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How does bleach work?

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Quick answer

Bleach works by releasing reactive chlorine and oxygen that attack the double bonds in coloured molecules. This breaks the bonds, so the molecule no longer absorbs visible light and appears colourless. The same reaction damages germs, making bleach a disinfectant.

Bleach works by oxidizing or chlorinating the coloured molecules that cause stains, breaking their chemical bonds so they no longer absorb visible light. This turns the molecules into smaller, colourless compounds that can be rinsed away.

Step by step

  1. Sodium hypochlorite in bleach reacts with water to form hypochlorous acid and other reactive species.
  2. These species attack the double bonds in the coloured molecules that cause stains.
  3. The double bonds break, splitting the molecule into smaller fragments that do not absorb visible light.
  4. The colour disappears, and the broken fragments are rinsed away with water.
  5. Meanwhile, the reactive species also oxidize proteins and DNA in germs, killing them.

Key things to understand

  1. Household bleach is a solution of sodium hypochlorite, which forms reactive hypochlorous acid and hypochlorite ions in water.
  2. These reactive oxidizing species attack the double bonds in coloured molecules, the part that absorbs light and gives the stain its colour.
  3. When those bonds break, the molecule splits into smaller pieces that no longer absorb visible light, so the stain looks colourless.
  4. Bleach also kills germs by damaging their proteins and DNA, which is why it is used as a disinfectant.
  5. Bleach works best on cotton and other natural fibres; on some synthetics it may not remove colour and can damage the fabric.

Labelled diagram

  1. Bleach in water

    Sodium hypochlorite releases reactive chlorine and oxygen species.

  2. Attack on stain

    Reactive species hit the double bonds in the coloured molecule.

  3. Bonds break

    The double bonds split, and the molecule fragments.

  4. Colour gone

    Fragments no longer absorb visible light, so the stain looks colourless.

How bleach removes colour

Real-world examples

  • A white cotton shirt stained with coffee can be soaked in diluted bleach to remove the brown colour.
  • In hospitals, bleach solutions are used to disinfect surfaces and kill bacteria like C. difficile.
  • Bleach is added to swimming pools to kill germs and keep the water clear.
  • Paper is sometimes bleached with chlorine dioxide to make it white for printing.

Common mistake

✗ Myth: Bleach removes colour by washing it away like soap.

✓ Fact: Bleach chemically changes the coloured molecule by breaking its light-absorbing bonds. Soap only lifts dirt; bleach actually destroys the colour.

People also ask

What does bleach do to stains?
Bleach oxidizes the coloured molecules in stains, breaking their chemical bonds so they no longer absorb visible light. The stain becomes colourless. It also kills germs by damaging their proteins and DNA. Bleach does not remove all stains; some dyes resist it, and it can damage certain fabrics.
Why is bleach dangerous to mix?
Mixing bleach with ammonia or acids releases toxic gases like chlorine or chloramine. These can cause severe breathing problems. Bleach should only be used alone or with water, in a ventilated area. Always read the label and never mix cleaning products.
How long does bleach take to kill germs?
Bleach kills most germs within minutes, but it must stay wet on the surface for the contact time on the label, often between one and ten minutes. The contact time depends on the concentration and the type of germ. Always follow the instructions on the bottle for safe and effective use.
Does bleach remove stains or just lighten them?
Bleach chemically destroys the coloured part of the stain molecule, so the stain is gone, not just lightened. However, if the stain is a dye that has bonded strongly to the fabric, bleach may only lighten it. Always test on a hidden area first.
Why does bleach smell so strong?
The smell comes mainly from hypochlorous acid and chlorine compounds formed as bleach reacts with dirt or acids. These fumes irritate the lungs, so bleach should only be used in well-ventilated areas. Never mix bleach with ammonia or vinegar, as this can release dangerous gases.
Can bleach be used on all fabrics?
No. Bleach is safe for cotton, linen, and some synthetics, but it can yellow or damage wool, silk, spandex, and some flame-resistant fabrics. Always check the care label. For coloured fabrics, use colour-safe bleach, which contains hydrogen peroxide instead of chlorine.
How does bleach kill bacteria?
Bleach kills bacteria by oxidizing their cell walls and destroying essential proteins and DNA. This stops the bacteria from functioning and reproducing. It is effective against many viruses and fungi too, but it must be left wet on the surface for a few minutes to work.

Part of the guide: Everyday Science: Light, Heat & Matter →