Answer: KSCN gives blood-red with Fe3+ (FeSCN2+); no colour with Fe2+; K4[Fe(CN)6] gives Turnbull's blue with Fe3+.
- A KSCN gives blood-red with Fe3+ (FeSCN2+); no colour with Fe2+; K4[Fe(CN)6] gives Turnbull's blue with Fe3+
- B Both Fe2+ and Fe3+ 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 Fe3+ (FeSCN2+); no colour with Fe2+; K4[Fe(CN)6] gives Turnbull's blue with Fe3+
Explanation: KSCN (potassium thiocyanate) forms an intensely blood-red complex [Fe(SCN)]2+ with Fe3+; Fe2+ 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.