Analysis of Inorganic Compounds

Full Notes

Test for Carbonate and Hydrogencarbonate Ions

Reagents:
Dilute hydrochloric acid (HCl) or any aqueous acid.

Observation:
Effervescence as carbon dioxide gas is released.

Equation examples:

• CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
• HCO₃⁻(aq) + H⁺(aq) → CO₂(g) + H₂O(l)

Confirmation of CO₂:
Bubble the gas through limewater (Ca(OH)₂). If CO₂ is present, limewater turns cloudy white due to formation of calcium carbonate.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Test for Sulfate Ions (SO₄²⁻)

Reagents:
Barium chloride solution (BaCl₂) acidified with dilute hydrochloric acid (HCl).

Observation:
White precipitate of barium sulfate (BaSO₄) forms if SO₄²⁻ is present.

Ionic equation:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Matt’s exam tip

The barium chloride solution must be acidified to remove carbonate ions (CO₃²⁻), which could also form a white precipitate with Ba²⁺ and give a false positive.

Test for Ammonium Ions (NH₄⁺)

Reagents:
Sodium hydroxide solution (NaOH)

Method:
Add NaOH(aq) to sample and warm the mixture gently.

Observation:
Ammonia gas (NH₃) is released — it has a sharp, choking smell and turns damp red litmus paper blue.

Ionic equation:
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

Summary

• Carbonates and hydrogencarbonates release carbon dioxide with acid which turns limewater cloudy.
• Sulfate ions give a white barium sulfate precipitate with acidified barium chloride.
• Ammonium ions release ammonia on warming with sodium hydroxide which turns damp red litmus paper blue.