Analytical Chemistry
Introduction
Analytical chemistry involves the identification of substances through chemical tests. In ICSE Class 10 Chemistry, you study how sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH) react with solutions of different metal salts to produce characteristic precipitates.
Action of NaOH on Salt Solutions
NaOH is added dropwise and then in excess to observe precipitate formation and solubility.
| Salt solution | Colour of precipitate | Solubility in excess NaOH | Inference |
|---|---|---|---|
| Ca²⁺ (Calcium) | White (Ca(OH)₂) | Insoluble | Ca²⁺ present |
| Fe²⁺ (Ferrous) | Dirty green (Fe(OH)₂) | Insoluble | Fe²⁺ present |
| Fe³⁺ (Ferric) | Reddish brown (Fe(OH)₃) | Insoluble | Fe³⁺ present |
| Cu²⁺ (Copper) | Pale blue (Cu(OH)₂) | Insoluble | Cu²⁺ present |
| Zn²⁺ (Zinc) | White (Zn(OH)₂) | Soluble (gives colourless solution) | Zn²⁺ present |
| Pb²⁺ (Lead) | White (Pb(OH)₂) | Soluble (gives colourless solution) | Pb²⁺ present |
| Al³⁺ (Aluminium) | White (Al(OH)₃) | Soluble (gives colourless solution) | Al³⁺ present |
Action of NH₄OH on Salt Solutions
NH₄OH is added dropwise and then in excess.
| Salt solution | Colour of precipitate | Solubility in excess NH₄OH | Inference |
|---|---|---|---|
| Ca²⁺ | No precipitate | — | Ca²⁺ present (no ppt with NH₄OH) |
| Fe²⁺ | Dirty green (Fe(OH)₂) | Insoluble | Fe²⁺ present |
| Fe³⁺ | Reddish brown (Fe(OH)₃) | Insoluble | Fe³⁺ present |
| Cu²⁺ | Pale blue (Cu(OH)₂) | Soluble (deep blue solution) | Cu²⁺ present |
| Zn²⁺ | White (Zn(OH)₂) | Soluble (colourless solution) | Zn²⁺ present |
| Pb²⁺ | White (Pb(OH)₂) | Insoluble | Pb²⁺ present |
| Al³⁺ | White (Al(OH)₃) | Insoluble | Al³⁺ present |
Amphoteric Nature of Al, Zn, Pb
Amphoteric substances can react as both acids and bases. Aluminium, zinc, and lead hydroxides are amphoteric.
Aluminium Hydroxide
Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O (Sodium meta-aluminate, soluble)
Al(OH)₃ + NH₄OH → No reaction (Al(OH)₃ is insoluble in NH₄OH)
Zinc Hydroxide
Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O (Sodium zincate, soluble)
Zn(OH)₂ + 4NH₄OH → Zn(NH₃)₄₂ + 4H₂O (Tetrammine zinc(II) hydroxide, soluble)
Lead Hydroxide
Pb(OH)₂ + 2NaOH → Na₂PbO₂ + 2H₂O (Sodium plumbite, soluble)
Pb(OH)₂ + NH₄OH → No reaction (Pb(OH)₂ is insoluble in NH₄OH)
Comparison: NaOH vs NH₄OH as Analytical Reagents
| Feature | NaOH | NH₄OH |
|---|---|---|
| Type | Strong base | Weak base |
| Precipitate with Ca²⁺ | White ppt | No ppt (distinguishes Ca²⁺) |
| Excess solubility of Cu²⁺ ppt | No | Yes (deep blue) |
| Excess solubility of Pb²⁺ ppt | Yes | No |
| Excess solubility of Al³⁺ ppt | Yes | No |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing Fe²⁺ and Fe³⁺ precipitate colours | Fe²⁺ = dirty green; Fe³⁺ = reddish brown |
| Forgetting which hydroxides are amphoteric | Al, Zn, Pb — their hydroxides dissolve in excess NaOH |
| Mixing NaOH and NH₄OH results for Cu²⁺ | Cu(OH)₂ dissolves in excess NH₄OH (deep blue) but NOT in NaOH |
| Thinking Ca²⁺ gives a ppt with NH₄OH | Ca(OH)₂ is relatively soluble — no ppt with NH₄OH |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: NaOH and NH₄OH test for cations, amphoteric nature of Al/Zn/Pb, comparison of reagents.
Marks Blueprint: NaOH test — 2 marks, NH₄OH test — 2 marks, Amphoteric nature — 2 marks.
Self-Test Questions
-
What is the colour of the precipitate formed when NaOH is added to (a) FeSO₄, (b) CuSO₄, (c) FeCl₃?
-
Which metal hydroxides dissolve in excess NaOH? Write an equation for one such reaction.
-
How would you distinguish between Al³⁺ and Pb²⁺ using NH₄OH?
-
Explain the amphoteric nature of zinc hydroxide with balanced equations.
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Why does Cu(OH)₂ dissolve in excess NH₄OH but not in excess NaOH? (Answer: NH₄OH forms a soluble complex [Cu(NH₃)₄]²⁺ (deep blue tetraamminecopper(II) ion). NaOH does not form such a complex, so Cu(OH)₂ precipitate remains.)
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Give the colour of the precipitate formed when NaOH is added to FeCl₃ solution. (Answer: Reddish-brown precipitate of Fe(OH)₃.)
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What happens when NH₄OH is added to ZnSO₄ solution dropwise then in excess? (Answer: White gelatinous precipitate of Zn(OH)₂ forms with dropwise addition, which DISSOLVES in excess NH₄OH forming a colourless soluble complex [Zn(NH₃)₄]²⁺.)
Practical Exam Tips
ICSE practical chemistry exams frequently test salt analysis. The qualitative analysis of cations using NaOH and NH₄OH is the most important practical topic. 'Students should create a systematic table: cation → colour with NaOH (dropwise) → colour with NaOH (excess) → colour with NH₄OH (dropwise) → colour with NH₄OH (excess). MEMORISE this table — it is directly tested in both theory (2 marks) and practical (5 marks).'
ICSE Theory Exam Pattern for Analytical Chemistry
| Question Type | Marks | What They Ask |
|---|---|---|
| State observation | 1-2 | "State what is observed when NaOH is added to CuSO₄ solution" |
| Identify the cation | 2-3 | Given observations with NaOH and NH₄OH, identify the metal ion |
| Distinguish between | 2-3 | "How will you distinguish between Fe²⁺ and Fe³⁺?" |
| Chemical equation | 1-2 | Write balanced equation for precipitate formation and dissolution |
- Which cation gives no precipitate with NH₄OH but gives a white precipitate with NaOH?
