Write the equation for the reaction where the aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide.

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Solution

In the Baeyer’s process, the bauxite ore is used to extract pure alumina.
Its first step involves the heating of the ore with caustic soda at $150^{°} C$ to form sodium aluminate.
Then sodium aluminate is diluted with water, then freshly prepared $A l {(O H)}_{3}$ is added to cause the precipitation and forming of $A l {(O H)}_{3}$ in the solution.
Then in the last step the precipitate is dried and ignited to form pure alumina. The reaction involved in this step is:
$2 A l {(O H)}_{3} \overset{△, 1100^{°} C}{\to} A l_{2} O_{3} + 3 H_{2} O A l u m i n i u m A l u m i n a V a p o u r s H y d r o x i d e$

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