JEE Main 2021 · 25 July · morning · Single correct · +4 / −1
In the leaching of alumina from bauxite, the ore expected to leach out in the process by reacting with NaOH is :
- (a) TiO₂
- (b) Fe₂O₃
- (c) ZnO
- (d) SiO₂
Answer and solution
Answer (d) SiO₂
Solution
Chemistry ma’am · 2 steps
Find the impurity actually present in bauxite that is acidic enough to dissolve in hot NaOH: that is silica.
- What leaching does. In the Bayer process, powdered bauxite is digested with hot concentrated NaOH. Amphoteric Al₂O₃ dissolves as sodium aluminate. Any impurity that is acidic or amphoteric dissolves along with it.
SiO₂ + 2NaOH → Na₂SiO₃ + H₂O
Oxide Nature Impurity in bauxite? Dissolves in NaOH? TiO₂ very weakly acidic, nearly inert yes no, stays in red mud Fe₂O₃ basic yes no, stays in red mud ZnO amphoteric no yes, but it is not present SiO₂ acidic yes yes, as sodium silicate - So. Silica is the bauxite impurity that leaches out with the alumina, as Na₂SiO₃. The answer is (d).
The idea to keep. Hot NaOH dissolves acidic and amphoteric oxides. In bauxite, Al₂O₃ goes into solution as aluminate and SiO₂ as silicate, while Fe₂O₃ and TiO₂ stay behind as red mud.
Answer from the NTA final answer key. Solution by PrepWiser.