Which pair of ions gives a blue colour in an aqueous state?
$V^{+4}, Ni^{+2}, Ti^{+3}, Co^{+2}, Fe^{+3}, Cu^{+2}$

The correct statement about iron includes:
$I$) The highest oxidation state of iron is $+ 6$ in $K_2FeO_4$.
$II$) The iron shows $+ 2$ oxidation state with six electrons in the $3d$ orbitals.
$III$) The common oxidation state of iron is $+ 3$ with five unpaired electrons in the $3d$ orbital.

The transition metal having the highest $3^{rd}$ ionisation enthalpy is:

Which of the following types of metals form the most effective catalysts?

Highest $(+7)$ oxidation state is shown by

Which of the following metals exhibits more than one oxidation state?