How Does Bleach Work? Chlorine, Oxygen and the Chemistry of Taking Colour Away

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.

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

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.

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.

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.

A common mistake is to treat bleach as a cleaner. It is not very good at removing dirt, grease or grime, and it can be defeated by them. Because hypochlorite reacts with almost any organic material, a surface covered in food residue, soil or body fluids uses up much of the bleach before it can reach the microbes hidden underneath. That is why hospital and food-hygiene guidance usually says to clean a surface with detergent and water first, rinse, and only then apply a disinfectant for the recommended contact time.

The same chemistry explains a familiar swimming-pool myth. Many people think a strong chlorine smell means a pool is extra clean, but a well-run pool smells of very little. The sharp odour, and much of the eye irritation, comes mostly from chloramines, formed when chlorine reacts with sweat, urine and other nitrogen-containing matter that swimmers bring in. A strong smell therefore suggests the chlorine is busy reacting with contamination rather than a pool with plenty to spare. Showering before swimming reduces the problem, because less organic matter enters the water in the first place.

Your immune system worked out the chemistry of bleach long before Berthollet did. Neutrophils, the most common white blood cells, swallow bacteria into a small sealed pocket and then flood it with toxic chemicals. One of the most powerful is hypochlorous acid, the same active substance that forms when household bleach is diluted, made by an enzyme called myeloperoxidase from hydrogen peroxide and the chloride in body fluids. The enzyme is green, and neutrophils contain so much of it that it gives pus, and the mucus of a heavy cold, part of their greenish tinge.

The acid form matters outside the body as well. In water, chlorine bleach exists in two forms that convert into each other depending on acidity: hypochlorous acid, which is uncharged and slips easily through bacterial membranes, and the hypochlorite ion, which is a far weaker disinfectant. Bottled bleach is kept strongly alkaline, mostly as the ion, because that keeps it stable on the shelf. Swimming pools are held close to neutral, at a pH of about 7.2 to 7.6, partly so that a good share of the chlorine is in the faster-acting acid form without making the water irritating to swimmers.

The bleach used on hair is not the chlorine bleach used for cleaning. Hair lighteners usually rely on hydrogen peroxide, often combined with ammonia or persulfate boosters, which together break down melanin, the pigment that colours hair. The alkaline mixture also swells the outer layer of each hair so the chemicals can reach the pigment inside.

Melanin breaks down in stages, which is why bleached dark hair often passes through red, orange and then yellow before it reaches pale blonde, and why colourists use toners to cancel unwanted warm shades. Because the process also damages the protein structure of the hair, repeated bleaching leaves it porous and fragile. Mixing hair products with household chlorine bleach would be dangerous and achieves nothing useful.

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.