Preparation and Properties of Bleaching Powder

Bleaching powder, also known as calcium hypochlorite, is a chemical compound with the formula $Ca(OCl)_2$. It is widely used as a disinfectant and a bleaching agent. In this article, we will delve into the preparation, chemical properties, and uses of bleaching powder.

Preparation of Bleaching Powder

Bleaching powder is prepared through the reaction of chlorine gas with dry slaked lime (calcium hydroxide). The process is known as the "chlorination of lime" and can be represented by the following chemical equation:

$$ Ca(OH)_2 + Cl_2 \rightarrow Ca(OCl)_2 + H_2O $$

The reaction is exothermic, and it is carried out in a large tower called a "bleaching powder tower" or "chloride of lime tower." Dry slaked lime is spread over the trays in the tower, and chlorine gas is passed through it. The chlorine reacts with the slaked lime to form bleaching powder.

Steps in the Preparation:

Slaking of Lime: Quicklime ($CaO$) is treated with water to produce slaked lime ($Ca(OH)_2$).

$$ CaO + H_2O \rightarrow Ca(OH)_2 $$
Chlorination: Dry slaked lime is exposed to chlorine gas.

$$ Ca(OH)_2 + Cl_2 \rightarrow Ca(OCl)_2 + H_2O $$
Collection: The product, bleaching powder, is collected and packed in moisture-proof containers to prevent decomposition.

Properties of Bleaching Powder

Bleaching powder exhibits a number of chemical and physical properties:

Physical Properties

It is a white powder with a strong smell of chlorine.
It is slightly soluble in water, and the solution is known as "chlorinated lime solution."
It is hygroscopic and must be stored in airtight containers to prevent moisture absorption.

Chemical Properties

Action of Water: Bleaching powder reacts with water to form a mixture of hypochlorous acid ($HClO$) and hydrochloric acid ($HCl$).

$$ Ca(OCl)_2 + H_2O \rightarrow Ca(OH)_2 + HClO $$
Bleaching Action: The bleaching action is due to the hypochlorous acid, which releases nascent oxygen.

$$ 2HClO \rightarrow 2HCl + O $$

The nascent oxygen ($O$) is a powerful oxidizing agent and is responsible for the bleaching effect.
Action with CO2: On exposure to carbon dioxide, bleaching powder undergoes a reaction to form calcium carbonate and chlorine.

$$ Ca(OCl)_2 + CO_2 \rightarrow CaCO_3 + Cl_2 $$
Action with Acids: When treated with acids, bleaching powder releases chlorine gas.

$$ Ca(OCl)_2 + 2HCl \rightarrow CaCl_2 + H_2O + Cl_2 $$

Uses of Bleaching Powder

Bleaching powder is used in various applications due to its bleaching and disinfecting properties:

As a bleaching agent in the textile and paper industries.
For disinfecting drinking water to make it free from microorganisms.
In the sanitation of swimming pools.
As an oxidizing agent in chemical industries.

Comparison Table: Bleaching Powder vs. Other Chlorine Compounds

Property	Bleaching Powder ($Ca(OCl)_2$)	Sodium Hypochlorite ($NaOCl$)	Chlorine Gas ($Cl_2$)
Appearance	White powder	Pale greenish-yellow liquid	Greenish-yellow gas
Solubility in Water	Slightly soluble	Highly soluble	Soluble
Stability	Less stable, decomposes on exposure to air	More stable in cold and dark conditions	Very reactive and toxic
Bleaching Action	Due to nascent oxygen	Due to hypochlorite ion	Direct bleaching by chlorine
Production Method	Chlorination of slaked lime	Reaction of chlorine with sodium hydroxide	Produced by electrolysis of saltwater

Conclusion

Bleaching powder is an important chemical compound with a wide range of applications. Its preparation involves the reaction of chlorine with slaked lime, and it possesses unique properties that make it suitable for bleaching and disinfecting. Understanding its properties and uses is crucial for its effective application in various industries.