Sure, let's analyze the given options to determine why water ([tex]$H_2O$[/tex]) has a much higher boiling point than methane ([tex]$CH_4$[/tex]), even though both molecules are approximately the same size.
1. Option A: Only metallic bonds exist between methane molecules.
- This option is incorrect because methane, being a simple molecular compound, does not exhibit metallic bonding. Metallic bonds are found in metals and alloys, not in molecular substances like methane.
2. Option B: Only Van der Waals forces exist between water molecules.
- This option is incorrect because water molecules are not held together solely by Van der Waals forces. Water molecules exhibit hydrogen bonding, which is much stronger than Van der Waals forces.
3. Option C: Only metallic bonds exist between water molecules.
- This option is incorrect because water molecules are held together by hydrogen bonds, not metallic bonds. Metallic bonds are characteristic of metals, not molecular compounds like water.
4. Option D: Only Van der Waals forces exist between methane molecules.
- This is the correct option. Methane molecules ([tex]$CH_4$[/tex]) are nonpolar and are held together by weak Van der Waals forces (specifically London dispersion forces). These forces are much weaker than the hydrogen bonds present between water molecules. Hydrogen bonds significantly increase the boiling point of water because they require more energy to break compared to Van der Waals forces.
Therefore, the reason water has a much higher boiling point than methane, despite the similar molecular sizes, is that only Van der Waals forces exist between methane molecules, whereas water molecules are held together by much stronger hydrogen bonds.
Hence, the correct answer is:
D. Only Van der Waals forces exist between methane molecules.
