How Does Bleach Work? Chlorine, Oxygen and the Chemistry of Taking Colour Away
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A splash of bleach turns a red shirt pink, a yellow stain white and a bacterial colony dead, and it does all three by the same reaction. Colour in a dye or a stain comes from molecules with long chains of alternating bonds that absorb visible light, and bleach breaks those chains so that the molecule no longer absorbs and the colour is gone; the same attack on the proteins and membranes of a microbe is what makes it a disinfectant. The chemistry was worked out in France in the 1780s, the household product has been the same since 1913, and the warning on the bottle about mixing it with other cleaners is not a formality.
What colour is
A substance is coloured because its molecules absorb some wavelengths of light and reflect the rest, and the molecules that absorb visible light are mostly ones with a chromophore, a stretch of atoms joined by alternating single and double bonds along which electrons are spread out and can be excited by a photon of visible energy. The longer the chain, the longer the wavelength absorbed; a short chain absorbs ultraviolet and looks colourless, a long one absorbs blue and looks orange. Dyes, the pigments in tomato sauce, grass, blood and coffee, and the melanin in a coloured fabric are all such chains, and breaking the chain anywhere along it, or adding atoms across the double bonds, shortens the stretch over which electrons can move, so the molecule stops absorbing visible light and appears white.
How bleach breaks it
Household bleach is sodium hypochlorite in water, at about five percent, and the hypochlorite ion is an oxidiser: it takes electrons from other molecules, and in doing so adds oxygen or chlorine across double bonds and breaks them. A chromophore that has been oxidised is a shorter chain, or two, and it is colourless; a stain of the same kind on a white shirt disappears for the same reason. The other bleaches do the same by other routes:
- •Chlorine bleach, sodium hypochlorite: strong, fast, cheap, and hard on fabric and colour; the product for whites, drains and disinfection
- •Oxygen bleach, hydrogen peroxide and the percarbonate in colour-safe powders: milder, releasing oxygen that oxidises stains without attacking most dyes, and breaking down to water
- •Sulphur dioxide and sulphites: reducing bleaches, which work by adding electrons rather than taking them, used for wool, silk, paper and wine
- •Sunlight: ultraviolet light breaks chromophores directly, which is why laundry was spread on grass and why curtains fade
Killing microbes
The same oxidation that destroys a chromophore destroys a cell. Hypochlorite attacks the proteins in a bacterial membrane and the enzymes inside it, oxidising the sulphur-containing amino acids that many enzymes depend on and rupturing the membrane, and it does it within seconds to most bacteria, viruses and fungi at concentrations of a few hundred parts per million; a capful in a bucket disinfects a surface, and the chlorination of drinking water since 1908 uses the same ion at about one part per million. It is indiscriminate, which is its virtue, since microbes cannot evolve resistance to having their proteins oxidised in the way they do to an antibiotic's specific attack, and its limitation, since it oxidises skin, lungs and fabric with equal indifference.
Why not to mix it
The warning on the bottle is about two reactions. Bleach mixed with an acid, which includes many toilet cleaners, limescale removers and vinegar, releases chlorine gas, the weapon of the First World War, which damages the lungs at low concentrations and has killed people cleaning bathrooms. Bleach mixed with ammonia, which is in some glass and floor cleaners, makes chloramines, which are less lethal and still send people to hospital. Bleach also decomposes slowly on its own, losing strength over months and faster in heat and light, which is why the bottle is opaque and why old bleach does not work; and it corrodes metal, which is why it is not used on stainless steel for long.
A short history
Cloth was bleached for millennia by soaking it in sour milk or lye and spreading it in the sun for weeks on bleaching fields, and the linen industry of the Netherlands and Scotland depended on acres of them. The French chemist Claude Berthollet found in 1785 that chlorine, discovered eleven years earlier by Scheele, bleached cloth in hours, and dissolved it in a potash solution to make a liquid, eau de Javel, named for the Paris suburb where it was made; the Scottish bleaching powder of 1799, chlorine absorbed into lime, made the process portable and cut the time from months to a day, which was as important to the cotton industry as the spinning jenny. Sodium hypochlorite in a bottle was sold as a household product from 1913, and the discovery that it killed germs, by Semmelweis's contemporaries in the 1840s and definitively in the Great War, when a solution of it was used to irrigate wounds, gave the laundry chemical its second job.
The takeaway
Bleach removes colour by oxidation, breaking the chains of alternating bonds in dye and stain molecules that absorb visible light so that they no longer do, and it kills microbes by oxidising the proteins and membranes that keep them alive, indiscriminately and without resistance developing. Chlorine bleach is sodium hypochlorite, oxygen bleach is peroxide and gentler on colours, mixing bleach with acid releases chlorine gas and with ammonia chloramines, and the chemistry dates from Berthollet's discovery of 1785 that chlorine could do in hours what sunlight did in weeks.