(a) Sodium hydroxide solution is added to solution A. A white precipitate is formed which is insoluble in excess sodium hydroxide solution. Name the metal ion present in solution A.
(b) When ammonium hydroxide is added to solution B, a pale blue precipitate is formed. This pale blue precipitate dissolves in excess ammonium hydroxide to give an inky blue solution. Name the cation present in solution B. What is the probable colour of solution B?

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Solution

(a) The metal ion present in solution A is the aluminium (Al³⁺) ion. Aluminium oxide or aluminium hydroxide reacts with sodium hydroxide solution to form an insoluble white precipitate of sodium aluminate.
$A l (O H_{3}) (s) + N a O H (a q) ⟶ N a A l O_{2} (s) + 2 H_{2} O (l)$
$A l_{2} O_{3} (s) + 2 N a O H (a q) ⟶ 2 N a A l O_{2} (s) + H_{2} O (l)$

(b) The metal ion present in solution B is the copper (Cu²⁺) ion. The probable colour of solution B should be deep blue. Cupric salts react with ammonium hydroxide to form a pale blue precipitate of copper hydroxide, which on further treatment with an excess of ammonium hydroxide forms soluble complex salt of copper that is deep blue in colour.
$C u S O_{4} (s) + 2 N H_{4} O H (a q) ⟶ C u {(O H)}_{2} (s) + {(N H_{4})}_{2} S O_{4} (s)$
$C u {(O H)}_{2} (s) + {(N H_{4})}_{2} S O_{4} (s) + 2 N H_{4} O H (a q) ⟶ C u {(N H_{3})}_{4} S O_{4} (s) + 4 H_{2} O (l)$

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