Science Questions

How does bleach bleach things?

Sat, 20th Oct 2012

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Question

Barbara, Wisbech asked:

I was wondering why bleach, when you put it on certain things and the colour disappears and yet, I can put it on my draining board and nothing happens to this. So where does the colour actually go?

Answer

Chris -   Bleach is a fairly common word which is used to describe a range of different things.  Some forms of bleach and the household bleach that you put down the loo and stuff has usually got a lot of chlorine in it.  In fact, the active chemical is a substance called sodium hypochlorite.  Its the same stuff that they put in swimming pools. 

Other forms of bleach include hydrogen peroxide the stuff that you bleach your hair with and you can also use for mouthwash and teeth whitening, and say, sterilising your contact lenses for example.  But they're called bleaches because they can react with things to take the colour out of them like hair. 

Now the way it works is that these molecules are reactive.  They're oxidising agents and things that have colour are usually big, chunky molecules that have lots of electrons going around the molecule, and when light comes in, the light waves which are little packets called photons interact with the electrons in substances and certain colours of light will be preferentially absorbed by those electrons whilst other colours will be reflected. 

So when you see something and it looks say, red, the reason it looks red is because all of the light hitting it is being absorbed by the molecules except the red colours which are being reflected back to you. 

When you mix a certain chemical with these oxidising agents like hypochlorite or contact lens solution hydrogen peroxide, the oxidising effect breaks open the molecule, damaging it or affecting its structure in some way, so that the electron cloud which was doing that absorbing now changes its shape. And that has the effect of stopping the chemical from absorbing certain colours.  So it goes white, instead of it absorbing certain colours and not others, now it reflects everything back at you and when you mix all the different colours together, you see white. 

So basically, you're changing the chemical structure of the thing that was giving the substance its colour in the first place.

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