Question

Bauxite ore is made up of ${ Al }_{ 2 }{ O }_{ 3 }+Si{ O }_{ 2 }+{ TiO }_{ 2 }+{ Fe }_{ 2 }{ O }_{ 3 }$. This ore is treated with conc. NaOH solution at 500K and 35 bar pressure for few hours and filtered, the species present are:
a) $NaAl{ (OH) }_{ 4 }$ only.
b) ${ Na }_{ 2 }Ti{ (OH) }_{ 6 }$ only
c) $NaAl{ (OH) }_{ 4 }$ and ${ Na }_{ 2 }Si{ O }_{ 3 }$
d) ${ Na }_{ 2 }Si{ O }_{ 3 }$ only

Answer 1

Hint: Bauxite ore is used for extracting Aluminium metal. It is purified by using Bayer process in which the ore is treated with concentrated sodium hydroxide at 500K and 35 bar pressure to dissolve the oxides of aluminium and silicon.

Complete step by step answer:
Bayer Process is used to purify the bauxite ore which is a mixture of ${ Al }_{ 2 }{ O }_{ 3 }+Si{ O }_{ 2 }+{ TiO }_{ 2 }+{ Fe }_{ 2 }{ O }_{ 3 }$. In this process, the first step involves the addition of a hot concentrated solution of sodium hydroxide to the ore. The alkali dissolve the oxides of aluminium (${ Al }_{ 2 }{ O }_{ 3 }$) and silicon ($Si{ O }_{ 2 }$), since these oxides are amphoteric and acidic in nature respectively while the oxides of Titanium ($Ti{ O }_{ 2 }$) and Iron (${ Fe }_{ 2 }{ O }_{ 3 }$) are basic in nature, therefore they do not react with sodium hydroxide and remain insoluble. The insoluble oxides are removed by filtration.
The reactions when sodium hydroxide reacts with the oxides of aluminium and silicon are given below:

${ Al }_{ 2 }{ O }_{ 3 }(s)+NaOH(conc.)\xrightarrow [ 500k ]{ 35\ bar } NaAl{ (OH) }_{ 4 }(aq)$

$Si{ O }_{ 2 }(s)+NaOH(conc.)\xrightarrow [ 500k ]{ 35\ bar } { Na }_{ 2 }Si{ O }_{ 3 }(aq)$

${ Al }_{ 2 }{ O }_{ 3 }$ is amphoteric in nature since it behaves both as an acid and as a base.

It reacts with hot dilute Hydrochloric acid and gives aluminium chloride. The reaction is given below:
${ Al }_{ 2 }{ O }_{ 3 }(s)+6HCl(dil.)\xrightarrow { \triangle } 2Al{ Cl }_{ 3 }(aq)+3{ H }_{ 2 }O(l)$

In $Si{ O }_{ 2 }$, the electronegativity of O and Si are very similar, therefore it does not have ionic character. Due to this it does not contain O as ${ O }^{ 2- }$ ions and hence it is not basic in nature since it is the ${ O }^{ 2- }$ ions present in an oxide give the oxide its basic nature. ${ O }^{ 2- }$ ions are a very strong base and have a high tendency to accept hydronium ions. In fact $Si{ O }_{ 2 }$ is acidic in nature. It reacts with calcium oxide which is basic in nature to give calcium silicate. The reaction is given below:

$Si{ O }_{ 2 }+CaO\longrightarrow CaSi{ O }_{ 3 }$

Therefore the correct option is (c) $NaAl{ (OH) }_{ 4 }$ and ${ Na }_{ 2 }Si{ O }_{ 3 }$.

Note: In the periodic table, the acidity of the oxides increases as we move across a period. The basicity of the oxides decreases as we move across a period. The most basic oxides lie on the left while the most acidic oxides lie on the right in the periodic table with the amphoteric oxides lying in the middle. However, this trend is true only for those oxides in which the elements belong to the third period of the periodic table and are present in their highest oxidation states.