`PCl_5` is kept in well stoppered bottles because

Options

A. It is highly volatile
B. It reacts readily with moisture
C. It reacts with oxygen
D. It is explosive

Explanation

To answer the question of why PCl5 (phosphorus pentachloride) is kept in well-stoppered bottles, we can analyze the options provided and the properties of PCl5.

### Step-by-Step Solution:

1. **Understanding PCl5**: PCl5 is a chemical compound that can react with various substances, including moisture (water vapor) in the air.

2. **Evaluating the Options**:
- **Option 1: It is highly volatile**: While PCl5 can sublime, this is not the primary reason for storing it in well-stoppered bottles.
- **Option 2: It reacts readily with moisture**: This is a significant property of PCl5. When it comes into contact with moisture, it reacts to form hydrochloric acid (HCl) and phosphoric acid (H3PO4).
- **Option 3: It reacts with oxygen**: PCl5 does not have a notable reaction with oxygen that would necessitate special storage.
- **Option 4: It is explosive**: PCl5 is not classified as an explosive substance.

3. **Chemical Reaction with Moisture**: The reaction of PCl5 with moisture can be represented as follows:
\[ \text{PCl}_5 + \text{H}_2\text{O} \rightarrow \text{H}_3\text{PO}_4 + 5\text{HCl} \]
This reaction shows that PCl5 decomposes in the presence of water, producing hydrochloric acid and phosphoric acid.

4. **Conclusion**: To prevent this unwanted reaction with moisture, PCl5 is stored in well-stoppered bottles. This ensures that moisture from the air does not enter the container and react with PCl5.

5. **Final Answer**: Therefore, the correct reason for keeping PCl5 in well-stoppered bottles is that it reacts readily with moisture.