Which of the following evolve hydrogen on reacting with cold dilute nitric acid ?

Options

A. Mg
B. Al
C. Fe
D. Cu

Explanation

To determine which of the given metals evolve hydrogen gas when reacting with cold dilute nitric acid (HNO3), we can analyze each metal's reaction with the acid.

### Step-by-Step Solution:

1. **Identify the Metals**: The metals given in the options are Magnesium (Mg), Aluminum (Al), Iron (Fe), and Copper (Cu).

2. **Reaction of Magnesium with Cold Dilute Nitric Acid**:
- The reaction can be written as:
\[ \text{Mg} + 2 \text{HNO}_3 \rightarrow \text{Mg(NO}_3)_2 + \text{H}_2 \]
- Here, magnesium reacts with nitric acid to produce magnesium nitrate and hydrogen gas. Therefore, magnesium evolves hydrogen gas.

3. **Reaction of Aluminum with Cold Dilute Nitric Acid**:
- The reaction can be written as:
\[ 8 \text{Al} + 4 \text{HNO}_3 \rightarrow 8 \text{Al(NO}_3)_3 + 3 \text{N}_2\text{O} + 15 \text{H}_2\text{O} \]
- In this case, aluminum reacts with nitric acid but does not produce hydrogen gas; instead, it forms water and nitrogen oxides.

4. **Reaction of Iron with Cold Dilute Nitric Acid**:
- The reaction can be written as:
\[ 4 \text{Fe} + 10 \text{HNO}_3 \rightarrow 4 \text{Fe(NO}_3)_3 + 3 \text{NO} + 5 \text{H}_2\text{O} \]
- Similar to aluminum, iron reacts with nitric acid to produce iron nitrate and water, without evolving hydrogen gas.

5. **Reaction of Copper with Cold Dilute Nitric Acid**:
- The reaction can be written as:
\[ 4 \text{Cu} + 10 \text{HNO}_3 \rightarrow 4 \text{Cu(NO}_3)_2 + 2 \text{NO}_2 + 2 \text{H}_2\text{O} \]
- Copper also does not evolve hydrogen gas when reacting with nitric acid; it produces copper nitrate, nitrogen dioxide, and water.

6. **Conclusion**: From the analysis, only magnesium evolves hydrogen gas when reacting with cold dilute nitric acid. Therefore, the correct answer is **Magnesium (Mg)**.

### Final Answer:
**Magnesium (Mg)** is the metal that evolves hydrogen gas when reacting with cold dilute nitric acid.