In the metallurgy of \(Al\), purified\(Al_2O_3\) is mixed with \(Na_3AlF_6\) or \(CaF_2\). The role of \(Na_3AlF_6\) or \(CaF_2\) is to:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Lower the melting point of Al2O3
Pure alumina melts at about 2323 K, far too high for a practical electrolysis; with cryolite or \( \ce{CaF2} \) the mix melts at about 1173 K and also conducts.
Why the other options are incorrect
Option (1) Decrease the conductivity — Incorrect. The additive increases the conductivity of the melt; a lower conductivity would hinder the electrolysis.
Option (3) Reduce Al2O3 — Incorrect. \( \ce{Al^{3+}} \) is reduced at the cathode by the electric current, not by cryolite.
Option (4) Oxidize Al2O3 — Incorrect. Aluminium in \( \ce{Al2O3} \) is already +3; the oxide ions are oxidised at the anode, again by the current.
NCERT (Pre-rationalised edition, 2022-23):
Aluminium
