Question

give on example for the change in substance's colour during a chemical reaction
write the chemical reaction for the answer

Answer 1

When a few drops of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.

Answer 2

Copper reacts with oxygen, H2O and CO2 to give copper carbonate, which changes color from brown to blue or blue green. Rusting, blackening of surfaces of cut vegetables and fruits are other examples of change of color in a chemical reaction.