(a) (i) When pyrolusite ore is fused with $ ext{KOH}$, in presence of air, a dark green coloured product 'A' is obtained which changes to purple coloured compound 'B' in acidic medium. (I) Write the formulae of 'A' and 'B'. (II) Write the ionic equation for the reaction when compound 'B' reacts with $ ext{Fe}^{2+}$ in acidic medium. (ii) Give reasons : (I) $ ext{Ce}^{4+}$ in aqueous solution is a good oxidising agent. (II) The actinoid contraction is greater from element to element than lanthanoid contraction. (III) $E^{\circ}_{\text{Zn}^{2+}/\text{Zn}}$ value is more negative than expected, whereas $E^{\circ}_{\text{Cu}^{2+}/\text{Cu}}$ is positive. OR (b) (i) While studying the periodic properties, Arti came across an abnormal behaviour in the atomic size of $ ext{Hf}$. She found that, even though $ ext{Hf}$ is placed below $ ext{Zr}$ in the same group, both have almost similar atomic sizes. (I) Which phenomenon is responsible for the above behaviour ? Define it. (II) Mention any other consequence of the above phenomenon. (ii) Give reasons for the following : (I) Transition metals exhibit catalytic properties. (II) Transition metals have high enthalpy of atomisation. (III) $ ext{Sc}$ is a transition element, while $ ext{Zn}$ is not.