Analytical Chemistry — Use of Ammonium & Sodium Hydroxide — Question 4
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| Action of Alkalis — on oxides and hydroxides of certain metals | |
|---|---|
| 19. Zinc oxide | ZnO + NaOH ⟶ ............... + ............... |
| 20. Zinc hydroxide | Zn(OH)2 + NaOH ⟶ ............... + ............... |
| 21. Lead [II] oxide | PbO + NaOH ⟶ ............... + ............... |
| 22. Lead hydroxide | Pb(OH)2 + NaOH ⟶ ............... + ............... |
| 23. Aluminium oxide | Al2O3 + NaOH ⟶ ............... + ............... |
| 24. Aluminium hydroxide | Al(OH)3 + KOH ⟶ ............... + ............... |
| Action of Alkalis — on oxides and hydroxides of certain metals | |
|---|---|
| 19. Zinc oxide | ZnO + 2NaOH ⟶ Na2ZnO2 + H2O |
| 20. Zinc hydroxide | Zn(OH)2 + 2NaOH ⟶ Na2ZnO2 + 2H2O |
| 21. Lead [II] oxide | PbO + 2NaOH ⟶ Na2PbO2 + H2O |
| 22. Lead hydroxide | Pb(OH)2 + 2NaOH ⟶ Na2PbO2 + 2H2O |
| 23. Aluminium oxide | Al2O3 + 2NaOH ⟶ 2NaAlO2 + H2O |
| 24. Aluminium hydroxide | Al(OH)3 + KOH ⟶ KAlO2 + 2H2O |
Chapter Overview: Analytical Chemistry
Analytical Chemistry deals with the identification and analysis of substances. In ICSE Class X, the focus is on qualitative analysis - identifying the acid radical (anion) and basic radical (cation) present in a given salt. Students learn systematic procedures to test for common cations (Pb2+, Cu2+, Fe2+, Fe3+, Zn2+, Ca2+, NH4+) and anions (Cl−, NO3−, SO42−, CO32−, S2−). The tests involve adding specific reagents and observing characteristic changes such as precipitate formation, gas evolution, colour changes, and flame tests. Understanding the action of NaOH and NH4OH on salt solutions is crucial for cation identification. For anion identification, dilute acids and specific reagents are used. Students must write balanced equations for all reactions observed during analysis. This chapter is highly practical and tested extensively in the practical examination, where students perform actual salt analysis.
Key Tests & Observations
| Ion | Test / Reagent | Observation |
|---|---|---|
| CO32− | Dil. HCl | Brisk effervescence; CO2 turns lime water milky |
| Cl− | Dil. HNO3 + AgNO3 | Curdy white precipitate (AgCl), soluble in NH4OH |
| SO42− | Dil. HCl + BaCl2 | White precipitate (BaSO4), insoluble in HCl |
| NO3− | Brown ring test (FeSO4 + conc. H2SO4) | Brown ring at junction of two layers |
| S2− | Dil. HCl | Rotten egg smell (H2S); turns lead acetate paper black |
| Cu2+ | NaOH / NH4OH | Pale blue ppt (insoluble in excess NaOH); deep blue with excess NH4OH |
| Fe3+ | NaOH / NH4OH | Reddish-brown ppt, insoluble in excess of both |
| Zn2+ | NaOH (excess) | White gelatinous ppt, soluble in excess NaOH (amphoteric) |
| Pb2+ | NaOH (excess) | White ppt, soluble in excess NaOH; also gives yellow ppt with KI |
| Ca2+ | NH4OH + (NH4)2C2O4 | White ppt of calcium oxalate |
Must-Know Concepts
- Flame tests: Na+ = golden yellow, K+ = lilac/violet, Ca2+ = brick red, Cu2+ = blue-green
- Action of NaOH on ammonium salts produces NH3 gas (pungent smell, turns moist red litmus blue)
- Pb2+ and Zn2+ hydroxides are amphoteric (dissolve in excess NaOH)
- Cu2+ gives deep blue solution with excess NH4OH (cuprammonium complex)
- Fe2+ gives dirty green ppt with NaOH; Fe3+ gives reddish-brown ppt
- For chloride test, add dil. HNO3 first (not HCl, as it would interfere)
Fe2+ vs Fe3+ Identification
| Test | Fe2+ (Ferrous) | Fe3+ (Ferric) |
|---|---|---|
| NaOH | Dirty green ppt | Reddish-brown ppt |
| K4[Fe(CN)6] | Turnbull's blue ppt | Prussian blue ppt |
| KSCN | No reaction (or pale green) | Blood red colouration |
Important Diagrams to Practice
- Flowchart for systematic analysis of anions (acid radicals)
- Flowchart for cation analysis using NaOH and NH4OH
- Brown ring test setup for nitrate identification
Common Mistakes
- Using HCl instead of HNO3 when testing for chloride (Cl− from HCl gives false positive)
- Confusing the precipitate colours of Fe2+ (dirty green) and Fe3+ (reddish-brown)
- Not mentioning "excess" when describing solubility of Zn(OH)2 or Pb(OH)2 in NaOH
- Forgetting to acidify the solution before adding BaCl2 for sulphate test
Scoring Tips
- Write observations in the format: Experiment → Observation → Inference
- Always write the balanced equation for every reaction observed
- Mention colour, state (ppt/gas/solution), and solubility in your observations
- For practical exams, practice the systematic procedure repeatedly
- Learn flame test colours as a mnemonic: "Nancy's Kitchen Caught Barium's Crimson Strontium"
Frequently Asked Questions
Why is dilute HNO3 used before the chloride test?
Dilute HNO3 is used to prevent interference from carbonate and sulphite ions, which would also form precipitates with AgNO3. HCl cannot be used because it would introduce Cl− ions and give a false positive.
How do you distinguish between Zn2+ and Pb2+?
Both give white precipitates with NaOH that dissolve in excess. However, Pb2+ gives a yellow precipitate (PbI2) with potassium iodide, while Zn2+ does not. Also, Pb2+ gives a white precipitate with dilute HCl (PbCl2), which is soluble in hot water.
What makes a hydroxide amphoteric?
An amphoteric hydroxide can react with both acids and bases. Zn(OH)2 and Pb(OH)2 are amphoteric because they dissolve in excess NaOH to form soluble zincate or plumbate ions, and also dissolve in dilute acids to form salts.