ICSE Class 10 Chemistry • Chapter 04
Analytical Chemistry: Branch of chemistry dealing with identification and separation of chemical substances.
In this chapter, we study the action of NaOH and NH₄OH on salt solutions to identify the cation (metal ion) present.
Procedure: Add NaOH solution drop by drop to the salt solution.
Observe: Formation and colour of precipitate (if any)
Then: Add excess NaOH and note if precipitate dissolves
| Salt Solution | Precipitate Formed | Colour | On Adding Excess NaOH | Ionic Equation |
|---|---|---|---|---|
| Calcium (Ca²⁺) | Ca(OH)₂ | White (milky) | Insoluble (No change) | Ca²⁺ + 2OH⁻ → Ca(OH)₂↓ |
| Iron(II) (Fe²⁺) | Fe(OH)₂ | Dirty/Pale Green | Insoluble (turns brown on standing) | Fe²⁺ + 2OH⁻ → Fe(OH)₂↓ |
| Iron(III) (Fe³⁺) | Fe(OH)₃ | Reddish Brown | Insoluble | Fe³⁺ + 3OH⁻ → Fe(OH)₃↓ |
| Copper (Cu²⁺) | Cu(OH)₂ | Pale Blue | Insoluble | Cu²⁺ + 2OH⁻ → Cu(OH)₂↓ |
| Zinc (Zn²⁺) | Zn(OH)₂ | White gelatinous | Soluble (forms sodium zincate) | Zn(OH)₂ + 2OH⁻ → ZnO₂²⁻ + 2H₂O |
| Lead (Pb²⁺) | Pb(OH)₂ | White | Soluble (forms sodium plumbite) | Pb(OH)₂ + 2OH⁻ → PbO₂²⁻ + 2H₂O |
| Aluminium (Al³⁺) | Al(OH)₃ | White gelatinous | Soluble (forms sodium aluminate) | Al(OH)₃ + OH⁻ → AlO₂⁻ + 2H₂O |
Amphoteric Hydroxides: Zn(OH)₂, Pb(OH)₂, Al(OH)₃ dissolve in EXCESS NaOH!
They react with both acids AND bases (amphoteric nature).
Procedure: Add NH₄OH solution drop by drop to the salt solution.
Observe: Formation and colour of precipitate
Then: Add excess NH₄OH and note if precipitate dissolves
| Salt Solution | Precipitate | Colour | On Adding Excess NH₄OH | Reason |
|---|---|---|---|---|
| Calcium (Ca²⁺) | No precipitate | - | - | Ca(OH)₂ slightly soluble |
| Iron(II) (Fe²⁺) | Fe(OH)₂ | Dirty Green | Insoluble | Does not form complex |
| Iron(III) (Fe³⁺) | Fe(OH)₃ | Reddish Brown | Insoluble | Does not form complex |
| Copper (Cu²⁺) | Cu(OH)₂ | Pale Blue | Soluble - Deep Blue solution | Forms [Cu(NH₃)₄]²⁺ complex |
| Zinc (Zn²⁺) | Zn(OH)₂ | White | Soluble - Colourless | Forms [Zn(NH₃)₄]²⁺ complex |
| Lead (Pb²⁺) | Pb(OH)₂ | White | Insoluble | Does not form complex |
| Aluminium (Al³⁺) | Al(OH)₃ | White | Insoluble | Does not form complex |
Cu²⁺ with excess NH₄OH → Deep Blue (Tetraamminecopper(II) complex)
Zn²⁺ with excess NH₄OH → Colourless (Tetraamminezinc(II) complex)
When NaOH is added to ammonium salts and heated:
NH₄Cl + NaOH → NaCl + H₂O + NH₃↑
Observation: Pungent smell of ammonia gas released
Test: Ammonia turns moist red litmus paper blue
| Cation | NaOH (limited) | NaOH (excess) | NH₄OH (limited) | NH₄OH (excess) |
|---|---|---|---|---|
| Cu²⁺ | Pale blue ppt | Insoluble | Pale blue ppt | Deep blue soln |
| Zn²⁺ | White ppt | Soluble | White ppt | Soluble |
| Pb²⁺ | White ppt | Soluble | White ppt | Insoluble |
| Al³⁺ | White ppt | Soluble | White ppt | Insoluble |
| Fe²⁺ | Dirty green ppt | Insoluble | Dirty green ppt | Insoluble |
| Fe³⁺ | Reddish brown ppt | Insoluble | Reddish brown ppt | Insoluble |
BOARD What do you observe when sodium hydroxide is added drop by drop and then in excess to solutions of: (i) Copper sulphate (ii) Zinc sulphate?
BOARD Identify the cation: A salt solution gives a pale blue precipitate with NaOH which is insoluble in excess. With NH₄OH, the precipitate dissolves in excess giving a deep blue solution.