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ICSE Class 10 Chemistry Question 8 of 10

Analytical Chemistry — Question 6

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Question 6

Distinguish by adding: Sodium hydroxide solution or ammonium hydroxide solution to

(a) Calcium salt solution and lead salt solution

(b) Lead nitrate solution and zinc nitrate solution

(c) Copper salt solution and ferrous salt solution

(d) Fe(II) salt solution and Fe(III) salt solution

(e) Ferrous nitrate and lead nitrate

Answer

(a) Ammonium hydroxide on reaction with lead salt solution gives chalky white precipitate of Pb(OH)2. No precipitation occurs on adding Ammonium hydroxide to Calcium salt solution even when it is added in excess.

Pb(NO3)2colourless+2NH4OH⟶Pb(OH)2white ppt↓+2NH4NO3\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2} ↓ + 2\text{NH}_4\text{NO}_3

(b) When ammonium hydroxide solution is added to each of the compounds, lead nitrate forms a chalky white precipitate of lead hydroxide [Pb(OH)2] which is insoluble in excess of ammonium hydroxide.

Pb(NO3)2colourless+2NH4OH⟶Pb(OH)2white ppt↓+2NH4NO3\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2} ↓ + 2\text{NH}_4\text{NO}_3

Whereas a gelatinous white precipitate of zinc hydroxide [Zn(OH)2] is formed in case of zinc nitrate, which is soluble in excess of ammonium hydroxide.

Zn(NO3)2 + 2NH4OH ⟶ 2NH4NO3 + Zn(OH)2 ↓

(c) On adding Sodium hydroxide to Copper salt pale blue coloured precipitate is obtained which is insoluble in excess of Sodium hydroxide. Ferrous salt solution gives a dirty green coloured precipitate with Sodium hydroxide which is insoluble in excess of NaOH.

CuSO4blue+2NaOHcolourless⟶Cu(OH)2↓pale blue ppt+ Na2SO4colourless\underset{\text{blue}}{{\text{CuSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2↓} + \underset{\text{colourless}}{\text{ Na}_2\text{SO}_4}

FeSO4pale green+2NaOHcolourless⟶Fe(OH)2↓dirty green ppt+Na2SO4colourless\underset{\text{pale green}}{{\text{FeSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{colourless}}{\text{Na}_2\text{SO}_4}

(d) Sodium hydroxide on reaction with Fe(II) salt gives dirty green coloured precipitate, while with Fe(III) salt solution it forms reddish brown precipitate. Both precipitates are insoluble in excess NaOH.

Fe(II) salt:

FeSO4pale green solution+2NaOH⟶Fe(OH)2↓dirty green ppt+Na2SO4colourless in solution\underset{\text{pale green solution}}{{\text{FeSO}_4}} + 2\text{NaOH} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{colourless in solution}}{\text{Na}_2\text{SO}_4}

Fe(III) salt :

FeCl3yellow+3NaOHcolourless⟶Fe(OH)3↓reddish brown ppt+3NaClcolourless\underset{\text{yellow}}{{\text{FeCl}_3}} + \underset{\text{colourless}}{3\text{NaOH}} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{colourless}}{3\text{NaCl}}

(e) Ammonium hydroxide on reaction with lead nitrate gives a chalky white insoluble precipitate, and with ferrous nitrate forms a dirty green ppt.

Pb(NO3)2colourless+2NH4OH⟶Pb(OH)2↓white ppt+2NH4NO3\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2↓} + 2\text{NH}_4\text{NO}_3

Fe(NO3)2colourless+2NH4OH⟶Fe(OH)2↓dirty green ppt+2NH4NO3 colourless\underset{\text{colourless}}{{\text{Fe(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{ colourless}}{2\text{NH}_4\text{NO}_3}

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. HClBrisk effervescence; CO2 turns lime water milky
Cl−Dil. HNO3 + AgNO3Curdy white precipitate (AgCl), soluble in NH4OH
SO42−Dil. HCl + BaCl2White precipitate (BaSO4), insoluble in HCl
NO3−Brown ring test (FeSO4 + conc. H2SO4)Brown ring at junction of two layers
S2−Dil. HClRotten egg smell (H2S); turns lead acetate paper black
Cu2+NaOH / NH4OHPale blue ppt (insoluble in excess NaOH); deep blue with excess NH4OH
Fe3+NaOH / NH4OHReddish-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)2C2O4White 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)
NaOHDirty green pptReddish-brown ppt
K4[Fe(CN)6]Turnbull's blue pptPrussian blue ppt
KSCNNo 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.