Chapter in a nutshell: Analytical chemistry identifies the ions in a salt. Adding sodium hydroxide (NaOH) or ammonium hydroxide (NH₄OH) drop by drop to a salt solution gives a coloured metal-hydroxide precipitate whose colour (and whether it dissolves in excess) reveals the metal cation. Zinc, aluminium and lead are amphoteric — their oxides/hydroxides react with both acids and alkalis. NaOH on an ammonium salt releases ammonia.
1. Analysis & Key Terms
- Analysis: determining the chemical components of a sample. Qualitative analysis = identifying which ions are present; quantitative analysis = finding how much.
- Reagent: a substance used to bring about a chemical test (NaOH and NH₄OH are key reagents here).
- Precipitate (ppt): an insoluble solid formed when two solutions react; its colour identifies the metal ion.
- The alkali is added drop by drop then in excess — added too fast, a precipitate that redissolves in excess may be missed.
2. Colours of Salts and Their Solutions
Salts of representative elements (Groups 1, 2, 13–17) are usually colourless/white; salts of transition elements (Groups 3–12) are usually coloured.| Ion | Colour |
|---|---|
| Cu²⁺ (cupric) | blue |
| Fe²⁺ (ferrous) | light/pale green |
| Fe³⁺ (ferric) | yellow / yellowish-brown |
| Ni²⁺ | green |
| Cr³⁺ | green |
| Mn²⁺ | pink/light pink |
| Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Zn²⁺, Pb²⁺, NH₄⁺ | colourless |
| MnO₄⁻ (permanganate) | purple/pink |
| Cr₂O₇²⁻ (dichromate) | orange |
| CrO₄²⁻ (chromate) | yellow |
3. Action of Sodium Hydroxide (NaOH) on Salt Solutions
NaOH added drop-wise precipitates the metal hydroxide; some dissolve in excess NaOH (amphoteric).| Cation | Precipitate (colour) | In excess NaOH |
|---|---|---|
| Ca²⁺ | Ca(OH)₂ — white | sparingly soluble |
| Fe²⁺ | Fe(OH)₂ — dirty green | insoluble |
| Fe³⁺ | Fe(OH)₃ — reddish brown | insoluble |
| Cu²⁺ | Cu(OH)₂ — pale blue | insoluble |
| Zn²⁺ | Zn(OH)₂ — gelatinous white | soluble (→ sodium zincate) |
| Pb²⁺ | Pb(OH)₂ — chalky white | soluble (→ sodium plumbite) |
- Ca(NO₃)₂ + 2NaOH → Ca(OH)₂↓ + 2NaNO₃
- FeSO₄ + 2NaOH → Fe(OH)₂↓ + Na₂SO₄
- FeCl₃ + 3NaOH → Fe(OH)₃↓ + 3NaCl
- CuSO₄ + 2NaOH → Cu(OH)₂↓ + Na₂SO₄
- ZnSO₄ + 2NaOH → Zn(OH)₂↓ + Na₂SO₄ ; then Zn(OH)₂ + 2NaOH(excess) → Na₂ZnO₂ (sodium zincate) + 2H₂O
- Pb(NO₃)₂ + 2NaOH → Pb(OH)₂↓ + 2NaNO₃ ; then Pb(OH)₂ + 2NaOH(excess) → Na₂PbO₂ (sodium plumbite) + 2H₂O
4. Action of Ammonium Hydroxide (NH₄OH) on Salt Solutions
NH₄OH is a weak base (few OH⁻). It precipitates most hydroxides; some dissolve in excess NH₄OH by forming soluble ammine (amino) complexes.| Cation | Precipitate (colour) | In excess NH₄OH |
|---|---|---|
| Ca²⁺ | no precipitate | (OH⁻ too low to precipitate) |
| Fe²⁺ | Fe(OH)₂ — dirty green | insoluble |
| Fe³⁺ | Fe(OH)₃ — reddish brown | insoluble |
| Cu²⁺ | Cu(OH)₂ — pale blue | soluble → deep/inky blue solution |
| Zn²⁺ | Zn(OH)₂ — gelatinous white | soluble (colourless) |
| Pb²⁺ | Pb(OH)₂ — chalky white | insoluble |
- FeSO₄ + 2NH₄OH → Fe(OH)₂↓ + (NH₄)₂SO₄
- FeCl₃ + 3NH₄OH → Fe(OH)₃↓ + 3NH₄Cl
- CuSO₄ + 2NH₄OH → Cu(OH)₂↓ + (NH₄)₂SO₄ ; then Cu(OH)₂ + (NH₄)₂SO₄ + 2NH₄OH(excess) → [Cu(NH₃)₄]SO₄ (tetrammine copper(II) sulphate, deep blue) + 4H₂O
- ZnSO₄ + 2NH₄OH → Zn(OH)₂↓ + (NH₄)₂SO₄ ; then Zn(OH)₂ + (NH₄)₂SO₄ + 2NH₄OH(excess) → [Zn(NH₃)₄]SO₄ + 4H₂O
- Pb(NO₃)₂ + 2NH₄OH → Pb(OH)₂↓ + 2NH₄NO₃
5. Special / Distinguishing Tests
- Cu²⁺ with excess NH₄OH → deep blue (tetrammine complex) — a characteristic test for copper.
- NaOH vs NH₄OH: NaOH dissolves Zn(OH)₂ and Pb(OH)₂; NH₄OH dissolves Zn(OH)₂ but not Pb(OH)₂. NH₄OH dissolves Cu(OH)₂ to deep blue; NaOH does not.
- Fe²⁺ vs Fe³⁺: dirty-green ppt (Fe²⁺) vs reddish-brown ppt (Fe³⁺) with either alkali.
- Zn²⁺ vs Pb²⁺ (both give white ppt soluble in NaOH): add NH₄OH in excess — Zn(OH)₂ dissolves, Pb(OH)₂ does not.
6. Action of NaOH on Ammonium Salts
Heating an ammonium salt with NaOH (a stronger base) releases ammonia gas (turns moist red litmus blue; pungent smell):- NH₄Cl + NaOH →(Δ) NaCl + H₂O + NH₃↑
- (NH₄)₂SO₄ + 2NaOH →(Δ) Na₂SO₄ + 2H₂O + 2NH₃↑
7. Action of Alkalis on Metals (Zn, Al, Pb)
Zinc, aluminium and lead react with hot concentrated caustic alkali, liberating hydrogen:- Zn + 2NaOH →(hot, conc.) Na₂ZnO₂ (sodium zincate) + H₂↑
- 2Al + 2NaOH + 2H₂O → 2NaAlO₂ (sodium meta-aluminate) + 3H₂↑
- Pb + 2NaOH → Na₂PbO₂ (sodium plumbite) + H₂↑
8. Amphoteric Oxides & Hydroxides
Amphoteric = reacts with both acids and alkalis to form salt + water. The amphoteric oxides/hydroxides in ICSE: ZnO/Zn(OH)₂, Al₂O₃/Al(OH)₃, PbO/Pb(OH)₂.- With acid (basic behaviour): ZnO + 2HCl → ZnCl₂ + H₂O
- With alkali (acidic behaviour): ZnO + 2NaOH → Na₂ZnO₂ + H₂O
- Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O ; Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O
- PbO + 2NaOH → Na₂PbO₂ + H₂O
9. Worked / Structured Examples (ICSE pattern)
Q1. A salt solution gives a reddish-brown ppt with NaOH, insoluble in excess. Identify the cation. Solution: Fe³⁺ (ferric ion) → Fe(OH)₃.Q2. A white ppt with NaOH dissolves in excess; with NH₄OH it also dissolves. Which cation? Solution: Zn²⁺ (Pb²⁺ would NOT dissolve in excess NH₄OH).
Q3. A blue solution gives a pale-blue ppt with NH₄OH that dissolves to a deep blue. Identify. Solution: Cu²⁺ (copper) — forms [Cu(NH₃)₄]²⁺.
Q4. Write the equation for the action of excess NaOH on zinc hydroxide. Solution: Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O.
Q5. Name the gas released when ammonium chloride is heated with NaOH; give its test. Solution: Ammonia (NH₃); turns moist red litmus blue / dense white fumes with HCl.
Q6. How do you distinguish a Pb²⁺ salt from a Zn²⁺ salt? Solution: Add NH₄OH in excess — Zn(OH)₂ dissolves (colourless), Pb(OH)₂ stays as a white ppt.
10. Key Terms — Quick Glossary
| Term | One-line definition |
|---|---|
| Analysis | finding the components of a sample. |
| Qualitative analysis | identifying the ions present. |
| Reagent | a substance used to perform a chemical test. |
| Precipitate | an insoluble solid formed in a reaction. |
| Amphoteric | reacts with both acids and alkalis. |
| Sodium zincate | Na₂ZnO₂, from Zn(OH)₂ + excess NaOH. |
| Sodium plumbite | Na₂PbO₂, from Pb(OH)₂ + excess NaOH. |
| Ammine complex | soluble complex (e.g. [Cu(NH₃)₄]²⁺) in excess NH₄OH. |
| Caustic alkali | strong base such as NaOH/KOH. |
| Soluble in excess | precipitate redissolving when more alkali is added. |
11. Common Mistakes to Avoid
- Forgetting that Pb(OH)₂ dissolves in excess NaOH but NOT in excess NH₄OH (key Zn/Pb distinction).
- Saying NH₄OH precipitates Ca²⁺ — it does not (too few OH⁻ ions).
- Mixing up Fe²⁺ (dirty green) and Fe³⁺ (reddish brown) precipitates.
- Writing the copper-ammine product as a simple hydroxide — it is the deep-blue [Cu(NH₃)₄]²⁺.
- Forgetting that Zn, Al, Pb give hydrogen with hot conc. alkali (amphoteric metals).
- Adding alkali too fast and missing a precipitate that redissolves in excess.
12. Likely Exam Questions (with crisp answers)
- What is qualitative analysis? → Identifying the ions/components present in a substance.
- Give the colour of (i) ferrous, (ii) ferric, (iii) copper salt solutions. → Pale green; yellow; blue.
- What ppt does Cu²⁺ give with NaOH and is it soluble in excess? → Pale blue Cu(OH)₂; insoluble in excess NaOH.
- Which hydroxides dissolve in excess NaOH? → Zn(OH)₂ and Pb(OH)₂ (and Al(OH)₃).
- How do you distinguish NaOH from NH₄OH solutions? → Add to a copper salt: NH₄OH gives a deep-blue solution in excess; NaOH gives an insoluble pale-blue ppt.
- Why does Ca²⁺ give no ppt with NH₄OH? → NH₄OH is weak; its OH⁻ concentration is too low to precipitate Ca(OH)₂.
- Name the gas evolved when an ammonium salt is heated with NaOH. → Ammonia.
- Define amphoteric oxide; give two examples. → Reacts with both acids and alkalis; ZnO, Al₂O₃ (also PbO).
- Write the reaction of aluminium with hot conc. NaOH. → 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂.
- What is observed when NaOH is added to FeSO₄ solution? → A dirty-green precipitate of Fe(OH)₂ forms.
- Name the complex formed when excess NH₄OH acts on copper sulphate. → Tetrammine copper(II) sulphate, [Cu(NH₃)₄]SO₄.
- Which metal salt gives a white ppt with NaOH that is sparingly soluble? → Calcium salt (Ca(OH)₂).
- Why is the alkali added drop by drop? → To observe the colour before any soluble-in-excess ppt redissolves.
- Name a metal whose hydroxide is soluble in excess of both NaOH and NH₄OH. → Zinc.
- State what is seen when caustic soda is added to lead nitrate, first little then excess. → White ppt of Pb(OH)₂ forms, then dissolves in excess to colourless sodium plumbite.
13. Chemical Equations & Formulas (quick reference)
NaOH on salts:- Ca(NO₃)₂ + 2NaOH → Ca(OH)₂↓ + 2NaNO₃
- FeSO₄ + 2NaOH → Fe(OH)₂↓ + Na₂SO₄
- FeCl₃ + 3NaOH → Fe(OH)₃↓ + 3NaCl
- CuSO₄ + 2NaOH → Cu(OH)₂↓ + Na₂SO₄
- ZnSO₄ + 2NaOH → Zn(OH)₂↓ + Na₂SO₄ ; Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O
- Pb(NO₃)₂ + 2NaOH → Pb(OH)₂↓ + 2NaNO₃ ; Pb(OH)₂ + 2NaOH → Na₂PbO₂ + 2H₂O
NH₄OH on salts:
- CuSO₄ + 2NH₄OH → Cu(OH)₂↓ + (NH₄)₂SO₄ ; Cu(OH)₂ + (NH₄)₂SO₄ + 2NH₄OH → [Cu(NH₃)₄]SO₄ + 4H₂O
- ZnSO₄ + 2NH₄OH → Zn(OH)₂↓ + (NH₄)₂SO₄ ; Zn(OH)₂ + (NH₄)₂SO₄ + 2NH₄OH → [Zn(NH₃)₄]SO₄ + 4H₂O
Ammonium salt + NaOH:
- NH₄Cl + NaOH →(Δ) NaCl + H₂O + NH₃↑
- (NH₄)₂SO₄ + 2NaOH →(Δ) Na₂SO₄ + 2H₂O + 2NH₃↑
Amphoteric metals + hot conc. alkali:
- Zn + 2NaOH → Na₂ZnO₂ + H₂↑
- 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂↑
- Pb + 2NaOH → Na₂PbO₂ + H₂↑
Amphoteric oxides/hydroxides:
- ZnO + 2HCl → ZnCl₂ + H₂O ; ZnO + 2NaOH → Na₂ZnO₂ + H₂O
- Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O ; Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O
- PbO + 2NaOH → Na₂PbO₂ + H₂O
Precipitate colours: Fe(OH)₂ dirty green · Fe(OH)₃ reddish brown · Cu(OH)₂ pale blue · Zn(OH)₂ white (sol. excess) · Pb(OH)₂ white (sol. excess NaOH) · Ca(OH)₂ white (sparingly sol.).
14. Cation-by-Cation Observation Summary (NaOH vs NH₄OH)
Read each as: solution colour → with a little alkali → with excess.- Calcium (Ca²⁺): colourless. NaOH → white Ca(OH)₂, sparingly soluble. NH₄OH → no precipitate.
- Ferrous (Fe²⁺): pale green. Both alkalis → dirty-green Fe(OH)₂, insoluble; slowly turns brown in air (→ Fe(OH)₃).
- Ferric (Fe³⁺): yellow/brown. Both → reddish-brown Fe(OH)₃, insoluble.
- Copper (Cu²⁺): blue. NaOH → pale-blue Cu(OH)₂, insoluble. NH₄OH → pale-blue ppt that dissolves in excess to deep/inky blue ([Cu(NH₃)₄]²⁺).
- Zinc (Zn²⁺): colourless. Both → white gelatinous Zn(OH)₂, soluble in excess of either.
- Lead (Pb²⁺): colourless. NaOH → chalky-white Pb(OH)₂, soluble in excess. NH₄OH → white ppt, insoluble in excess.
15. Flame Tests (cation identification)
| Metal ion | Flame colour |
|---|---|
| Sodium (Na⁺) | golden yellow |
| Potassium (K⁺) | lilac / pale violet |
| Calcium (Ca²⁺) | brick red |
| Strontium (Sr²⁺) | crimson |
| Barium (Ba²⁺) | apple green |
| Copper (Cu²⁺) | bluish green |
16. Common Anion (Acid-Radical) Tests — quick reference
| Anion | Test | Observation |
|---|---|---|
| Carbonate CO₃²⁻ | dilute acid | effervescence; CO₂ turns lime water milky |
| Sulphide S²⁻ | dilute acid | H₂S (rotten-egg smell); blackens lead-acetate paper |
| Sulphate SO₄²⁻ | BaCl₂ + dil. HCl | white ppt BaSO₄ (insoluble) |
| Chloride Cl⁻ | AgNO₃ + dil. HNO₃ | white ppt AgCl (soluble in NH₄OH) |