Which of the following gas always shows positive deviation from ideal gas behaviour ?

Options

A. `H_(2)`
B. `CO_(2)`
C. `NH_(3)`
D. `CH_(4)`

Answer

To determine which gas always shows positive deviation from ideal gas behavior, we need to understand the concept of the compressibility factor (Z).

### Step-by-Step Solution:

1. **Understanding Ideal Gas Behavior**:
- An ideal gas follows the ideal gas law, represented as PV = nRT. For ideal gases, the compressibility factor (Z) is equal to 1.
- When Z = 1, the gas behaves ideally.

2. **Positive and Negative Deviations**:
- If Z > 1, the gas shows positive deviation from ideal behavior. This means the gas occupies more volume than predicted by the ideal gas law.
- If Z < 1, the gas shows negative deviation from ideal behavior, meaning it occupies less volume than predicted.

3. **Identifying Gases with Positive Deviation**:
- We need to identify which gases consistently exhibit Z > 1. Common gases that show positive deviation include hydrogen (H2) and helium (He).
- In the options provided (H2, CO2, NH3, CH4), we need to analyze their behavior.

4. **Analyzing the Given Gases**:
- **H2 (Hydrogen)**: Known to show positive deviation due to weak intermolecular forces.
- **CO2 (Carbon Dioxide)**: Typically shows negative deviation at high pressures due to strong intermolecular attractions.
- **NH3 (Ammonia)**: Exhibits strong hydrogen bonding, leading to negative deviation.
- **CH4 (Methane)**: Generally behaves closer to ideal but can show deviations depending on conditions.

5. **Conclusion**:
- Among the gases listed, hydrogen (H2) is the one that consistently shows positive deviation from ideal gas behavior.

### Final Answer:
The gas that always shows positive deviation from ideal gas behavior is **Hydrogen (H2)**.