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Analysis of Zinc Chloride (ZnCl₂)

Class 12 Chemistry Practical

How to do salt analysis · Viva · Quiz

Aim

To analyse the given inorganic salt for one acidic and one basic radical.

Preliminary Test

ExperimentObservationInference
Physical examinationWhite deliquescent solid; odourlessNot copper
SolubilitySoluble in waterSoluble chloride
Dry heatingMay give white fumes of ZnO on very strong heating; no NH₃Not NH₄Cl
Dilute H₂SO₄No CO₂Not carbonate
Conc. H₂SO₄HCl gas; white fumes with NH₄OHCl⁻ may be present
Flame testNo characteristic alkali-earth colourNot Ba / Ca / Sr

Test of Anion (Cl⁻)

ExperimentObservationInference
Heat a pinch of salt with conc. H₂SO₄Colourless gas with a pungent smell; dense white fumes with NH₄OHCl⁻ may be present
Confirmatory Tests
MnO₂ / conc. H₂SO₄
Heat the salt with conc. H₂SO₄ and a pinch of MnO₂		Greenish-yellow chlorine gas is evolvedCl⁻ is indicated
Silver nitrate
Acidify the soda extract with dilute HNO₃ and add AgNO₃, then NH₄OH		Curdy white ppt of AgCl, soluble in NH₄OH, reappears with dilute HNO₃Cl⁻ is confirmed
Chromyl chloride
Heat salt + K₂Cr₂O₇ + conc. H₂SO₄. Pass vapours into NaOH; acidify with acetic acid and add lead acetate		Yellow chromyl chloride vapours; yellow ppt of PbCrO₄Cl⁻ is confirmed (chromyl chloride test)

Ionic equations

  • NaCl + H₂SO₄ (conc.) → NaHSO₄ + HCl ↑
  • MnO₂ + 4HCl → MnCl₂ + Cl₂ ↑ + 2H₂O
  • Ag⁺ + Cl⁻ → AgCl ↓ (white)
  • AgCl + 2NH₄OH → [Ag(NH₃)₂]Cl + 2H₂O

Test of Cation (Zn²⁺)

ExperimentObservationInference
Pass H₂S through the solution made alkaline with NH₄OH (after Group III)White ppt of ZnSGroup IV (Zn²⁺) may be present
Confirmatory Tests
Sodium hydroxide
Add NaOH dropwise, then in excess, and warm		White ppt of Zn(OH)₂ dissolves in excess NaOHZn²⁺ is confirmed
Potassium ferrocyanide
Neutralise and add K₄[Fe(CN)₆]		Bluish-white ppt of zinc ferrocyanideZn²⁺ is confirmed

Ionic equations

  • Zn²⁺ + H₂S → ZnS ↓ (white) (ammoniacal)
  • Zn(OH)₂ + 2OH⁻ → [Zn(OH)₄]²⁻

Result

The given salt contains Zn²⁺ as the cation (basic radical) and Cl⁻ as the anion (acidic radical). The salt is Zinc Chloride (ZnCl₂).

Precautions

  1. Use small quantities of salt and reagents; do not use excess.
  2. Keep the mouth of the test tube away from the face while heating.
  3. Use freshly prepared FeSO₄ for the brown ring test.
  4. Nessler’s reagent is toxic (mercury); handle with care and do not pipette by mouth.

Viva voce

  1. Why Group IV?

    ZnS ppts with H₂S in ammoniacal medium.

  2. Why excess NaOH?

    Amphoteric zinc hydroxide dissolves.

  3. Why AgNO₃?

    Chloride anion.

  4. Difference from ZnSO₄?

    Anion tests: chloride vs BaCl₂ sulphate.

  5. Difference from AlCl₃ / alum?

    Al is Group III gelatinous hydroxide and blue lake.

  6. Why white fumes with NH₄OH in the anion test?

    HCl + NH₃ → NH₄Cl smoke.

  7. Why chromyl chloride?

    Second confirmatory for chloride.

  8. Why not Nessler?

    Cation is Zn²⁺.

Related salts