When water is added to bleaching powder (calcium oxychloride), which gas is released as a product of the reaction?

Introduction / Context:
Bleaching powder is widely used as a disinfectant and bleaching agent in homes, swimming pools, and water treatment systems. Chemically, it is often represented as calcium oxychloride, a mixture that can release chlorine gas under appropriate conditions. Understanding what happens when water comes into contact with bleaching powder helps explain its bleaching and disinfecting action and is a common question in basic inorganic chemistry and environmental science.

Given Data / Assumptions:

• Bleaching powder is mainly calcium oxychloride, commonly written as CaOCl2.
• The question considers the effect of adding water to bleaching powder.
• The options list oxygen, hydrogen, carbon dioxide, and chlorine gases.
• We assume typical conditions where bleaching powder reacts with water to release active chlorine.

Concept / Approach:
When bleaching powder is treated with water, it undergoes reactions that lead to the formation of hypochlorous acid and calcium hydroxide. In many practical situations and simplified exam explanations, it is stated that bleaching powder in the presence of water releases chlorine, which is responsible for its bleaching and germicidal properties. More precisely, bleaching action is due to nascent oxygen released from hypochlorous acid, but the initial reaction is usually written to show chlorine being liberated. Among the given options, chlorine is the only gas directly associated with bleaching powder and water in basic school level chemistry.

Step-by-Step Solution:
Step 1: Recall the formula and nature of bleaching powder. Bleaching powder is CaOCl2 and acts as a source of chlorine in water. Step 2: Consider what happens on adding water. On exposure to water and carbon dioxide or mild acids, bleaching powder releases chlorine gas. Step 3: Connect this to everyday use. The smell of chlorine in treated water and swimming pools comes from chlorine released by bleaching powder. Step 4: Match this with the options and select chlorine as the gas released.

Verification / Alternative check:
Textbooks often summarise the reaction as CaOCl2 + H2O + CO2 → CaCO3 + 2HCl, followed by HCl reacting to release Cl2, or they present a simplified version directly showing chlorine liberation. In school level discussions, bleaching powder is described as giving chlorine when it reacts with water or acids. This chlorine is responsible for killing germs and bleaching coloured substances. Oxygen, hydrogen, and carbon dioxide are not typically described as gases that come directly from the reaction of bleaching powder with water in this context, confirming chlorine as the correct answer.

Why Other Options Are Wrong:
Option A (Oxygen): Although bleaching action involves nascent oxygen at a later stage, the gas directly associated with bleaching powder reacting with water in exam style questions is chlorine, not oxygen. Option B (Hydrogen): There is no standard reaction where hydrogen gas is produced simply by adding water to bleaching powder. Option C (Carbon dioxide): CO2 may be involved in reactions with bleaching powder but is not the gas commonly said to be released when water is added to it.

Common Pitfalls:
Learners sometimes confuse the detailed mechanism of bleaching, which involves nascent oxygen, with the gas named in simpler exam questions. Another source of confusion is mixing up bleaching powder with other oxidising agents or with processes like electrolysis that produce oxygen or hydrogen. To avoid this, remember the simple association: bleaching powder plus water (and mild acid or CO2) releases chlorine, which then leads to bleaching action through formation of hypochlorous acid and nascent oxygen.

Final Answer:
When water is added to bleaching powder, the gas released is Chlorine.