The substance that has the lowest boililng point is:

Options

A. `HCl`
B. `H_(2)S`
C. `PH_(3)`
D. `SiH_(4)`

Answer

To determine which substance has the lowest boiling point among the given options (HCl, H2S, PH3, and SiH4), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Compounds and Their Properties**:
- The compounds given are:
- HCl (Hydrochloric acid)
- H2S (Hydrogen sulfide)
- PH3 (Phosphine)
- SiH4 (Silane)

2. **Understand the Nature of Each Compound**:
- HCl is a polar molecule and exhibits dipole-dipole interactions.
- H2S is also a polar molecule with dipole-dipole interactions.
- PH3 is a polar molecule but has weaker dipole-dipole interactions compared to HCl and H2S.
- SiH4 is a non-polar molecule and primarily exhibits weaker London dispersion forces.

3. **Compare the Types of Intermolecular Forces**:
- Polar molecules (HCl, H2S, PH3) have stronger dipole-dipole interactions compared to the London dispersion forces in non-polar molecules (SiH4).
- The stronger the intermolecular forces, the higher the boiling point.

4. **Research the Boiling Points**:
- HCl: 188 K
- H2S: 216 K
- PH3: 185 K
- SiH4: 161 K

5. **Analyze the Boiling Points**:
- From the boiling points, SiH4 has the lowest boiling point at 161 K.
- The other compounds (HCl, H2S, and PH3) have higher boiling points due to stronger intermolecular forces.

6. **Conclusion**:
- Therefore, the substance with the lowest boiling point among the given options is SiH4.

### Final Answer:
The substance that has the lowest boiling point is **SiH4** (Option 4).

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