The molecules water (H2O) and methane (CH4) have similar masses, but methane changes from a gas to liquid at -161 degrees C. Water becomes a gas at 100 degrees C. what could account for the difference?

Think about hydrogen bonding. Present in water. Essentially non-existent in CH4.

The difference in boiling points between water and methane can be explained by examining their molecular structures and intermolecular forces.

Water molecules are polar, which means they have a positive charge on the hydrogen atom and a negative charge on the oxygen atom. This creates permanent dipole moments within the water molecule. In the liquid phase, water molecules form hydrogen bonds with neighboring water molecules. Hydrogen bonds are strong intermolecular forces that require significant energy to break, hence requiring a higher temperature to convert water from a liquid to a gas.

On the other hand, methane molecules are nonpolar, as the carbon atom shares equal electronegativity with the hydrogen atoms. Methane molecules do not form strong intermolecular forces like hydrogen bonds. Instead, they exhibit weaker London dispersion forces, which arise due to temporary fluctuations in electron distribution within the molecule. These forces are relatively weaker than hydrogen bonds, requiring less energy to overcome and thus resulting in a lower boiling point for methane.

In summary, the difference in boiling points between water and methane can be attributed to the presence of hydrogen bonding in water and the weaker London dispersion forces in methane. The polar nature and ability to form hydrogen bonds in water leads to stronger intermolecular attractions, thereby requiring a higher temperature to convert water to a gas compared to methane.