Tests for anions (Edexcel GCSE Chemistry Combined Science): Revision Notes

Tests for anions

1. Test for Carbonate Ions (CO₃²⁻)

Reagents: Hydrochloric acid ($HCl$) and limewater (calcium hydroxide solution).

Observation: If CO₂ is present, the limewater turns cloudy/milky.

Equation:

$$CO₃²⁻ (aq) + 2H⁺ (aq) → CO₂ (g) + H₂O (l)$$ $$CO₂ (g) + Ca(OH)₂ (aq) → CaCO₃ (s) + H₂O (l)$$

($CaCO₃$ causes the milky appearance)

2. Test for Sulphate Ions (SO₄²⁻)

Reagents: Hydrochloric acid ($HCl$) and barium chloride ($BaCl$₂).

Observation: If sulphate ions are present, a white precipitate of barium sulphate (BaSO₄) will form.

Equation:

$$SO₄²⁻ (aq) + Ba²⁺ (aq) → BaSO₄ (s)$$

3. Test for Halide Ions (Cl⁻, Br⁻, I⁻)

Reagents: Nitric acid ($HNO₃$) and silver nitrate $(AgNO₃$).

Observations:

• Chloride (Cl⁻): A white precipitate of silver chloride (AgCl) forms.
• Bromide (Br⁻): A cream precipitate of silver bromide (AgBr) forms.
• Iodide (I⁻): A yellow precipitate of silver iodide (AgI) forms. Equations:

$$Cl⁻ (aq) + Ag⁺ (aq) → AgCl (s)$$ $$Br⁻ (aq) + Ag⁺ (aq) → AgBr (s)$$ $$I⁻ (aq) + Ag⁺ (aq) → AgI (s)$$

Summary of Results:

Anion	Reagent(s)	Observation	Equation
Carbonate (CO₃²⁻)	HCl + Limewater	Limewater turns cloudy	CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
Sulphate (SO₄²⁻)	HCl + BaCl₂	White precipitate (BaSO₄)	SO₄²⁻ + Ba²⁺ → BaSO₄
Chloride (Cl⁻)	HNO₃ + AgNO₃	White precipitate (AgCl)	Cl⁻ + Ag⁺ → AgCl
Bromide (Br⁻)	HNO₃ + AgNO₃	Cream precipitate (AgBr)	Br⁻ + Ag⁺ → AgBr
Iodide (I⁻)	HNO₃ + AgNO₃	Yellow precipitate (AgI)	I⁻ + Ag⁺ → AgI