Q.(a)

(a) Gravity separation exploits a density difference to wash away lighter gangue with flowing water, and concentrated sulphuric acid's strong affinity for water lets it char sugar and decompose formic acid by removing the elements of water. OR (b) silicones' water-repellent and insulating properties give them varied industrial uses, and the Fischer-Tropsch process turns coal-derived syngas into synthetic hydrocarbon fuel.

(a)(i) Gravity separation process: this concentration method exploits the difference in density (specific gravity) between the valuable ore particles and the lighter, unwanted gangue. The finely crushed ore is washed with a strong, controlled current of water (often on an inclined, vibrating table — a hydraulic classifier or a jig). The heavier ore particles settle down and are collected, while the lighter siliceous/earthy gangue particles are carried away by the flowing water. This method is used for concentrating oxide ores such as haematite ($Fe_2O_3$) and tinstone ($SnO_2$).

(a)(ii) $H_2SO_4$ as a dehydrating agent (with examples): concentrated sulphuric acid has a very strong affinity for water and can remove the elements of water (H and O, in a 2:1 ratio) even from organic compounds that do not contain free water, by extracting hydrogen and oxygen from the compound's own structure. Example 1: it chars cane sugar, extracting the water content and leaving behind a black mass of carbon: $C_{12}H_{22}O_{11} \xrightarrow{conc.\ H_2SO_4} 12C + 11H_2O$ Example 2: it dehydrates formic acid to carbon monoxide: $HCOOH \xrightarrow{conc.\ H_2SO_4} CO + H_2O$

OR (b)(i) Uses of silicones (any three): (1) as water-repellent/water-proofing coatings for fabrics, wood, and paper (due to their low surface tension); (2) as electrical insulators, owing to their good dielectric strength and thermal stability; (3) as high-temperature lubricating greases/oils, stable over a wide temperature range; (4) as sealants and adhesives in construction and automotive applications; (5) in surgical and cosmetic implants, due to their biocompatibility.

…