Question

Colourless solutions of the following four salts are placed separately in four different test tubes and a strip of copper is dipped in each one of these. Which solution will turn blue?

(A) $\mathrm{KNO}_3$

(B) $\mathrm{AgNO}_3$

(C) $\mathrm{Zn}\left(\mathrm{NO}_3\right)_2$

(D) $\mathrm{ZnSO}_4$

Answer 1

$\mathrm{As} \mathrm{Cu}$ is more reactive than $\mathrm{Ag}$.

$\mathrm{Cu}+2 \mathrm{AgNO}_3 \rightarrow \mathrm{Cu}\left(\mathrm{NO}_3\right)_2+2 \mathrm{Ag}$

$\mathrm{Cu}^{2+}$ will give blue solution. $\mathrm{Cu}$ is below in electrochemical series than $\mathrm{K}$ and $\mathrm{Zn}$ hence, no reaction takes place with $\mathrm{KNO}_3, \mathrm{Zn}\left(\mathrm{NO}_3\right)_2$ and $\mathrm{ZnSO}_4$.

So, The correct option is (B).