Match List I with List II (Colour): (A) Mg(NH4)PO4 (B) K3[Co(NO2)6] (C) MnO(OH)2 (D) Fe4[Fe(CN)6]3 with (I) brown (II)…

🧠 Standard Inorganic Colours You Should Know

| Compound | Colour | Common name / context | |---|---|---| | (A) $\mathrm{Mg(NH_4)PO_4}$ | white | Magnesium ammonium phosphate; classic gravimetric ppt for Mg²⁺ | | (B) $\mathrm{K_3[Co(NO_2)_6]}$ | yellow | Sodium cobaltinitrite test for K⁺ (gives yellow ppt $\mathrm{K_3[Co(NO_2)_6]}$) | | (C) $\mathrm{MnO(OH)_2}$ | brown | Mn(IV) species; basis of Winkler dissolved-O₂ test | | (D) $\mathrm{Fe_4[Fe(CN)_6]_3}$ | blue | Prussian blue; iron(III) hexacyanoferrate(II) |

Match: A-II (white), B-III (yellow), C-I (brown), D-IV (blue).

🗺️ Why Each Is the Colour It Is

• $\mathrm{Mg(NH_4)PO_4}$: Mg²⁺ has no d-electrons → no d–d transitions → white.
• $\mathrm{K_3[Co(NO_2)_6]}$: Co(III) d⁶ LS with NO₂⁻ → small allowed transitions → yellow.
• $\mathrm{MnO(OH)_2}$: Mn(IV), d³, with O²⁻/OH⁻ ligands → broad absorption → brown.
• Prussian blue: charge-transfer (Fe²⁺ ↔ Fe³⁺ intervalence) → intense deep blue.

⚡ Diagnostic Tests Worth Memorising

| Cation | Reagent | Coloured product | |---|---|---| | Mg²⁺ | $\mathrm{(NH_4)_2HPO_4 + NH_3}$ | white $\mathrm{Mg(NH_4)PO_4}$ | | K⁺ | $\mathrm{Na_3[Co(NO_2)_6]}$ | yellow $\mathrm{K_3[Co(NO_2)_6]}$ | | Fe³⁺ | $\mathrm{K_4[Fe(CN)_6]}$ | blue Prussian blue |

⚠️ Don't Confuse Prussian Blue and Turnbull's Blue

Both are very dark blue and chemically identical (the same mixed-valence iron–cyanide compound), but historically named by the test direction:

• Prussian blue: Fe³⁺ + ferrocyanide.
• Turnbull's blue: Fe²⁺ + ferricyanide.

$\boxed{\text{Answer: (2) A-II, B-III, C-I, D-IV}}$