Halogen acid used in the preparation of aqua regia is ………….

Answer

To solve the question regarding the halogen acid used in the preparation of aqua regia, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Aqua Regia**: Aqua regia is a highly corrosive mixture of acids that is capable of dissolving noble metals such as gold and platinum. It is known for its ability to dissolve metals that are typically resistant to other acids.

2. **Identify the Components of Aqua Regia**: Aqua regia is prepared by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) in a specific ratio. The common ratio used is 3 parts of hydrochloric acid to 1 part of nitric acid.

3. **Recognize the Halogen Acid**: Among the acids used in the preparation of aqua regia, hydrochloric acid (HCl) is a halogen acid. Halogen acids are acids that contain a halogen atom, and hydrochloric acid is one of the most common examples.

4. **Conclusion**: Since hydrochloric acid is the halogen acid used in the preparation of aqua regia, we can conclude that the answer to the question is hydrochloric acid (HCl).

### Final Answer:
The halogen acid used in the preparation of aqua regia is hydrochloric acid (HCl).

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