Which of the following acts as pickling agent ?

Options

A. `HNO_3`
B. `H_2 SO_4`
C. `HCl`
D. `HNO_2`

Answer

To determine which of the given options acts as a pickling agent, we need to understand the concept of pickling and the role of acids in this process.

### Step-by-Step Solution:

1. **Define Pickling Process**:
- The pickling process involves treating a metal surface with a solution to remove impurities such as stains, rust, and oxides. This is commonly done in metalworking to prepare the surface for further processing or finishing.

**Hint**: Remember that pickling is primarily used to clean metal surfaces.

2. **Identify Pickling Agents**:
- Pickling agents are typically strong acids that can effectively dissolve oxides and contaminants on metal surfaces. Common acids used in pickling include sulfuric acid and hydrochloric acid.

**Hint**: Strong acids are usually effective in removing rust and stains from metals.

3. **Analyze Given Options**:
- The options given are:
1. Nitric Acid (HNO3)
2. Sulfuric Acid (H2SO4)
3. Hydrochloric Acid (HCl)
4. Nitrous Acid (HNO2)

4. **Evaluate Each Option**:
- **Nitric Acid (HNO3)**: While it can be used for some metal treatments, it is not commonly used as a pickling agent due to its oxidizing properties.
- **Sulfuric Acid (H2SO4)**: This is a strong acid and is widely used in pickling processes to remove rust and impurities from metals.
- **Hydrochloric Acid (HCl)**: This acid is also used in pickling, particularly for iron and steel, but is less common than sulfuric acid in industrial applications.
- **Nitrous Acid (HNO2)**: This acid is not typically used as a pickling agent.

5. **Conclusion**:
- Based on the analysis, the most effective and commonly used pickling agent among the options is **Sulfuric Acid (H2SO4)**.

### Final Answer:
The correct option that acts as a pickling agent is **Sulfuric Acid (H2SO4)**.

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