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NCERT Class XII Chemistry
Chapter - General Principle of Process of Isolation of Elements
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Question : 29 of 31
Marks: +1, -0
What is the significance of leaching in the extraction of aluminium?
Solution:  
Leaching is significant as it helps in removing the impurities likeSiO2,Fe2O3SiO_2, Fe_2O_3, etc. from the bauxite ore.
Al2O3(s)↙Alumina+2NaOH(aq)+3H2O(l) →473−523K2Na[Al( OH)4 ](aq)↙Sodiummeta−aluminate{{Al_2O_3}_(s)} ↙{ Alumina }+ {2NaOH}_({aq} )+3 {H}_{2} {O}_{(l)}\ \xrightarrow[]{473-523K}{2 {Na} [ {Al}(\ {OH})_{4}\ ]_{(a q)}} ↙{Sodium meta-aluminate}
SiO2(l)Silica+2 NaOH(aq)→473−523 KNa2SiO3(aq)Sodium silicate+H2O(l)\underset{Silica}{SiO_{2(l)}} + 2\ NaOH_{(aq)} \xrightarrow[]{473-523\ \text{K}} \underset{\text{Sodium silicate}}{Na_2SiO_{3(aq)}} + H_2O_{(l)}
2Na[Al(OH)4](aq)+2CO2(g)⟶Al2O3⋅xH2O(s)↓+2NaHCO3(aq)2 Na [Al(OH)_4]_\text{(aq)} + 2 CO_{2\text{(g)}} \longrightarrow Al_2 O_3 \cdot x H_2 O_\text{(s)} \downarrow + 2 NaHCO_3\text{(aq)}
 Al2O3⋅xH2O(s)→1473 KAl2O3(s)+xH2O(g)\ {Al}_{2} {O}_{3} \cdot x {H}_{2} {O}_{(s)} \xrightarrow[]{1473\ \mathrm{K}}{Al}_{2} {O}_{3( {s})}+x {H}_{2} {O}_{(g)}
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