
- Dissolve in water, add freshly prepared FeSO₄ solution
- Carefully pour concentrated H₂SO₄ down the side of the tube
- A brown ring at the junction confirms NO₃⁻
The brown ring is [Fe(NO)(H₂O)₅]²⁺ — an iron nitrosyl complex.
Sulphide (S²⁻):
- Add dilute HCl → rotten egg smell (H₂S gas)
- Lead acetate paper turns black (PbS forms)
Phosphate (PO₄³⁻):
- Add excess ammonium molybdate ((NH₄)₂MoO₄) in nitric acid, heat → canary yellow precipitate (ammonium phosphomolybdate)
Cation (Basic Radical) Identification — Group System
The classical systematic analysis uses precipitation reactions to separate cations into 6 groups. Each group is precipitated by a specific group reagent.
Group 0 — NH₄⁺ (Ammonium)
Test before adding other reagents (some procedures add NaOH, which would remove NH₄⁺).
Test: Add excess NaOH, heat → pungent smell (NH₃ gas). Moist red litmus turns blue.
Group I — Pb²⁺, Hg₂²⁺ (Mercurous), Ag⁺
Group reagent: Dilute HCl
These ions form insoluble chlorides:
- AgCl (white) — soluble in NH₄OH
- PbCl₂ (white) — soluble in hot water
- Hg₂Cl₂ (white, calomel) — turns grey/black with NH₄OH (disproportionation)
Group II — Pb²⁺, Hg²⁺, Cu²⁺, Bi³⁺, Cd²⁺, As³⁺/⁵⁺, Sb³⁺/⁵⁺, Sn²⁺/⁴⁺
Group reagent: H₂S in dilute HCl (acidic medium)
These sulphides are insoluble in dilute HCl:
- CuS (black), HgS (black), PbS (black), Bi₂S₃ (brown)
- As₂S₃ (yellow), Sb₂S₃ (orange)
Confirmatory test for Cu²⁺: Add excess NH₃ → deep blue solution (tetraamminecopper complex [Cu(NH₃)₄]²⁺).
Group III — Fe³⁺, Al³⁺, Cr³⁺
Group reagent: NH₄OH + NH₄Cl (ammonium buffer)
These cations precipitate as hydroxides in mildly alkaline solution (Group II sulphides don’t precipitate here because concentration of S²⁻ is too low):
- Fe(OH)₃ (reddish-brown)
- Al(OH)₃ (white, gelatinous) — soluble in excess NaOH (amphoteric)
- Cr(OH)₃ (green) — soluble in excess NaOH (amphoteric)
Confirmatory test for Fe³⁺:
- KSCN (potassium thiocyanate) → blood red colour [Fe(SCN)₃]
- K₄[Fe(CN)₆] (potassium ferrocyanide) → Prussian blue ↓
Confirmatory test for Al³⁺:
- Add aluminium (Al salt) + aluminon (ammonium aurin tricarboxylate) → red lake precipitate
Group IV — Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺
Group reagent: H₂S in ammoniacal solution (NH₄OH + NH₄Cl)
Sulphides precipitate in basic solution:
- CoS (black), NiS (black), MnS (pink/buff), ZnS (white)
Confirmatory test for Zn²⁺: White precipitate of ZnS dissolves in dilute HCl but not in excess NaOH (distinguishes Zn from Group III hydroxides which dissolve in NaOH).
Group V — Ca²⁺, Ba²⁺, Sr²⁺
Group reagent: Ammonium carbonate ((NH₄)₂CO₃)
These precipitate as white carbonates.
Differentiation by flame test:
- Ba²⁺: apple green flame
- Ca²⁺: brick red flame
- Sr²⁺: crimson red flame
Group VI — Mg²⁺, Na⁺, K⁺
These are not precipitated by any of the above reagents.
Confirmatory test for Mg²⁺: Add NH₄OH + disodium hydrogen phosphate (Na₂HPO₄) → white crystalline precipitate of magnesium ammonium phosphate (MgNH₄PO₄).
Confirmatory tests for Na⁺, K⁺: Flame tests (golden yellow for Na; violet for K, viewed through cobalt blue glass).
Important Specific Tests
For Fe²⁺ vs Fe³⁺ distinction: Use K₃[Fe(CN)₆] (potassium ferricyanide) for Fe²⁺ → Turnbull’s blue precipitate. Use K₄[Fe(CN)₆] (potassium ferrocyanide) for Fe³⁺ → Prussian blue precipitate. Use KSCN for Fe³⁺ → blood red. KSCN with Fe²⁺ gives no distinctive colour.
Solved Examples
Example 1 — Identifying an Unknown Salt (CBSE Level)
Q: An unknown white salt dissolves in water to give a colourless solution. The flame test gives a golden yellow colour. Addition of dilute H₂SO₄ gives brisk effervescence with colourless gas that turns lime water milky. Identify the salt.
Solution:
- White salt → common cation (several possibilities)
- Golden yellow flame → Na⁺ (sodium cation)
- Gas with dilute H₂SO₄ that turns lime water milky → CO₂ → anion is CO₃²⁻
Salt = Sodium Carbonate (Na₂CO₃)
Example 2 — Distinguishing Similar Salts (JEE Level)
Q: Three salts A, B, C are white solids. A gives golden yellow flame, B gives brick red flame, C gives violet flame (through cobalt glass). When treated with dilute H₂SO₄: A gives no gas; B gives a colourless gas turning lime water milky; C gives no gas. With AgNO₃ solution, all three give white precipitates that dissolve in NH₄OH. Identify A, B, C.
Solution:
-
Flame colours: A = Na⁺; B = Ca²⁺; C = K⁺
-
A (Na⁺) + no gas with H₂SO₄ + white ppt with AgNO₃ soluble in NH₄OH → NaCl (sodium chloride)
-
B (Ca²⁺) + CO₂ with H₂SO₄ + white ppt with AgNO₃ → CaCl₂? Wait — CO₂ from H₂SO₄ means CO₃²⁻. CaCO₃ + H₂SO₄ → CaSO₄ + CO₂. But CaCO₃ is insoluble in water… let’s reconsider.
Actually, Ca(HCO₃)₂ is soluble in water. And with dilute H₂SO₄: Ca(HCO₃)₂ + H₂SO₄ → CaSO₄ + 2H₂O + 2CO₂. But CaSO₄ doesn’t give white ppt with AgNO₃.
Re-examine: if the salt has Cl⁻ and CO₃²⁻… Revisiting — the white ppt with AgNO₃ (soluble in NH₄OH) confirms Cl⁻. If B is CaCl₂, it shouldn’t give CO₂ with dilute H₂SO₄. Perhaps the gas source is different.
Teaching note: This example illustrates that qualitative analysis requires careful systematic reasoning, not jumping to conclusions. In real analysis, each observation must be consistent with all others. Contradictory observations usually mean the test was not performed carefully or there’s a mixture present.
Example 3 — Anion Chain (NEET Style)
Q: A salt solution gives a brown ring with FeSO₄ + concentrated H₂SO₄. What anion is confirmed?
Solution: The brown ring test confirms nitrate (NO₃⁻). The brown ring is the complex [Fe(NO)(H₂O)₅]²⁺ formed when Fe²⁺ reduces NO₃⁻ to NO in concentrated acid, and NO then complexes with Fe²⁺.
Exam-Specific Tips
CBSE Class 11/12: The practical examination tests anion and cation identification. Know the 6 cation groups and their group reagents. Know the key confirmatory tests: Fe³⁺ (KSCN → blood red), Cu²⁺ (NH₃ → deep blue), Cl⁻ (AgNO₃ → white ppt soluble in NH₄OH), SO₄²⁻ (BaCl₂ → white ppt insoluble in HCl), NO₃⁻ (brown ring test).
JEE Main: The chemistry underlying each test is important. Understand why AgCl is soluble in NH₄OH (formation of [Ag(NH₃)₂]⁺ complex), why the brown ring forms (Fe²⁺ + NO₃⁻ → Fe³⁺ + NO; then NO + Fe²⁺ → [Fe(NO)]²⁺ complex), why Al(OH)₃ dissolves in both acid AND excess NaOH (amphoteric).
Common Mistakes to Avoid
Mistake 1 — Confusing AgBr and AgCl: Both are white/pale precipitates with AgNO₃. The key distinction: AgCl is soluble in dilute NH₄OH; AgBr is only slightly soluble in concentrated NH₄OH; AgI is insoluble in NH₄OH. Always state the solubility in ammonia when confirming Cl⁻.
Mistake 2 — Using NaOH before testing for NH₄⁺: If the salt contains NH₄⁺ and you add NaOH at the start of cation analysis, you’ll boil off the NH₄⁺ and miss it entirely. Always test for NH₄⁺ in an original sample before any other additions.
Mistake 3 — BaSO₄ vs BaSO₃: Both give white precipitates with Ba²⁺. Distinguish: BaSO₄ (sulphate) is insoluble in dilute HCl; BaSO₃ (sulphite) dissolves in dilute HCl with effervescence (SO₂ gas). This tells you whether the anion is SO₄²⁻ or SO₃²⁻.
Mistake 4 — Prussian blue vs Turnbull’s blue: K₄[Fe(CN)₆] + Fe³⁺ → Prussian blue (Berlin blue). K₃[Fe(CN)₆] + Fe²⁺ → Turnbull’s blue. Both are actually the same compound (KFe[Fe(CN)₆]), but the reaction tells you which iron state is present in the original solution.
Practice Questions
Q1: Which cation gives a blood red colour with KSCN? Write the reaction.
Fe³⁺ gives a blood red colour with potassium thiocyanate (KSCN):
FeCl₃ + 3KSCN → Fe(SCN)₃ + 3KCl (blood red)
More accurately in solution: [Fe(H₂O)₆]³⁺ + SCN⁻ → [Fe(SCN)(H₂O)₅]²⁺ + H₂O (blood red complex)
This is the confirmatory test for Fe³⁺. Even trace amounts of Fe³⁺ give an intensely red colour.
Q2: A salt gives a yellow precipitate with ammonium molybdate in HNO₃. Which anion is present?
Yellow precipitate with ammonium molybdate in HNO₃ confirms phosphate (PO₄³⁻). The yellow precipitate is ammonium phosphomolybdate: (NH₄)₃[PMo₁₂O₄₀]. This is the confirmatory test for phosphate ions.
Q3: Explain why CO₂ first turns lime water milky and then clears it if excess CO₂ is passed.
When CO₂ is first passed into Ca(OH)₂ (lime water), calcium carbonate forms as an insoluble white precipitate: Ca(OH)₂ + CO₂ → CaCO₃ ↓ (white) + H₂O
When excess CO₂ is passed, the insoluble CaCO₃ reacts with CO₂ and water to form soluble calcium bicarbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble)
The solution clears. This same chemistry explains why limestone caves form: rainwater (containing dissolved CO₂) dissolves CaCO₃ rock. And stalactites form when Ca(HCO₃)₂ loses CO₂ and deposits CaCO₃.
Q4: A colourless solution gives a white precipitate with AgNO₃. The precipitate is soluble in excess NH₄OH. What anion is present?
White precipitate with AgNO₃ that dissolves in NH₄OH confirms Cl⁻ (chloride): Ag⁺ + Cl⁻ → AgCl ↓ (white) AgCl + 2NH₃ → [Ag(NH₃)₂]⁺ + Cl⁻ (dissolves in excess ammonia due to diamine silver complex formation)
If the precipitate were pale yellow (Br⁻ → AgBr) it would be only slightly soluble in concentrated NH₃. If yellow (I⁻ → AgI), it would be insoluble in NH₃.
FAQs
Q: Why is lead (Pb²⁺) in both Group I and Group II? Pb²⁺ is primarily a Group II cation, but PbCl₂ is slightly soluble in cold water — so some Pb²⁺ may precipitate with Group I. This is why lead analysis requires confirmation steps. In most systematic schemes, Pb²⁺ is confirmed in Group II after thorough Group I analysis.
Q: What is the difference between a group test and a confirmatory test? A group test precipitates a whole group of cations (e.g., H₂S in acidic medium precipitates all Group II cations). It tells you which group to look in. A confirmatory test is specific — it uniquely identifies one ion (e.g., KSCN gives blood red only with Fe³⁺, not with other Group III cations).
Q: Can qualitative analysis be automated? Yes — modern analytical chemistry uses techniques like atomic absorption spectroscopy (AAS), ICP-OES (inductively coupled plasma-optical emission spectroscopy), and ion chromatography for rapid, automated ion identification and quantification. Classical wet analysis is still taught because it builds deep chemical intuition about ion chemistry and reactivity.