Question 28

Aluminium sulphate is treated with excess of caustic soda and ammonium hydroxide separately

Solution

Both reagents first give a white precipitate of $$\text{Al(OH)}_3$$. With excess NaOH it dissolves as sodium aluminate, $$\text{Al(OH)}_3 + \text{NaOH} \rightarrow \text{NaAlO}_2 + 2\text{H}_2\text{O}$$, giving a clear solution. $$\text{Al(OH)}_3$$ does not dissolve in excess ammonium hydroxide, so only caustic soda gives a clear solution.

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