Question

State for what purpose bleaching powder is used in water treatment plants.

Answer 1

Hint: Bleaching powder dissolved in water produces less than five percent available chlorine. Bleaching powder is used in the process of disinfection and this chemical is also known as disinfectant which means the chemical which kills bacteria.

Complete step by step answer:
The chemical name of bleaching powder is calcium hypochlorite, which is an inorganic chemical compound with a chemical formula
 $CaOC{{l}_{2}}+{{H}_{2}}O\to CaC{{l}_{2}}+HCl+HOCl$
$HOCl\to HCl+[O]$
 This is also called calcium oxychloride.

There are two processes involved in a water treatment plant,
1. Removal of macro-organisms
2. Removal of bacteria
The process of removal of bacteria is the disinfection process, meaning that by adding chemical substances to water by the removal of microorganisms like bacteria, etc.
In this process, the chemical substances which are added are disinfectants.
Examples for disinfectants,
Bleaching powder, chlorine gas, chloramines, ozone gas

The process and reactions take place when bleaching powder added to water as follows:
1. Bleaching powder is enriched chlorine content. When it is added to water, it releases chlorine molecules into the water.
2. The concentration of chlorine in water does not exceed 5%. If it does excess chlorine is lost in sediment.
3. The weight of bleaching powder to be added to a tank to prepare a chlorine solution of a given strength can be calculated by given equation,
$W=\dfrac{1000 x V x C}{S}$
Where V= volume in a tank in Litres,
C= concentration of solution required in percent,
S= strength of powder in percent weight chlorine.
4. The following chemical reactions take place when bleaching powder is added to the water treatment plant.
 $CaOC{{l}_{2}}+{{H}_{2}}O\to CaC{{l}_{2}}+HCl+HOCl$
$HOCl\to HCl+[O]$
[O] Is nascent oxygen, which kills bacteria in water.
This nascent oxygen is highly reactive and reacts with different biomolecules. This is the strongest oxidizing agent and removes organic matter.

Note: More amount of bleaching powder will cause the hardness of water. Hence, the concentration of solution doesn't exceed 5%. For example, to get a 0.25% chlorine solution, 1gm of bleaching powder dissolved in one liter of water.