Q.(a)

(a) Gravity separation concentrates an ore by density difference in flowing water, and Mond's process purifies nickel via the volatile carbonyl $Ni(CO)_4$; (b) the inert pair effect explains why heavier p-block elements favour a lower oxidation state, and boric acid has several everyday/industrial uses.

(a)(i) Gravity separation (levigation) method:

This method of ore concentration is based on the difference in density (specific gravity) between the ore particles and the unwanted earthy/rocky impurities (gangue), and is used mainly for oxide ores (e.g. haematite, $Fe_2O_3$) that are typically heavier than the gangue. The powdered ore is washed on a sloping, vibrating table (a hydraulic classifier or jigging apparatus) in a strong current of water. The lighter gangue particles are carried away by the flowing water, while the heavier ore particles, being denser, settle down and are collected near the top/near the water inlet of the sloping surface. This is a purely physical, gravity-based concentration step, carried out before the actual metallurgical extraction (reduction) begins.

(a)(ii) Mond's process (refining nickel):

Impure nickel (obtained after the initial extraction/smelting steps, still contaminated with iron, cobalt etc.) is heated in a stream of carbon monoxide gas at a relatively low temperature of about 330-350 K. Under these conditions, nickel selectively combines with $CO$ to form a volatile compound, nickel tetracarbonyl:

$Ni(s) + 4CO(g) \xrightarrow{330-350\,K} Ni(CO)_4(g)$

The impurities (iron, cobalt, etc.) do not form such volatile carbonyls under these conditions and are left behind as a solid residue. The volatile $Ni(CO)_4$ vapour is then carried to a separate chamber and heated to a higher temperature, about 450-470 K, at which it decomposes back into pure nickel metal and carbon monoxide gas (which is recycled to the first stage):

$Ni(CO)_4(g) \xrightarrow{450-470\,K} Ni(s) + 4CO(g)$

This gives nickel of very high purity and is a classic example of purification via a volatile compound.

(b)(i) Inert pair effect: …