Which of the following metals produce `H_(2)` gas on reaction with cold water?

Options

A. Hg
B. Sn
C. Al
D. Ca

Explanation

To determine which of the given metals produce hydrogen gas (H₂) when they react with cold water, we can analyze each option step by step.

### Step 1: Analyze each metal's reactivity with water

1. **Mercury (Hg)**:
- Mercury does not react with cold water. It is a heavy metal and is generally unreactive with water under normal conditions.
- **Hint**: Consider the reactivity series of metals; mercury is low on the list.

2. **Tin (Sn)**:
- Tin also does not react with cold water. It is a relatively stable metal and does not readily react with water at room temperature.
- **Hint**: Think about the common uses of tin; it is often used in coatings and does not corrode easily.

3. **Aluminium (Al)**:
- Aluminium can react with water, but the reaction is very slow due to the protective oxide layer that forms on its surface. However, when the oxide layer is removed or if the water is hot, aluminium can react to produce hydrogen gas.
- **Hint**: Remember that aluminium requires certain conditions to react, such as heat or the removal of its oxide layer.

4. **Calcium (Ca)**:
- Calcium reacts readily with cold water to form calcium hydroxide (Ca(OH)₂) and liberates hydrogen gas (H₂). The reaction can be represented as:
\[ \text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \]
- **Hint**: Consider the position of calcium in the reactivity series; it is quite reactive with water.

### Step 2: Conclusion
Based on the analysis, the only metal from the options provided that produces hydrogen gas when reacting with cold water is **Calcium (Ca)**.

### Final Answer:
**Option D: Calcium** is the correct answer as it produces hydrogen gas on reaction with cold water.