Question

why the solution of silver nitrate becomes blue in colour after sometime when copper turning are added to it.

Answer 1

When copper turnings are added to silver nitrate solution, the solution becomes brown in color after sometime because copper is more reactive than silver so it displaces silver from silver nitrate solution and form copper nitrate solution.

Answer 2

Copper on being dipped in Silver Nitrate solution forms Copper Nitrate (it displaces Silver due to their position in the reactivity series). The Copper Nitrate formed is blue in colour in water(thus the solution turns blue from colourless).

Now if you want to know why it is blue and not other other colour then then, the answer is that when light passes through the Copper Nitrate solution all the colours other than the blue colour gets abosrbed (the electrons absorb their energies). This leads to only blue colour reaching our eye and hence it looks to us as blue in colour.