Understanding Bleaching Powder and Water Reaction

Bleaching powder is a chemical compound widely used for bleaching cotton and linen in the textile industry, as a disinfectant, and for oxidizing purposes. Its chemical name is calcium oxychloride, and its formula is often represented as \( \text{CaOCl}_2 \). While this is a simplified formula, its actual composition is more complex, being a mixture containing calcium hydroxide, calcium chloride, and calcium hypochlorite.

When water is added to bleaching powder, it undergoes hydrolysis. This reaction leads to the formation of calcium hydroxide and the release of hypochlorous acid (\( \text{HClO} \)). Hypochlorous acid is unstable and decomposes to release nascent oxygen and hydrochloric acid, or in the presence of excess chloride ions (present in bleaching powder), it can decompose to release chlorine gas.

Chemical Reaction of Bleaching Powder with Water

The key reaction involving calcium hypochlorite component (\( \text{Ca(ClO)}_2 \)) when water is added is:

\( \text{Ca(ClO)}_2\text{(s)} + \text{2H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{2HClO(aq)} \)

The hypochlorous acid (\( \text{HClO} \)) then further reacts or decomposes.

A simplified overall reaction showing the release of chlorine gas, particularly if there are other chlorine-containing compounds or acidic conditions:

\( \text{CaOCl}_2\text{(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Cl}_2\text{(g)} \)

This equation clearly shows that chlorine gas (\( \text{Cl}_2 \)) is released when water reacts with bleaching powder.

Identifying the Released Gas

Based on the chemical reaction, the gas produced when water is added to bleaching powder is chlorine gas. This is why bleaching powder has a distinct smell of chlorine and is effective as a disinfectant and bleaching agent (due to the oxidizing power of hypochlorous acid or nascent oxygen derived from it, or chlorine itself).

Analyzing Other Options

• Carbon dioxide: This gas is not produced from the reaction of bleaching powder and water. Carbon dioxide is typically released from reactions involving carbonates or combustion.
• Oxygen: While nascent oxygen is involved in the bleaching action derived from hypochlorous acid decomposition (\( \text{HClO} \rightarrow \text{HCl} + \text{[O]} \)), the primary gas visibly released with the characteristic smell upon adding water is chlorine gas, especially considering the typical composition and reaction conditions.
• Hydrogen: Hydrogen gas is usually produced from reactions involving metals with acids or water under specific conditions (e.g., electrolysis of water). This reaction does not yield hydrogen gas.

Therefore, the gas released when water is added to bleaching powder is chlorine.

Revision Table: Key Concepts

Additional Information: Uses of Bleaching Powder

Bleaching powder is a versatile compound with several important applications:

• Bleaching: Used in the textile industry to bleach cotton and linen and in the paper industry to bleach wood pulp.
• Disinfectant: Used for disinfecting drinking water, swimming pools, and sewage due to the oxidizing power of the chlorine compounds it releases.
• Oxidizing Agent: Used as an oxidizing agent in various chemical industries.
• Chloroform Production: It was historically used in the production of chloroform.

The release of chlorine gas upon contact with water or even atmospheric carbon dioxide (forming \( \text{HClO} \) which decomposes) is key to its disinfectant and bleaching properties.