The role of cryolite in the metallurgy of aluminium is:

The correct answer is Option (2) → to lower the melting point of mixture

Aluminium is obtained by the electrolysis of fused alumina (Hall–Héroult process). Pure \( \ce{Al2O3} \) melts above 2300 K and conducts poorly when fused. It is therefore mixed with cryolite, Na₃AlF₆ (or \( \ce{CaF2} \)), which lowers the melting point of the mix (to about 1240 K) and brings conductivity. The fused mixture is then electrolysed with a steel cathode and graphite anodes:

    \( \displaystyle \text{Cathode: }\ce{Al^3+(melt) + 3e^- -> Al(l)} \)

    \( \displaystyle \text{Anode: }\ce{C(s) + O^2-(melt) -> CO(g) + 2e^-} \)

    \( \displaystyle \text{Anode: }\ce{C(s) + 2O^2-(melt) -> CO2(g) + 4e^-} \)

Why the other options are incorrect

• Option (1) to increase the solubility of mixture — Incorrect. Alumina does dissolve in the molten cryolite, but the purpose of adding cryolite is the lower melting point and better conductivity of the melt, not a higher “solubility of the mixture”.

• Option (3) to remove impurities from the mixture — Incorrect. Impurities are removed earlier, when bauxite is leached with NaOH to give pure alumina (Baeyer process); cryolite does not purify.

• Option (4) to prevent froth formation — Incorrect. Froth is a feature of froth flotation, used to concentrate sulphide ores; it plays no part in the electrolysis of alumina.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Aluminium – “In the metallurgy of aluminium, purified Al₂O₃ is mixed with Na₃AlF₆ or CaF₂ which lowers the melting point of the mix and brings conductivity.”