Why does the addition of oxygen render materials colourless? [closed]








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In our book, it is mentioned that $\ce{H2O2}$ (hydrogen peroxide) is used as a bleaching agent because of its oxidizing nature:
\begin{align}
 \ce{H2O2 &-> H2O + [O]}\\
 \ce{\text{coloured material} + [O] &-> \text{colourless material}}
\end{align}


Why does the second equation happen? In other words, what role does oxygen play in the absorption of light here?




Many organic dyes (arguably, *all* of them) produce color due to electron transitions in [conjugated $\pi$ systems](https://en.wikipedia.org/wiki/Conjugated_system). Strong oxidants (not just the peroxide, mind you, but also hypochlorite bleach and others) snap the double bonds and thus destroy these conjugated systems. So the color vanishes.


This is not applicable to *inorganic* dyes, which usually would not get bleached by peroxide. On the contrary, some transition metals ($\ce{Ti^4+,\;Cr^6+}$, and some others) which have little or no color by themselves would produce intensely colored complexes when hydrogen peroxide is added. These reactions are sometimes used in analytic chemistry to discover their presence.


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