Which of the following reaction produces hydrogen ?

Options

A. `Mg+H_(2)O`
B. `H_(2)S_(2)O_(7)+H_(2)O `
C. `BaO_(2)+HCl `
D. `Na_(2)O_(2) + HCl `

Explanation

To determine which of the given reactions produces hydrogen gas (H2), we will analyze each reaction step by step.

### Step-by-Step Solution:

1. **Analyze the First Reaction:**
- Reaction: \( \text{Mg} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2 \)
- Balancing: \( \text{Mg} + 2 \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2 \)
- **Conclusion:** This reaction produces hydrogen gas (H2). So, this option is correct.

2. **Analyze the Second Reaction:**
- Reaction: \( \text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \)
- Balancing: \( \text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2 \text{H}_2\text{SO}_4 \)
- **Conclusion:** This reaction does not produce hydrogen gas. So, this option is incorrect.

3. **Analyze the Third Reaction:**
- Reaction: \( \text{BaO}_2 + \text{HCl} \rightarrow \text{BaCl}_2 + \text{H}_2\text{O}_2 \)
- Balancing: \( \text{BaO}_2 + 2 \text{HCl} \rightarrow \text{BaCl}_2 + \text{H}_2\text{O}_2 \)
- **Conclusion:** This reaction does not produce hydrogen gas. So, this option is incorrect.

4. **Analyze the Fourth Reaction:**
- Reaction: \( \text{Na}_2\text{O}_2 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}_2 \)
- Balancing: \( \text{Na}_2\text{O}_2 + 2 \text{HCl} \rightarrow 2 \text{NaCl} + \text{H}_2\text{O}_2 \)
- **Conclusion:** This reaction does not produce hydrogen gas. So, this option is incorrect.

### Final Conclusion:
The only reaction that produces hydrogen gas is the **first reaction**: \( \text{Mg} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2 \).