Question

When a metal rod $M$ is dipped into an aqueous colourless concentrated solution of compound $N$, the solution turns light blue. Addition of aqueous $NaCl$ to the blue solution gives a white precipitate $O$. Addition of aqueous $N{H}_3$ dissolves $O$ and gives an intense blue solution.
The metal rod $M$ is
(1) $Fe$
(2) $Cu$
(3) $Ni$
(4) $Co$

• A. Fe
• B. Cu
• C. Ni
• D. Co

Answer 1

Answer: (B)

Appearance of blue colour on addition of the metal rod $M$ to the solution of $N$ is an indication that metal may be $Cu$. Later formation of white precipitate on addition of $NaCl$ and dissolution of this precipitate in aqueous ammonia confirm that while precipitate is of $AgCl$. This implies that the solution $N$ is of $AgN{O}_3$. This confirm that the metal $M$ is only $Cu$.