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Thread: How does bleach work?

  1. #1 How does bleach work? 
    Forum Professor Daecon's Avatar
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    I'm hoping this is appropriate for the "Chemistry" forum

    I was wondering, how does bleach work when it removes colouration from fabrics, what's the process that happens to the dyes and pigments?

    When bleach removes a stain, does it actually remove the substances that the stain is made of, or does it just whiten the colour?


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    Bullshit Intolerant PhDemon's Avatar
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    Bleaches usually contain a strong oxidising agent (usually sodium hypochlorite or similar) which "oxidise the shit out of"1 (technical term) organics and other stuff that might stain fabrics. The breakdown products may then be easier to remove or more soluble and easier to get rid of but this isn't necessarily the case.

    1. or breaks down the organic molecules so the part that absorbs visible light no longer does so if you're being picky


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    Forum Professor jrmonroe's Avatar
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    There's a really great website all about color (the ways colors are created, how, why, etc), and I thought I could find it quickly, but it's not showing in the search results and I don't remember it's name. I don't have time now, but I'll continue to look later.
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  5. #4  
    exchemist
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    Quote Originally Posted by Daecon View Post
    I'm hoping this is appropriate for the "Chemistry" forum

    I was wondering, how does bleach work when it removes colouration from fabrics, what's the process that happens to the dyes and pigments?

    When bleach removes a stain, does it actually remove the substances that the stain is made of, or does it just whiten the colour?
    Just to expand a bit on PhDemon's reply, organic compounds with a colour must absorb light in the visible region of the spectrum. The absorption involves an electron making a transition from a low energy molecular orbital to a higher energy one. The energy levels of molecular orbitals depend on their shape and the numbers of atoms involved. Generally speaking dyes and other coloured organic materials have systems of "conjugated" bonds (alternate single and double bonds in conventional drawings of them), which provide a system of orbitals involving multiple atoms, and these often have orbital transitions of low enough energy to occur in the visible.

    A bleach will often "saturate" one or more of the double bonds, by adding atoms across it. This changes the orbital system to one in which the transitions occur at higher energy, in the UV. The new substance thus formed therefore has no colour. Compounds that release active oxygen or chlorine are typically used in bleaches as they easily add across double bonds.

    This, by the way, is why bleaches generally cannot remove inorganic stains due to rust or other metal-containing compounds, as the colour in these arises from different bonding that is not susceptible to these addition reactions.

    I generalise and I'm sure there are example of bleaches that work via different chemical mechanisms, but in all cases the aim is to break up the bonding scheme that provides the visible absorption.

    So to answer your final question directly, the bleach reacts with the coloured compound and changes it to one that is not coloured. It does not have to remove it physically.
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