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Q.(a) When bauxite is treated with sodium hydroxide solution, what happens to :
(i) the aluminium oxide,
(ii) the iron (III) oxide.

Dear student
(i)Bauxite ore is a mixture of hydrated aluminium oxides and compounds of other elements such as iron. In the Bayer process, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution (caustic soda) at a temperature of 150 to 200 °C.
This process converts the aluminium oxide (Al2O3) in the ore to soluble sodium aluminate, NaAlO2. The reaction is:
                                                  Al2O3 + 2 NaOH → 2 NaAlO2 + H2O
(ii) The undissolved waste after the aluminium compounds are extracted, bauxite tailings, contains iron oxides, silica, some unreacted alumina. To remove them the alkaline solution was cooled and treated by bubbling carbon dioxide through it, a method by which aluminium hydroxide precipitates:
                                 2 NaAlO2 + 3 H2O + CO2 → 2 Al(OH)3 + Na2CO3

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Aluminium oxide dissolves forming sodium meta aluminate. Iron oxide forms a reddish brown ppt.
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