Question

In which reaction colourless become green?

Answer 1

Answer:

The only sulfate which is aqueous and less reactive than Iron is H2SO4. So if you drop an Iron piece in an aqueous solution of H2SO4, then you can expect this reaction.

Fe +  H2SO4 -----> FeSO4 + H2

Here the color of the solution changes from colorless (H2SO4) to green (FeSO4).