Superconductors are derived from compounds of

The colour of $K_2Cr_2O_7$ changes from red-orange to lemon-yellow on treatment with aqueous $KOH$ because of:

Which from the following is the general formula of the compound obtained when lanthanoid $(Ln)$ reacts with carbon at elevated temperature?

Match the following:

List-$I$ (Aquated Ion)	List-$II$ (Colour)
$A. Ni^{2+}$	$I. \text{Violet}$
$B. Fe^{3+}$	$II. \text{Blue}$
$C. Mn^{3+}$	$III. \text{Yellow}$
$D. V^{4+}$	$IV. \text{Red}$
	$V. \text{Green}$

The reagent used for toning in photography is

Identify the correct $(T)$ and incorrect $(F)$ statements from the following:
$(a)$ $MnO_4^{2-}$ is diamagnetic.
$(b)$ $MnO_4^-$ is green and $MnO_4^{2-}$ is purple.
$(c)$ $K_2Cr_2O_7$ is yellow.
$(d)$ $K_2Cr_2O_7$ is orange.