Qualitative Analysis

This note summarizes the key qualitative tests for identifying common anions, cations, gases, and organic compounds by observing characteristic reactions such as precipitate formation, color changes, and gas behaviors.

1. Tests for Anions
  • 🔹 Carbonate (CO₃²⁻):

    🔹 Add dilute acid; effervescence of CO₂ gas occurs. Test gas with limewater, which turns milky.

  • 🔹 Chloride (Cl⁻):

    🔹 Add silver nitrate; white precipitate forms, soluble in dilute ammonia.

  • 🔹 Iodide (I⁻):

    🔹 Add silver nitrate; yellow precipitate forms, insoluble in dilute ammonia.

  • 🔹 Nitrate (NO₃⁻):

    🔹 Warm with iron(II) sulfate and sulfuric acid; brown ring forms.

  • 🔹 Sulfate (SO₄²⁻):

    🔹 Add barium chloride with dilute acid; white precipitate forms.

2. Tests for Cations
  • 🔹 Ammonium (NH₄⁺):

    🔹 Add sodium hydroxide and warm; ammonia gas released, turns damp red litmus blue.

  • 🔹 Calcium (Ca²⁺):

    🔹 Flame test: brick-red flame; white precipitate with sodium carbonate.

  • 🔹 Copper (Cu²⁺):

    🔹 Flame test: green/blue flame; blue precipitate with sodium hydroxide.

  • 🔹 Iron (Fe²⁺):

    🔹 Green precipitate with sodium hydroxide.

  • 🔹 Iron (Fe³⁺):

    🔹 Brown precipitate with sodium hydroxide.

  • 🔹 Lead (Pb²⁺):

    🔹 White precipitate with sodium iodide; yellow precipitate with potassium chromate.

  • 🔹 Zinc (Zn²⁺):

    🔹 White precipitate with sodium hydroxide, soluble in excess to form colorless solution.

3. Tests for Gases
  • 🔹 Ammonia (NH₃):

    🔹 Sharp smell; turns damp red litmus blue.

  • 🔹 Carbon dioxide (CO₂):

    🔹 Turns limewater milky.

  • 🔹 Chlorine (Cl₂):

    🔹 Pungent smell; bleaches damp litmus paper white.

  • 🔹 Hydrogen (H₂):

    🔹 Produces ‘pop’ sound with lit splint.

  • 🔹 Oxygen (O₂):

    🔹 Relights a glowing splint.

4. Qualitative Analysis of Organic Compounds
  • 🔹 Test for Unsaturation:

    🔹 Add bromine water; orange color disappears if double bond present.

  • ⚠️ Confusing solubility of silver halide precipitates in ammonia (e.g., thinking all dissolve).
  • ⚠️ Mixing up flame test colors between calcium, copper, and other metals.
  • ⚠️ Assuming all gases have distinctive smells (some do not).
  • ⚠️ Believing that all precipitates are insoluble in excess reagent.
  • ⚠️ Thinking the brown ring test works without proper heating or layering technique.

  • 👉 Memorize key colors of precipitates and flame tests distinctly.
  • 👉 Perform tests carefully, noting whether precipitates dissolve in excess reagent.
  • 👉 Use proper techniques for gas tests (e.g., collecting gas over water or using damp litmus).
  • 👉 Practice writing balanced ionic equations where applicable.
  • 👉 Remember to confirm gases by their characteristic tests (pop for H₂, relight for O₂).
  • 👉 For organic tests, explain disappearance of bromine color due to addition across double bonds.

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