Question

Which of the following metal strips when placed in colourless AgNO₃ solution, turns the solution blue?
(1) Fe
(2) Cu
(3) Mg
(4) Zn

Answer 1

Copper is more reactive than silver, it displaces silver from silver nitrate solution and forms blue coloured solution of copper nitrate.

$2\underset{\left(\mathrm{brown}\right)}{\mathrm{Cu}}\left(\mathrm{s}\right)+\underset{\left(\mathrm{colourless}\right)}{{\mathrm{AgNO}}_3}\left(\mathrm{aq}\right)\to \underset{\left(\mathrm{blue}\right)}{\mathrm{Cu}({\mathrm{NO}}_3{)}_2}\left(\mathrm{aq}\right)+\underset{\left(\mathrm{grey}\right)}{\mathrm{Ag}}\left(\mathrm{s}\right)$

Hence, the correct answer is option 2.