Mohr's salt is ______ in colour.

The correct answer is Option (3) → Green

Mohr’s salt is the double salt ferrous ammonium sulphate, \( \ce{FeSO4} \)·\( \ce{(NH4)2SO4} \)·\( \ce{6H2O} \). Like every double salt it dissociates completely in water into simple ions: \( \ce{Fe^{2+}} \), \( \ce{NH4^{+}} \) and \( \ce{SO4^{2-}} \). The \( \ce{NH4^{+}} \) and \( \ce{SO4^{2-}} \) ions are colourless, so the colour comes from the hydrated Fe²⁺ ion.

Fe (Z = 26) is [Ar]3d⁶4s²; removing the two 4s electrons gives \( \ce{Fe^{2+}} \) = [Ar]3d⁶, a partly filled d subshell. A d–d transition absorbs light in the red region of the visible spectrum, and the hydrated \( \ce{Fe^{2+}} \) ion appears pale green. Mohr’s salt crystals and their solution are therefore green.

NCERT (2026-27 edition):
Formation of Coloured Ions – …

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