Which of the following are examples of double salt? (A) FeSO4(NH4)2SO4 6H2O (B) CuSO4 4NH3 H2O (C) K2SO4 Al2(SO4)3…

🧠 The Aqueous Dissociation Test

Double salt vs coordination compound is settled by one question: when you dissolve it in water, does the metal-ligand bond survive?

• Double salt — fully dissociates into all constituent simple ions. No coordination sphere persists.
• Coordination compound — the complex ion holds together; the metal stays bound to its ligands.

Mohr's salt and potash alum are the canonical double salts; both fall apart in water. Tetraamminecopper sulfate and potassium ferrocyanide hold their coordination spheres firmly — they're complexes.

🗺️ Tag Each Compound

(A) $\mathrm{FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O}$ — Mohr's salt. In water → $\mathrm{Fe^{2+}}$, $\mathrm{NH_4^+}$, $\mathrm{SO_4^{2-}}$. Double salt ✓ (B) $\mathrm{CuSO_4 \cdot 4NH_3 \cdot H_2O}$ — actually $[\mathrm{Cu(NH_3)_4}]\mathrm{SO_4 \cdot H_2O}$. The deep blue $[\mathrm{Cu(NH_3)_4}]^{2+}$ ion survives — coordination compound. (C) $\mathrm{K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O}$ — potash alum. In water → $\mathrm{K^+}$, $\mathrm{Al^{3+}}$, $\mathrm{SO_4^{2-}}$. Double salt ✓ (D) $\mathrm{Fe(CN)_2 \cdot 4KCN}$ — really $\mathrm{K_4[Fe(CN)_6]}$, potassium ferrocyanide. The $[\mathrm{Fe(CN)_6}]^{4-}$ ion is very stable — coordination compound.

So A and C only.

⚡ The "Colour Test"

If the dissolved solution gives the characteristic colour of the free metal ion (pale blue $\mathrm{Cu^{2+}}$, pale green $\mathrm{Fe^{2+}}$), the bond broke — it was a double salt. If the colour is something new (deep blue tetraammine, yellow ferrocyanide), a coordination sphere is still intact. Real lab habit, fast on paper.

⚠️ Hydrate Water Counts as a Salt Component

Don't confuse "$\cdot \, n\mathrm{H_2O}$" (water of crystallisation, present in both double salts and many complex salts) with the deciding factor. Whether a compound is a double salt is decided by what dissociates in solution, not by how many waters it carries in the solid.

$\boxed{\text{Answer: (1) A and C only}}$