During the preparation of hydrogen chloride gas on a humid day the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to

Options

A. absorb the evolved gas
B. moisten the gas
C. absorb moisture from the gas
D. absorb `Cl^-` ions from the evolved gas

Explanation

**Step-by-Step Solution:**

1. **Understanding the Context**: When preparing hydrogen chloride (HCl) gas, especially on a humid day, it is important to ensure that the gas is dry. Humidity can introduce water vapor into the gas, which can affect its purity and reactivity.

2. **Role of the Guard Tube**: A guard tube is used in the setup to prevent moisture from entering the gas stream. This is crucial for maintaining the quality of the hydrogen chloride gas.

3. **Calcium Chloride as a Dehydrating Agent**: Calcium chloride (CaCl₂) is placed in the guard tube because it is a strong dehydrating agent. This means it has the ability to absorb water vapor from the surrounding environment.

4. **Mechanism of Action**: As hydrogen chloride gas passes through the guard tube containing calcium chloride, the calcium chloride absorbs any moisture present in the gas. This process helps to dry the gas before it is collected or used in further reactions.

5. **Conclusion**: Therefore, the role of calcium chloride in the guard tube is to act as a dehydrating agent, ensuring that the hydrogen chloride gas is free from moisture.

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