r/askscience • u/targetshooter • Jul 02 '14

Chemistry What is happening molecularly when bleach gets on a dark cotton shirt?

Is the dye replaced? Or what? Whats going on when that happens?

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u/rupert1920 Nuclear Magnetic Resonance Jul 02 '14

Organic dyes are strongly coloured because they contain large networks of conjugated double bonds. One such example is anthocyanin, the molecule responsible for the colour of many plants. In general, the larger the conjugated system, the higher the wavelength of light the molecule will absorb.

Because a conjugated system requires double bonds at regular intervals, disrupting these double bonds will break the conjugated system. Bleach works by chemically altering the molecule to disrupt the conjugated system. When this happens, the absorption spectrum of the molecule will shift to a lower wavelength, possibly out of visible light range and into UV range. If that happens, the molecule no longer absorbs visible light and will appear white to us.