Bleaching powder becomes useless on exposure to air because

Options

A. It evolves `Cl_(2)` by decomposition
B. It absorbs `CO_(2)` to form `CaCO_(3)`
C. It is affected by `O_(2)` in air
D. None of the above

Explanation

**Step-by-Step Solution:**

1. **Understanding Bleaching Powder**: Bleaching powder is chemically known as calcium hypochlorite, with the formula \( \text{Ca(OCl)}_2 \). It is commonly used as a bleaching agent and disinfectant.

2. **Exposure to Air**: When bleaching powder is exposed to air, it undergoes certain reactions that can render it ineffective.

3. **Reaction with Carbon Dioxide**: One of the primary reactions that occur when bleaching powder is exposed to air is its reaction with carbon dioxide (\( \text{CO}_2 \)). The carbon dioxide in the air reacts with the calcium hypochlorite to form calcium carbonate (\( \text{CaCO}_3 \)) and chlorine gas (\( \text{Cl}_2 \)).

\[ \text{Ca(OCl)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{Cl}_2 \]

4. **Formation of Calcium Carbonate**: The formation of calcium carbonate is significant because it reduces the amount of active bleaching agent (chlorine) available in the bleaching powder. Calcium carbonate is not a bleaching agent, thus making the bleaching powder less effective.

5. **Conclusion**: Therefore, the correct reason why bleaching powder becomes useless on exposure to air is that it absorbs carbon dioxide to form calcium carbonate, which is represented by option B.

**Final Answer**: Bleaching powder becomes useless on exposure to air because it absorbs carbon dioxide to form calcium carbonate (Option B).

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