Q25 / 160 · AP EAMCET 2018 (24 Apr Shift 1) · Chemistry

The pair of oxides, which can be leached out, when powdered bauxite containing certain impurities is digested with concentrated solution of $\mathrm{NaOH}$ at $473-523 \mathrm{~K}$ temperature and $35-36$ bar pressure are

  • A
    $\mathrm{TiO}_2, \mathrm{SiO}_2$
  • Correct answer
    B
    $\mathrm{SiO}_2, \mathrm{Al}_2 \mathrm{O}_3$
  • C
    $\mathrm{SiO}_2, \mathrm{Fe}_2 \mathrm{O}_3$
  • D
    $\mathrm{Al}_2 \mathrm{O}_3, \mathrm{Fe}_2 \mathrm{O}_3$

Solution

Bauxite is the most important ore of aluminium contains only $30-60 \% \mathrm{Al}_2 \mathrm{O}_3$, the rest being a mixture of $\mathrm{SiO}_2, \mathrm{Fe}_2 \mathrm{O}_3$ and $\mathrm{TiO}_2$. The powdered bauxite is treated with concentrated solution of $\mathrm{NaOH}$ resulting in the dissolution of $\mathrm{Al}_2 \mathrm{O}_3$ and $\mathrm{SiO}_2$ while the other oxides $\left(\mathrm{Fe}_2 \mathrm{O}_3\right.$ and $\mathrm{TiO}_2$ ) being basic remain insoluble in $\mathrm{NaOH}$. Hence, only $\mathrm{SiO}_2$ and $\mathrm{Al}_2 \mathrm{O}_3$ leached out.

Hence, the correct answer is option: 'B'.

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