When i was at younger age, my mother used to be very angry when i was home from primary school with my "yellowish brown" school uniform which was actually in sharp and clean white before setting off to school in the morning! She was really sick and tired of washing my school uniform as it will be dirty again very soon :) i could often see her black face especially when the dirty shirt was again with some stubborn stains. Do you have the experience like mine? :)
However, i did not get an black eye everytime because of their existences :
Yea~ bleaching agents!As we know, bleaching agents are bottlled of solution which used to lighten or remove stains or colours. Chemically speaking, bleaching agents contain certain composition of active chemical compounds or molecules that react with various type of substrates such as stains, dyes to remove or decolourize them on textiles or fabrics.
As what you've seen , There are two main types of common household bleaching agents which are 'chlorine' bleaches and 'oxygen' bleaches. Although of the different manufacturing ingredients, both of them function in the same way - decolourising substances through oxidation process.
Oxygen bleaches is one of a common strong bleaching agent that contains the chemical compound of hydrogen peroxide(H2O2) or other peroxide releasing compounds as the main ingredient. Hydrogen peroxide is a pale blue weak acid which has strong oxidising properties. The commercial brand "Vanish O²" on the market is one of the example of oxydative bleaches.Structural formula of Hydrogen peroxide:
However, we'll only focus on CHLORINE bleach here and now ...o.O
"Chlorox", the commercial bleaching agent which most of us will have it at home is in fact falls into the category of "Chlorine" bleaches as it involves chlorine molecules in its process of manufacturing which then finally produces a chemical compound known as Sodium Hypochlorite(NaOCl) - the active ingredient of all Chlorine bleaching agents.
Structural formula of Sodium hypochlorite
:
Molecular formula = NaOCl
Sodium hypochlorite, also known as Sodium chlorate(I) is a chemical compound consists of Sodium, Chlorine and Oxygen which has been used for bleaching and disnfecting purposes for centuries.
How is it produced?Earlier in 1789, Hypochlorite had been produced by passing chlorine gas through a solution of sodium carbonate in Javel, France, due to that the product which is a weak acid solution of Sodium hypochlorite was known as "Javel water" Or "Eau de Javel".This less efficient method of production is then replaced by another alternative method of E.S.Smith later in the end of 19th century.In this method, Chlorine gas and caustic soda is first obtained through the hydrolysis of brine solution before the chlorination process, that is to mix both products to form Sodium hypochlorite. Nowadays, the method has been improved and so called the Hooker process. This process is now the large scale industrial method in the production of commercial bottled Sodium hypochloride solution in various trademarks. Throughout the process, Chlorine gas is to react with cool and diluted aqueous Sodium hydroxide to yield Sodium hypochloride, Sodium Chloride and water as the by-products. Electrolysis process is involved in the separation of charges in cathodeand anode.Cl2(g) + 2 NaOH(aq) <--------------> NaClO(aq) + NaCl(aq) + H2O(l)Commercial household bleach usually contain 3 - 6 % of Sodium hypoclorite. Different formulation certainly will have products of different strength but the strength of the solution might be gradually decreased due to long storage too!
Why do substances have colours?Well~ We'll start with a little Physic + Chemistry :D
A substance has a colour because it contains Chromophore!A Chromophore is the a group of atoms make up a dye molecule with different functional group of that is largely responsible for its colour or pigmentation. It is the region where there is a difference of energy level falls in a range of visible spectrum between two different molecular orbitals created by a series of single and double bond alternately. A molecule has a colour when it absorbs and reflects others with certain wavelengths of visible light as the light can hit the chromophore and being absorbed throuugh exciting an electron from its ground state! 
Fabric dyes, also known as Azo compounds, food coloring, flower pigmentations and many more substances contains chromophores!Chromophores contain unsaturated groups like C=O and -N=N- extended from arene rings. Within the structure, the unsaturated group often appears in a system of electron delocarization. All Azo Compounds, with the colur of the shirts you're wearing inclusive, have the general equation of R-N=N-R' arrangement! ^^
Here are a few examples of Azo dyes' structural formula:
Yellow Azo dye looks like this!
Basic Chrysoidine Dye (orange)
An Advance Red Dye ^^
Do you know how the stain on your shirt is being removed?
Get to know ~*The mechanism of bleaching*~Substances can be decolourised through either oxidation or reduction process. For reducing agents, sodium dithionite can be one of the examples while most commercial bleaches today are oxidising agents with chlorine and oxydative bleaches inclusive.
Sodium hypochloride is highly reactive and unstable when it's pure thus we often buy it in the form of diluted aqueous solution. When it is being mixing with water, hypochlorite will decompose to form Hydrogen ions, Chloride ions and a powerful oxidising agent known as Hypochloric acid(HClO) which has a excellent bleaching and disinfecting effect.The equation of the process is as the following :Cl2(aq) + H2O(l) ---------------> H+(aq) + Cl-(aq) + HClO(aq)
Structural formula of HClO:
Hypochlorous acid is then partially dissociates in aqueous solution and forms the anion known as Hypochlorite (ClO-) :
HClO(aq) ----------------> ClO- + H+Next, Hypochlorite tends to decompose into chloride and oxygen :ClO- -------------------> Cl- + OThe oxygen is in highly reactive form which then leads to bleaching and anti-bacteria effects when reacting with other organic substances.
As oxidising agents, Chlorine and oxydative bleaches cause the substance it reacting with to be oxidised while itself being reduced in the bleaching process. Recall that oxidation is losing one or more electrons whereas reduction is gaining of one or more electrons. During the process, the bleaching agent undergoes reduction by gaining activated electrons which can absorb visible lights from the substances and hold them to itself. The oxidation and reduction process occur simultaneously . In other words, the bleaching process is actually a redox reaction!
You may have a look at the following simplified diagram to help in illustrating the process :
The hypochlorite ion (OCl-) is reduced to chloride ions and hydroxide ions during reduction. It is then forms a basic solution as it receives electrons from the coloured material.Equation :
OCl- + 2e- + HOH -------------------> Cl- + 2OH
Besides that, Chlorine and oxydative bleaches works by breaking the series of alternating chemical bonds of the chromophores so that the molecules are changed into a different form and thus destroy the ability of the chromophores in absorption of visible light causing the substance to be gradually decolorised. By destroying chemical bonds, large stain molecules can be broken down to smaller and water-soluble fragments which eventually to be removed much easily by detergent during washing process.
Some pictures for you :)
Stain molecules has it colour's origin in the double bond connections.
Bleach molecules breaking up the double bond connections!
Double bond connections have been broken!
AND~
Stain has been decolourised!!
In addition, besides chemicals in bleaching agents, high energy such as photons in sunlight can also distrupt chemical bonds in chromophores for decolourising effects. This is what was done by many of the people in the past to bleach out colours from substances!
Apart from being household and laundry products, bleaching agents are very useful in many industries such as bleaching of wood pulps, beeswax, flour, tissue papers, skin, cellulose, disinfectant for swimming pool and a number of other industries!
