The chemistry of bleaching powder

SCIENCE FOR YOUTH

BY HIYA ISLAM

science

The marvels of basic bleach are widely known. Its list of uses runs long for both of its liquid and powdered forms; cleaning pool water, sterilising municipal water supplies, removing stains in laundry and textile industries, disinfecting medical equipment, producing the infamous chloroform and so on. However, as with all other products, these come with expiration dates, after which the product will no longer be able to save your favorite shirt from the clutches of ketchup.

Bleaching agents aka oxidative agents do their magic by oxidation.  Primarily divided into two types, chlorine-based and oxygen-based, these work by playing with the light-reflecting properties of chemical bonds found in stains. Chlorine-based bleaching agents have an active ingredient, sodium hypochlorite (NaOCl) while oxygen-based ones owe its strength to hydrogen peroxide (H2O2). As soon as drops of bleach soak a garment, it brutally attacks the chromophores. Chromophores are a part of the dye’s molecule that is responsible for the color you see. To illustrate, the hideous ketchup stain is red in color and therefore it cannot absorb the wavelength at around 650 mm (which is the region for red in the visible spectrum of light). In other terms, all other wavelengths are absorbed by the stain or by the electrons involved in certain bonds, to be specific. It is these bonds that are oxidised and destroyed rendering no wavelength to be absorbed. As no light is absorbed, it appears just like the rest of the shirt. The remains of ketchup are still present, only that now you can’t see ‘em! By using detergents, of course, it can be removed permanently. NaOCl is a more powerful oxidising agent than H2O2. Peroxide or oxygen-based bleach is used in laundry and is unable to break the ‘stronger’ dyes of the clothing ensuring zero color damage.

Another question yet remains unanswered. How does bleach work as a disinfectant? Bleach can kill a lot of dangerous bacteria, viruses and fungi (like, Vibrio Cholerae, MRSA, influenza, HIV etc.). It is an efficient method of killing microbes as bacteria can grow resistant to antibiotics which, also, cannot be used to terminate viruses. Rather than using it straight from the bottle it is advised to dilute it with water for better efficacy. Often, a 10% bleach solution is used. What happens is that bleach has the same effect on bacteria as extreme heat does; it coagulates protein essentials for metabolism. Proteins, such as enzymes, lose their 3D structure and cannot take part in reactions causing the cell to die eventually.

One fascinating phenomenon relative to bleaching powders is ‘sun bleaching’ or technically, photodegradation. This is when the color of an object placed under natural sunlight fades away. This occurs due to high-energy UV rays disrupting the bonds of chromophores. Maybe, next time you can try bleaching your whites under the sun.

 

 

 

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