Answered by Dr. Cathy Riddle, Chemist, Idaho National Laboratory
Bleach is an oxidizing agent. This means that the bleach will react in certain ways with certain groups of chemicals. Natural stains, as well as dyes, are produced from chemical compounds called chromophores.
Chromophores are generally molecules with long chains of alternating “double” and “single” bonds between that can absorb light at specific wavelengths and therefore cause colors. Bleach breaks the bonds holding the molecular chains together and the color absorbing properties of the dyes are lost since oxygen reacts easily with the chromophores to eliminate the portion of its structure that causes the color.
Not everything can be bleached though; some metals and metal ions such as iron (Fe) are not as easily oxidized by bleach and will simply change color which is why bleaching a blood stain (blood contains Fe) will only change it to a light rusty-colored stain!
So what, exactly, happens to that ketchup stain on your white t-shirt when you bleach it? In order to understand how chlorine bleach makes a stain “disappear,” we need to understand how colors work.
Light is both a particle and a wave; its particles, called photons, travel in waves that have a particular length. Not all wavelengths of light are visible to the human eye: infrared light wavelengths are too long for our eyes to see, and ultraviolet wavelengths are too short. The wavelengths we can see are between 400 and 700 nanometers, and they appear as color to us. For example, when light with a wavelength of about 475 nanometers hits the retina in your eye, you perceive the color blue. The light that comes from the ketchup stain on your t-shirt to your retina has a wavelength of about 650 nanometers, which makes it appear red.
Every material has a different atomic make-up and different materials will absorb and reflect different wavelengths of light depending on their atomic structure.
Bleach works by releasing oxygen molecules in a process called oxidation. The oxygen molecules released by bleach break up the chemical bonds of chromophores. The changed chromophore molecules either reflect no color or a color outside the visible spectrum. This absence of color is seen by our eyes as white.
Ready to see how bleach works firsthand? You’ll want to get some help from an adult before trying this fun science experiment.
Materials: food coloring, water, household bleach, dropper, glass or jar, spoon
- Fill a glass or jar about halfway full with water.
- Add a few drops of food coloring. Stir.
- Add drops of bleach until the color starts to disappear. You can stir to make the reaction faster. Continue adding bleach until the color is gone.
- Add a few drops of another color. What happens?
The color doesn’t spread out the same way as it did when coloring was added to pure water. It forms swirls, which may disappear if there is enough bleach in the water. Bleach oxidizes or reacts with the chromophore or color molecules in food coloring. Although the pigment molecule remains, its shape changes so that it can’t absorb/reflect light the same way, so it loses its color as a result of the chemical reaction.