Aluminium sulphate [Al₂(SO₄)₃] is the salt formed from aluminium hydroxide (weak base) and sulphuric acid (strong acid). In qualitative salt analysis, Al³⁺ is identified by the white gelatinous precipitate formed with NaOH, and SO₄²⁻ is identified by the white precipitate formed with BaCl₂ in the presence of dilute HCl. Since Al₂(SO₄)₃ is formed from a weak base and a strong acid, its aqueous solution is acidic (pH < 7).
Aluminium sulphate formula: Al₂(SO₄)₃ — 2 Al³⁺ and 3 SO₄²⁻.
Formed from weak base Al(OH)₃ + strong acid H₂SO₄ → acidic salt (pH < 7 in solution).
Salt analysis test for Al³⁺: white gelatinous precipitate with NaOH (dissolves in excess).
Salt analysis test for SO₄²⁻: white precipitate with BaCl₂, insoluble in HCl.
Al(OH)₃ is amphoteric — dissolves in both acid and excess NaOH.
BaSO₄ precipitate is insoluble in dilute HCl — confirms sulphate (distinguishes from sulphite/carbonate).
Used in water treatment as a coagulant to remove suspended matter.
Used in paper making, dyeing (mordant), and as a fire retardant.
Chemical formula: Al₂(SO₄)₃ Chemical name: Aluminium sulphate Common name: Alum of sulphate, papermakers' alum, filter alum
Formation: 2Al(OH)₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 6H₂O
Composition: • Cation: Al³⁺ (aluminium, tripositive) • Anion: SO₄²⁻ (sulphate, dinegative) • Ratio: 2 Al³⁺ : 3 SO₄²⁻ (to balance charges: 2×(+3) = 6 = 3×(−2) ✓)
Molar mass: 342.15 g/mol Solubility: Soluble in water
Nature of solution: • Formed from weak base Al(OH)₃ + strong acid H₂SO₄ • Aqueous solution is acidic (pH < 7) due to hydrolysis of Al³⁺: Al³⁺ + 3H₂O ⇌ Al(OH)₃ + 3H⁺
Test for Al³⁺ (aluminium ion):
NaOH test: Al³⁺ + 3OH⁻ → Al(OH)₃↓ (white gelatinous precipitate) With excess NaOH: Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻ (soluble — dissolves) Observation: White precipitate dissolves in excess NaOH (amphoteric)
Ammonia test: Al³⁺ + 3NH₃ + 3H₂O → Al(OH)₃↓ + 3NH₄⁺ Observation: White precipitate, insoluble in excess NH₃
Litmus/pH: Acidic solution (due to hydrolysis)
Test for SO₄²⁻ (sulphate ion):
Physical properties: • White crystalline solid • Soluble in water • Hydrated form: Al₂(SO₄)₃·18H₂O • Aqueous solution is acidic
Chemical properties: • Amphoteric nature (Al³⁺ forms Al(OH)₄⁻ with excess base) • Hydrolysis: Al₂(SO₄)₃ + 6H₂O ⇌ 2Al(OH)₃ + 3H₂SO₄
Uses:
Water treatment — coagulant that removes suspended particles Al³⁺ + 3H₂O → Al(OH)₃ (gelatinous floc) + 3H⁺ Al(OH)₃ floc traps suspended dirt → settles out
Paper making — as a sizing agent (aluminium rosin soap)
Fire retardants — decomposes to Al₂O₃ and SO₃ at high temperature
Food additive (E520) — pickling agent
Dyeing — mordant to fix dyes to fabric
Sewage treatment — precipitation of phosphates
The formula of aluminium sulphate is Al₂(SO₄)₃. It contains 2 aluminium ions (Al³⁺) and 3 sulphate ions (SO₄²⁻). It is formed by: 2Al(OH)₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 6H₂O.
Aluminium sulphate gives an acidic solution in water (pH < 7). It is formed from a weak base (Al(OH)₃) and a strong acid (H₂SO₄). The Al³⁺ ion hydrolyses in water to produce H⁺ ions: Al³⁺ + 3H₂O ⇌ Al(OH)₃ + 3H⁺.
Al³⁺ is identified by adding NaOH solution: Al³⁺ + 3OH⁻ → Al(OH)₃↓ (white gelatinous precipitate). With excess NaOH, the precipitate dissolves: Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻ (soluble). This amphoteric dissolution in excess NaOH confirms Al³⁺.
Sulphate is identified by the BaCl₂ test. Add dilute HCl to the solution, then add BaCl₂ solution. A white precipitate of BaSO₄ that is insoluble in dilute HCl confirms sulphate: Ba²⁺ + SO₄²⁻ → BaSO₄↓ (white, insoluble in HCl).
Aluminium sulphate (Al₂(SO₄)₃) is used as a coagulant in water treatment. It hydrolyses to form gelatinous aluminium hydroxide (Al(OH)₃), which traps suspended clay, dirt, and colloids into large flocs that settle to the bottom, making the water clear.
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