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Which among the following statement(s) is(are) true for the extraction of aluminium from bauxite?
$(A)$ Hydrated $Al _2 O _3$ precipitates, when $CO _2$ is bubbled through a solution of sodium aluminate.
$(B)$ Addition of $Na _3 AlF _6$ lowers the melting point of alumina.
$(C)$ $CO _2$ is evolved at the anode during electrolysis.
$(D)$ The cathode is a steel vessel with a lining of carbon.
$A,B,C$
$A,B,C,D$
$A,B$
$A,C$
Solution
$(A)$ $2 Na \left[ Al ( OH )_4\right]_{( aq .)}+ CO _2 \longrightarrow Na _2 CO _3+ H _2 O +2 Al ( OH )_3(\downarrow)$
or
$Al _2 O _3 .2 H _2 O \text { (ppt) }$
$(B)$ Function of $Na _3 AlF _6$ is to lower the melting point of electrolyte.
$(C)$ During electrolysis of $Al _2 O _3$, the reactions at anode are :
${\left[2 Al ^{3+}(\ell)+3 O ^{2-}(\ell) \xrightarrow{\text { At mode }} O _2( gas )+2 e ^{-}\right]}$
$C \text { (graphite })+ O _2 \longrightarrow CO (\uparrow)+ CO _2(\uparrow)$
$(D)$ The steel vessel with a lining of carbon acts as cathode.
