CGPET 2013 Solved Paper

Most of the compounds of transition metals are coloured in solid as well as solution state. Colour in transition metal ions is usually due to electron transition within the $d$-shell. Intensely coloured ions with the metal in its highest oxidaton state (e.g., $\mathrm{Mn}^{7+}$, $\mathrm{Cr}^{6+}, \mathrm{Fe}^{6+})$ derive the colour from electrons transitions between the metal and the oxygen atom.Iron(i) $\mathrm{Fe}^{2+}$ (ferrous ion) is green.(ii) $\mathrm{Fe}^{3+}$ (ferric ion) is pale yellow in colour.Chromium(i) The colour of chromic ion, i.e., $\mathrm{Cr}^{3+}$ depends around the ligand around the ion. Aqueous solution contains the violet octahedral hexaaquachromium (III) ion, but when some of the water ligands are replaced by other species, such $\mathrm{Cr}^{3+}$ ions are green.$[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})_6]^{3+} (\mathrm{aq}) + 3 \mathrm{OH}^{-} (\mathrm{aq})$Violet,octahedral$\rightarrow [\mathrm{Cr}(\mathrm{OH})_3(\mathrm{H}_2\mathrm{O})_6] (\mathrm{s}) + 3 \mathrm{H}_2\mathrm{O} (\mathrm{l})$Green, octahedral (ii) In chromate ion $(\mathrm{CrO}_4^{2-}) , \mathrm{Cr}$ occurs in $+6$ oxidation state and it is yellow in colour.