Bleach refers to a wide range of chemical substances that are used mainly in order to whiten fabrics, clean surfaces and disinfect water supplies.
This range of substances works by oxidizing the molecules that either stain our clothes/furniture, or are part of unwanted germs. They can be divided into two main products:
- Chlorine-based bleach: These function by releasing elemental chlorine. Elemental chloride on its own is also a form of bleach, although it is much more dangerous to handle.
- Peroxide-based bleach: They release oxygen, which does the oxidation itself. Oxygen is derived from the decomposition of peroxide compounds (R-O-O-R).
White is often seen as a symbol of purity and virtue (even the ancient Egyptians and Greeks shared this view ), thus there was a need for effective fabric whitening methods.
Since the ancient times, people used a widely available "bleaching product", the sun itself (the small amounts of UV radiation that reach the surface of the earth, break up bonds of stain molecules that are responsible for the colours that we perceive).
Later in ancient rome, before the invention of soap, other alkaline products were used as cleaning agents, such as urine (which breaks down to ammonia). These only removed the grease stains from the cloth, before it was left to dry and whiten in the sun.
Ultimately, from wood ashes a better cleaning product was made which was lye. Lye is used as the main ingredient is soap making.
In the 17th century, the Dutch were leading the bleaching market. They invented a new method of bleaching which involved additions of bases and acid to the cloth. Firstly, the fabrics were soaked in lye in order to remove grease stains and then washed clean. Then they were spread out in specific places, called bleachfields. There, on the grass, they got whitened under the sun. This process was repeated several times. Lastly, sour milk was placed on the fabric in order to neutralize the bases added prior and once again a wash took place. This process was not only physically demanding, but also took months to finish.
Thankfully, after the discovery of elemental Chlorine by Karl Wilhelm Scheele, another scientist came up with a faster way to bleach clothes. He was Claude Berthollet, who made in 1789, what is known as "Eau de Javel" (commonly known as liquid bleach), which is a widely used bleach in laundry, cleaning and water treatment. That bleach consists of a solution of 3-6% NaClO.
Later, in 1798 Charles Tennant made a solution of known as bleaching powder. Peroxide-based bleach only became popular in the 20th century, after being made in a century ago.
Household bleach mainly contains sodium hypochloride, NaOCl (eau de Javel). It is an unstable white powder that dissolves in water to give a yellow stable solution.
It also contains small amounts of NaoH, and . The first method of producing bleach involved a reaction between chloride gas and sodium carbonate (). Later, sodium carbonate was replaced by caustic soda (soda lye), NaOH, a cheaper substance that also made the production more efficient.
In this reaction, is reaction with a solution of NaOH giving a spontaneous reaction:
The raw materials of the reaction, chloride and caustic soda come from the electrolysis of brine.
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