Which of the following melts below 298 k.

Options

A. `NaCl_((s))`
B. `Si_((s))`
C. `Ar_((s))`
D. `Na_((s))`

Answer

To determine which of the given compounds melts below 298 K (25 °C), we will analyze each option based on their types of solids and their melting points.

### Step-by-Step Solution:

1. **Identify the Options**:
We have four compounds to analyze:
- NaCl (Sodium Chloride)
- Silica (SiO2)
- Argon (Ar)
- Sodium (Na)

2. **Classify the Types of Solids**:
- **NaCl**: This is an ionic solid. Ionic solids typically have moderate to high melting points due to the strong electrostatic forces between the ions.
- **Silica (SiO2)**: This is a covalent solid. Covalent solids generally have very high melting points because of the strong covalent bonds throughout the structure.
- **Argon (Ar)**: This is a molecular solid (or can be considered as a noble gas). Molecular solids usually have low melting points due to weak intermolecular forces (like Van der Waals forces).
- **Sodium (Na)**: This is a metallic solid. Metallic solids can have a range of melting points, but sodium has a relatively low melting point compared to other metals.

3. **Determine Melting Points**:
- **NaCl**: Melting point is approximately 801 K (528 °C) – does not melt below 298 K.
- **Silica (SiO2)**: Melting point is approximately 1713 K (1440 °C) – does not melt below 298 K.
- **Argon (Ar)**: Melting point is approximately 83.8 K (−189.3 °C) – melts below 298 K.
- **Sodium (Na)**: Melting point is approximately 371 K (98 °C) – does not melt below 298 K.

4. **Conclusion**:
Among the options, only Argon (Ar) has a melting point below 298 K. Therefore, the correct answer is Argon.

### Final Answer:
**Argon (Ar)** is the only compound that melts below 298 K.

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