Acid used in formation of `H_(2)` gas?

Options

A. Concentrated `H_(2)SO_(4)`
B. Dilute `H_(3)PO_(4)`
C. Dilute `H_(2)CO_(3)`
D. Dilute `HCl`

Answer

To determine the acid used in the formation of hydrogen gas (H₂), we can analyze the options provided and understand the reactions involved.

### Step-by-Step Solution:

1. **Identify the Acids**: The question provides four options which are different acids. We need to evaluate each one to see if it can produce hydrogen gas when reacted with a suitable metal.

2. **Evaluate Dilute Hydrochloric Acid (HCl)**:
- Dilute HCl is a strong acid that can react with metals like zinc (Zn) or magnesium (Mg).
- The reaction can be represented as:
\[ \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \uparrow \]
- This reaction produces hydrogen gas, indicating that dilute HCl can indeed be used to form H₂.

3. **Evaluate Concentrated Sulfuric Acid (H₂SO₄)**:
- Concentrated H₂SO₄ is a strong acid but it does not produce hydrogen gas when reacting with metals. Instead, it produces sulfur dioxide (SO₂).
- Therefore, concentrated H₂SO₄ cannot be the correct answer.

4. **Evaluate Dilute Phosphoric Acid (H₃PO₄)**:
- Dilute H₃PO₄ is a weak acid and does not readily react with metals to produce hydrogen gas.
- Thus, it cannot be the correct answer.

5. **Evaluate Dilute Carbonic Acid (H₂CO₃)**:
- Dilute H₂CO₃ is also a weak acid and does not produce hydrogen gas when reacting with metals.
- Therefore, this option is also incorrect.

6. **Conclusion**: After evaluating all options, we find that only dilute hydrochloric acid (HCl) is capable of producing hydrogen gas when reacted with metals. Therefore, the correct answer is option D.

### Final Answer:
The acid used in the formation of H₂ gas is **dilute HCl** (option D).

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