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ICSE Class 10 · Chemistry · Chapter 4

Analytical Chemistry: Uses of Ammonium Hydroxide and Sodium Hydroxide

5 topics · 15 questions

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Ammonium Hydroxide (NH₄OH) - Introduction

  • Formula: NH₄OH (also written as NH₃ + H₂O). Weak base, aqueous solution of ammonia.
  • Preparation: NH₃ (ammonia gas) dissolved in water: NH₃ + H₂O ⇌ NH₄OH
  • Nature: Weak alkali, partial ionization. Pungent smell of ammonia.
  • Use as Reagent: Important analytical reagent to identify metal ions based on precipitate colors.
  • Physical Properties: Colorless solution, pungent odor, turns red litmus blue.
  • Chemical Properties: Reacts with acids to form ammonium salts. Example: NH₄OH + HCl → NH₄Cl + H₂O

Action of NH₄OH on Metal Salt Solutions

  • Calcium (Ca²⁺): No precipitate or white precipitate (slight). CaCl₂ + 2NH₄OH → Ca(OH)₂ + 2NH₄Cl
  • Lead (Pb²⁺): White precipitate of Pb(OH)₂, insoluble in excess. Pb(NO₃)₂ + 2NH₄OH → Pb(OH)₂↓ (white) + 2NH₄NO₃
  • Zinc (Zn²⁺): White precipitate, soluble in excess giving colorless solution. ZnSO₄ + 2NH₄OH → Zn(OH)₂↓ (white). Excess: Zn(OH)₂ + 4NH₄OH → [Zn(NH₃)₄](OH)₂ (soluble)
  • Iron(II) (Fe²⁺): Dirty green precipitate, insoluble in excess. FeSO₄ + 2NH₄OH → Fe(OH)₂↓ (dirty green) + (NH₄)₂SO₄
  • Iron(III) (Fe³⁺): Reddish-brown precipitate, insoluble in excess. FeCl₃ + 3NH₄OH → Fe(OH)₃↓ (reddish-brown) + 3NH₄Cl
  • Copper (Cu²⁺): Blue precipitate, soluble in excess giving deep blue solution. CuSO₄ + 2NH₄OH → Cu(OH)₂↓ (pale blue). Excess: Cu(OH)₂ + 4NH₄OH → [Cu(NH₃)₄](OH)₂ (deep blue)

Sodium Hydroxide (NaOH) - Introduction

  • Formula: NaOH. Common name: Caustic soda.
  • Nature: Strong alkali, complete ionization in water.
  • Physical Properties: White deliquescent solid, absorbs moisture from air.
  • Preparation: Electrolysis of brine (NaCl solution) - Chlor-alkali process.
  • Use as Reagent: Important analytical reagent for identification of metal ions.
  • Chemical Properties: Neutralizes acids, reacts with metals, reacts with ammonium salts liberating NH₃.

Action of NaOH on Metal Salt Solutions

  • Calcium (Ca²⁺): White precipitate, slightly soluble. CaCl₂ + 2NaOH → Ca(OH)₂↓ (white) + 2NaCl
  • Lead (Pb²⁺): White precipitate, soluble in excess. Pb(NO₃)₂ + 2NaOH → Pb(OH)₂↓ (white). Excess: Pb(OH)₂ + 2NaOH → Na₂PbO₂ + 2H₂O (sodium plumbite - soluble)
  • Zinc (Zn²⁺): White precipitate, soluble in excess. ZnSO₄ + 2NaOH → Zn(OH)₂↓ (white). Excess: Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O (sodium zincate - soluble)
  • Aluminum (Al³⁺): White precipitate, soluble in excess. AlCl₃ + 3NaOH → Al(OH)₃↓ (white). Excess: Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O (sodium aluminate - soluble)
  • Iron(II) (Fe²⁺): Dirty green precipitate, insoluble in excess. Turns brown on standing (oxidation). FeSO₄ + 2NaOH → Fe(OH)₂↓ (dirty green) + Na₂SO₄
  • Iron(III) (Fe³⁺): Reddish-brown precipitate, insoluble in excess. FeCl₃ + 3NaOH → Fe(OH)₃↓ (reddish-brown) + 3NaCl
  • Copper (Cu²⁺): Blue precipitate, insoluble in excess. CuSO₄ + 2NaOH → Cu(OH)₂↓ (blue) + Na₂SO₄

Comparison and Identification Summary

  • Key Difference: NH₄OH forms complex ions (soluble) with Zn²⁺ and Cu²⁺. NaOH forms simple salts (zincate, plumbite soluble in excess).
  • Cu²⁺ Distinction: Blue ppt with both, but NH₄OH excess gives deep blue solution; NaOH excess keeps blue ppt.
  • Zn²⁺ Distinction: White ppt with both, soluble in excess of both, but nature of solution different (complex vs zincate).
  • Fe²⁺ vs Fe³⁺: Green ppt = Fe²⁺; Reddish-brown ppt = Fe³⁺. Both insoluble in excess reagent.
  • Amphoteric Behavior: Zn(OH)₂, Pb(OH)₂, Al(OH)₃ dissolve in excess NaOH (amphoteric hydroxides).
  • Practical Application: Used in qualitative analysis to identify unknown metal cations in solution.

Quiz

Test your knowledge

Multiple choice

  1. Question 1 of 15Knowledge
    The formula of ammonium hydroxide is:
  2. Question 2 of 15Knowledge
    When NH₄OH is added to copper sulphate solution, the precipitate formed is:
  3. Question 3 of 15Knowledge
    The blue precipitate of Cu(OH)₂ dissolves in excess NH₄OH to give:
  4. Question 4 of 15Knowledge
    NaOH is also known as:
  5. Question 5 of 15Comprehension
    When NaOH is added to zinc sulphate solution, a white precipitate is formed which is:
  6. Question 6 of 15Knowledge
    The color of Fe(OH)₃ precipitate is:
  7. Question 7 of 15Knowledge
    Which of the following hydroxides is amphoteric?
  8. Question 8 of 15Knowledge
    Fe(OH)₂ precipitate is:
  9. Question 9 of 15Assertion-Reasoning
    Assertion (A): Cu(OH)₂ dissolves in excess NH₄OH but not in excess NaOH. Reason (R): NH₄OH forms soluble complex with Cu²⁺.
  10. Question 10 of 15Application
    Which reagent is used to distinguish between Fe²⁺ and Fe³⁺?
  11. Question 11 of 15Knowledge
    Pb(OH)₂ dissolves in excess:
  12. Question 12 of 15Knowledge
    The nature of NaOH is:
  13. Question 13 of 15Knowledge
    Al(OH)₃ precipitate dissolves in excess NaOH to form:
  14. Question 14 of 15Application
    Which ion gives a white precipitate with NH₄OH that is insoluble in excess?
  15. Question 15 of 15Knowledge
    NaOH is prepared industrially by:

0 of 15 answered