Identification of Ions and Gases

1. Tests to Identify Anions

(a) Carbonate (CO₃²⁻)

• Principle: Carbonates react with dilute acids to produce carbon dioxide gas.
• Procedure:
  1. Add dilute hydrochloric acid (HCl) or dilute nitric acid (HNO₃) to the solid carbonate or its solution.
  2. Observe effervescence (bubbling) due to gas evolution.
  3. Test the gas by bubbling it through limewater (aqueous calcium hydroxide, Ca(OH)₂).
• Positive Result: Limewater turns milky due to the formation of insoluble calcium carbonate.
• Equation:
  • CO₃²⁻ + 2H⁺ → CO₂(g) + H₂O(l)
  • Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

(b) Halides (Cl⁻, Br⁻, I⁻)

• Principle: Halide ions form precipitates with silver nitrate in acidic medium.
• Procedure:
  1. Acidify the test solution with dilute nitric acid (to remove interfering anions like CO₃²⁻ or OH⁻).
  2. Add aqueous silver nitrate (AgNO₃).
• Results:
  • Cl⁻ → White precipitate (AgCl), soluble in dilute NH₃.
  • Br⁻ → Cream precipitate (AgBr), partially soluble in concentrated NH₃.
  • I⁻ → Yellow precipitate (AgI), insoluble in NH₃.
• Equation Example:
  Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

(c) Nitrate (NO₃⁻)

• Principle: Nitrate ions can be reduced to ammonia using aluminium and alkali.
• Procedure:
  1. Add a small piece of aluminium foil to the test solution.
  2. Add aqueous sodium hydroxide (NaOH).
  3. Warm the mixture gently.
  4. Test the gas evolved with damp red litmus paper.
• Positive Result: Ammonia gas is produced, turning damp red litmus paper blue.
• Equation:
  3NO₃⁻ + 8Al + 5OH⁻ + 18H₂O → 3NH₃(g) + 8[Al(OH)₄]⁻

(d) Sulfate (SO₄²⁻)

• Principle: Sulfate ions produce insoluble barium sulfate with barium salts.
• Procedure:
  1. Acidify the solution with dilute nitric acid.
  2. Add aqueous barium nitrate (Ba(NO₃)₂).
• Positive Result: White precipitate of BaSO₄ forms, insoluble in acids.
• Equation:
  Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

(e) Sulfite (SO₃²⁻)

• Principle: Sulfites can be oxidised by acidified potassium manganate(VII).
• Procedure:
  1. Add a few drops of acidified KMnO₄ (purple) to the solution containing sulfite ions.
  2. Observe the colour change.
• Positive Result: Purple KMnO₄ is decolourised due to oxidation of sulfite to sulfate.
• Equation:
  5SO₃²⁻ + 2MnO₄⁻ + 6H⁺ → 5SO₄²⁻ + 2Mn²⁺ + 3H₂O

2. Tests for Aqueous Cations

Using Aqueous Sodium Hydroxide (NaOH) and Aqueous Ammonia (NH₃)

3. Tests to Identify Gases

4. Flame Tests for Metal Cations