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Bleach

We bleach our clothes. We bleach our hair. We bleach our teeth. We bleach our skin. We bleach our food.

We use bleaches to disinfect and deodorize.

Some bleaches smell awful. Some, like lemon juice and sunshine, are refreshing.

Color and bleach
Colored substances contain molecules with chromophores , areas of the molecule that have double bonds between carbon atoms or oxygen atoms. A good example is beta carotene , and that section goes into more detail on how molecules become colored.

Bleaches attack these chromophores in one of two ways.

Oxidizing bleaches like sodium hypochlorite break the molecules at the double bond. This results in either a shorter molecule that does not absorb visible light, or a molecule whose chromophore is either shorter or non-existant. A shorter chromophore will absorb light of a shorter wavelength than visible light (such as ultraviolet light), and so does not appear colored.

Reducing bleaches such as lemon juice (in combination with sunlight) or sulfur dioxide, convert the double bonds in the chromophore into single bonds, eliminating its ability to absorb visible light. Sometimes the reaction is reversible, where oxygen in the air reacts with the molecule to repair the chromophore, and the stain returns.

Bleaching clothing
Laundry bleaches fall into two categories. The first is what are called "chlorine" bleaches. The second are "oxygen" bleaches.

While pure chlorine gas will bleach colors, in laundry bleaches, sodium hypochlorite or calcium hypochlorite are actually used, and they work by releasing oxygen, not chlorine. The chlorine remains in solution, either as sodium chloride (table salt), or calcium chloride.

These bleaches are made by bubbling chlorine gas through a solution of sodium hydroxide (lye) or calcium hydroxide (quicklime).

Chlorine gas can be released if the bleach is mixed with an acid. To prevent this from happening, commercial bleaches leave extra alkalies in the solution to keep the pH very high (pH 12). This small amount of extra lye in the solution, along with the caustic nature of the hypochlorite itself, is what eats away the cloth if undiluted bleach is spilled on the clothing.

Another chlorine bleach often used is sodium dichloroisocyanurate.

Oxygen bleaches also work by releasing oxygen. Hydrogen peroxide is the active ingredient, either as itself, or as a product of reacting another ingredient with water to release hydrogen peroxide.

Oxygen bleaches such as sodium carbonate peroxide (also called sodium percarbonate), sodium peroxide, or sodium perborate are made by reacting molecules with hydrogen peroxide. When the result is added to water, the hydrogen peroxide is released.

Borax also works by releasing hydrogen peroxide into the water.

Most oxygen bleaches work best in hot water. Additives such as tetra acetyl ethylene diamine allow the hydrogen peroxide to work in warm water (50 C).

Bleaching hair
Ultraviolet light from the sun is the most common hair-bleaching agent. Lemon juice is sometimes added to speed up the process of reducing the double bonds in hair pigments to single bonds.

However, the most famous hair bleach is hydrogen peroxide of peroxide blonde fame. Unlike sunlight and lemon juice, peroxide is an oxidizing bleach, and its effects are less easily undone.

The calcium hypochlorite or sodium dichloroisocyanurate used to disinfect swimming pools also bleaches hair, although (contrary to popular belief) it does not turn the hair green. It bleaches the hair, allowing the green copper sulfate in the water to show in the hair. The copper sulfate comes from the reaction of the copper pipes in the plumbing to the sulfuric acid used to neutralize the alkalies in the chlorination chemicals.

Bleaching teeth
Dental bleaches are found in whitening toothpastes and in whitening gels or strips applied to the teeth.

In toothpastes, sodium carbonate peroxide is generally used.

In gels and strips, carbamide peroxide is used, often with tetra acetyl ethylene diamine as a bleach activator.

All of these products owe their bleaching action to the hydrogen peroxide that is liberated when they are applied.

Bleaching skin
Skin lighteners, freckle and age spot removers, and other remedies for hyperpigmentation are not actually bleaches like the others listed so far.

The active ingredient is hydroquinone, which inhibits melanin formation when applied to the skin. Since the effect is easily reversed by exposure to sunlight or ultraviolet light, a sunscreen is usually included in the formula.

Bleaching food
Wheat flour normally becomes white by normal oxidation in air when stored for a few weeks. To speed up the process, benzoyl peroxide is used as a bleaching agent.

Sulfur dioxide is a reducing bleaching agent that is used to preserve dried fruits.

Disinfecting
Oxidizing bleaches kill microbes by reacting with cell membranes and cell proteins. The most widely used is sodium hypochlorite for household and hospital uses, and calcium hypochlorite for drinking water and swimming pool disinfecting.
By Simon Quellen Field

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