Q.(a)

(a) Cryolite serves as a flux/solvent lowering alumina's melting point in aluminium extraction, and iodine enables a volatile-compound vapour-phase purification of zirconium; interhalogens, as a class, are covalent and more reactive than the parent halogens. OR (b) Borate is identified by a characteristic green-edged flame test, and borax has wide industrial/domestic uses from flux to antiseptic.

(a)(i)(1) Role of cryolite in aluminium extraction (Hall-Heroult process): pure alumina ($Al_2O_3$) has a very high melting point ($\approx2323\ K$), which would make electrolysis prohibitively energy-intensive. Cryolite, $Na_3AlF_6$, is mixed with the alumina to (i) lower the melting point of the mixture to around $1140\ K$, making electrolysis practical, and (ii) increase the electrical conductivity of the molten mixture, improving the efficiency of electrolysis.

(a)(i)(2) Role of iodine in refining zirconium (van Arkel process): impure zirconium is heated with iodine vapour in an evacuated vessel at a moderate temperature, forming volatile zirconium tetraiodide, $ZrI_4$, while non-volatile impurities are left behind. This vapour is then passed over a tungsten filament heated to a much higher temperature ($\approx1800\ K$), where $ZrI_4$ decomposes, depositing pure zirconium on the filament and releasing iodine gas, which is recycled to react with more impure metal.

(a)(ii) Properties of interhalogen compounds: (1) they are more reactive than the halogens themselves (except $F_2$), because the $X-X'$ bond between two different halogens is weaker than the $X-X$ bond of the more electronegative halogen; (2) they are covalent in nature (not ionic); (3) most are diamagnetic; (4) they readily hydrolyse in water to give a halide ion and an oxyhalide/oxyacid of the larger, less electronegative halogen.

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