Why do oxidation-reduction reactions cause a color change?

Answer 1

Take for example TiO2 ,the common white pigment found in paint, in this the titanium is in oxidation state 4+ and has no electrons in its d orbitals. When it is reduced to 3+ by the addition of an electron it has a violet color. This color is due to the fact that it absorbs light energy equivalent to the difference in the energy levels of its d orbitals and appears then as violet. There are many other examples in transition metal chemistry try Cotton and Wilkinson for a fuller answer . Many redox reactions show a color change because in many redox reactions dissolved ions are being precipitated out of solution, and metals are being dissolved, forming new ions. Therefore there is an exchange of the type of ions that are present in the solution and some ions have different colors than other ones. For instance, if you dip a piece of copper metal (solid) in a solution of aqueous silver nitrate (dissolved), then a redox reaction will occur. The silver ions will be reduced and become silver metal, precipitating out of the solution, and the copper metal will be oxidized to form copper ions in solution. There is a color change associated with this particular reaction because silver ions are colorless, but copper ions in solution have a nice blue color. And so you see the solution go from colorless to blue due to the exchange of silver ions for copper ions.

Answer 2

it depends on whats in the chemicals