$Cu^{2+}$ aqueous solution has . . . . . . colour.

Give the formulas of the oxocations of the $3d$ series metals.

The yellow colour solution of $Na_2CrO_4$ changes to orange-red on passing $CO_2$ gas due to the formation of

Fusion of $MnO_2$ with $KOH$ in presence of $O_2$ produces a salt $W$. Alkaline solution of $W$ upon electrolytic oxidation yields another salt $X$. The manganese containing ions present in $W$ and $X$,respectively,are $Y$ and $Z$. Correct statement$(s)$ is (are)
$(1)$ $Y$ is diamagnetic in nature while $Z$ is paramagnetic
$(2)$ Both $Y$ and $Z$ are coloured and have tetrahedral shape
$(3)$ In both $Y$ and $Z$,$\pi$-bonding occurs between $p$-orbitals of oxygen and $d$-orbitals of manganese.
$(4)$ In aqueous acidic solution,$Y$ undergoes disproportionation reaction to give $Z$ and $MnO_2$.

When a small amount of vanadium is added to steel,it becomes:

The color of $KMnO_4$ is due to: