Qualitative analysis

Test for carbonate ions (CO₃²⁻)

Add dilute acid (H⁺, e.g. HCl) to the sample.

CO₃²⁻ + 2H⁺ → CO₂(g) + H₂O

Observation:
Effervescence (bubbles of gas)
CO₂ can be confirmed by turning limewater milky

Test for sulfate ions (SO₄²⁻)

First acidify the solution, then add barium ions (Ba²⁺, e.g. BaCl₂ solution).

SO₄²⁻ + Ba²⁺ → BaSO₄(s)

Observation:
White precipitate of barium sulfate (BaSO₄)
Indicates presence of sulfate ions

Test for halide ions (Cl⁻, Br⁻, I⁻)

Add silver nitrate solution (Ag⁺) to acidified sample:

• Ag⁺ + Cl⁻ → AgCl(s) (white)

• Ag⁺ + Br⁻ → AgBr(s) (cream)

• Ag⁺ + I⁻ → AgI(s) (yellow)
  

Then confirm using aqueous ammonia:

• Cl⁻ dissolves in dilute NH₃

• Br⁻ dissolves in concentrated NH₃

• I⁻ insoluble

Test for ammonium ions (NH₄⁺)

Add warm sodium hydroxide solution (NaOH).

NH₄⁺ + OH⁻ → NH₃(g) + H₂O

Observation:
Ammonia gas produced; Sharp smell
Turns damp red litmus paper blue

Sequence of tests

Tests must be done in this order:

1. Carbonate (CO₃²⁻) – acid test first

2. Sulfate (SO₄²⁻) – then Ba²⁺ test

3. Halides (Cl⁻, Br⁻, I⁻) – Ag⁺ test last
   
   BaCO₃ and Ag₂SO₄ are insoluble, so earlier ions must be removed/identified first to avoid confusion.