Role of Cryolite in Aluminum Electrolysis

Steps:

• Recognize the process as Hall-Héroult electrolysis for aluminum extraction, where alumina dissolves in molten cryolite.
• Recall cryolite (Na₃AlF₆) lowers the melting point of the electrolyte to ~950°C and ionizes to provide conducting ions.
• Analyze options: A addresses ion provision; B misstates impurity handling; C and D confuse electrode reactions.
• Select A as it matches cryolite's electrolytic role.

Why A is correct:

• Cryolite ionizes in molten form to Na⁺, Al³⁺, and F⁻ ions, increasing ion concentration for electrical conduction in the electrolyte.

Why the others are wrong:

• B: Cryolite dissolves alumina but does not react with impurities to form slag; impurities are removed separately.
• C: The carbon cathode does not react significantly with oxygen and requires no regular replacement.
• D: The carbon anode reacts with oxygen, but the statement is incorrect in this context as A is the precise role highlighted.

Final answer: A