Q.A double salt which contains fourth period alkali metal (A) on heating at 500K gives (B). aqueous solution of (B) gives white precipitate with BaCl₂ and gives a red colour compound with alizarin. Identify A and B.

Step 1. The double salt containing a 4th-period alkali metal is potash alum, K₂SO₄.Al₂(SO₄)₃.24H₂O -- potassium (K) is the 4th-period alkali metal, matching the alum family member the chapter details in Section 2.2.9. So A = potash alum.

Step 2. Heated at 500 K, potash alum loses its water of hydration and swells into burnt alum: K₂SO₄.Al₂(SO₄)₃.24H₂O →(500 K)→ K₂SO₄.Al₂(SO₄)₃ + 24H₂O. So B = burnt alum, K₂SO₄.Al₂(SO₄)₃.

Step 3. Burnt alum's aqueous solution contains sulphate ions (from both the K₂SO₄ and Al₂(SO₄)₃ components); sulphate ions give a white precipitate with barium chloride, BaCl₂ + SO₄²⁻ → BaSO₄↓(white) + 2Cl⁻ -- matching the white-precipitate clue in the question. …

Identify the 4th-period alkali metal (potassium) to fix which alum is meant, trace what it becomes on heating to 500 K (Section 2.2.9), then explain both dia …

• Identifying B as the fully-decomposed, red-hot product (K₂SO₄ + Al₂O₃ + 3SO₃) instead of the intermediate 'burnt alum' stage -- the question specifies heating at 500 K specifically, which th …