Answer: KSCN gives blood-red with Fe 3+ (FeSCN2+); no colour with Fe 2+ ; K 4 [Fe(CN) 6 ] gives Turnbull's blue with Fe 3+.
- A KSCN gives blood-red with Fe<sup>3+</sup> (FeSCN2+); no colour with Fe<sup>2+</sup>; K<sub>4</sub>[Fe(CN)<sub>6</sub>] gives Turnbull's blue with Fe<sup>3+</sup>
- B Both Fe<sup>2+</sup> and Fe<sup>3+</sup> are said to give an identical colour reaction with every common test reagent under typical conditions
- C A simple pH measurement taken of the dissolved iron salt solution alone according to standard textbooks
- D A standard flame test performed by observing the colour of the emitted light in general practice as frequently described
Correct answer: A. KSCN gives blood-red with Fe<sup>3+</sup> (FeSCN2+); no colour with Fe<sup>2+</sup>; K<sub>4</sub>[Fe(CN)<sub>6</sub>] gives Turnbull's blue with Fe<sup>3+</sup>
Explanation: KSCN (potassium thiocyanate) forms an intensely blood-red complex [Fe(SCN)]2+ with Fe<sup>3+</sup>; Fe<sup>2+</sup> gives no colour with KSCN.
Aqueous solutions of transition metal ions show characteristic colours caused by electrons absorbing visible light to jump between split d-orbitals; the exact colour depends on the metal, its oxidation state, and surrounding ligands.
Concept context
Transition metals (d-block) and lanthanides/actinides (f-block). Known for coloured compounds, variable oxidation states, complex formation, and catalytic properties. Frequently tested in JEE and NEET.