Which one has the lowest boiling point ?

Options

A. `H_2 O`
B. `H_2 S`
C. `H_2 Se`
D. `KNO_3`

Answer

To determine which compound has the lowest boiling point among the given options, we can analyze the boiling points of each compound based on their bonding types and intermolecular forces.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We need to identify the compounds in the question. The options include potassium nitrate (KNO3), water (H2O), hydrogen selenide (H2Se), and hydrogen sulfide (H2S).

2. **Classify the Compounds**:
- **Potassium Nitrate (KNO3)**: This is an ionic compound.
- **Water (H2O)**: This is a covalent compound with strong hydrogen bonding.
- **Hydrogen Selenide (H2Se)**: This is a covalent compound with weaker Van der Waals forces.
- **Hydrogen Sulfide (H2S)**: This is also a covalent compound with even weaker Van der Waals forces compared to H2Se.

3. **Analyze Boiling Points**:
- **Ionic Compounds**: Generally have high boiling points due to strong electrostatic forces. KNO3 will have the highest boiling point.
- **Water (H2O)**: Has a high boiling point (100°C) due to strong hydrogen bonds.
- **Hydrogen Selenide (H2Se)**: Has a boiling point of approximately -41.25°C, which is higher than H2S.
- **Hydrogen Sulfide (H2S)**: Has a boiling point of approximately -60°C, which is lower than H2Se.

4. **Compare the Boiling Points**:
- KNO3: High boiling point (ionic compound)
- H2O: High boiling point (due to hydrogen bonding)
- H2Se: Boiling point of -41.25°C
- H2S: Boiling point of -60°C

5. **Conclusion**:
From the analysis, hydrogen sulfide (H2S) has the lowest boiling point among the given compounds.

### Final Answer:
The compound with the lowest boiling point is **hydrogen sulfide (H2S)**.