Water has a significantly higher boiling point than methane and carbon dioxide due to the presence of hydrogen bonding. The boiling point of a substance is directly related to the strength of the intermolecular forces present.

Methane (CH₄) is a nonpolar molecule and only exhibits London Dispersion Forces (LDF). These forces are relatively weak, resulting in a low boiling point (-161 °C).

Carbon dioxide (CO₂) is a nonpolar molecule and also only exhibits London Dispersion Forces (LDF). While CO₂ has a slightly higher molar mass than methane, the LDFs are still relatively weak, leading to a low boiling point (-78 °C).

Water (H₂O) is a polar molecule and exhibits both London Dispersion Forces and, crucially, hydrogen bonding. The oxygen atom is much more electronegative than the hydrogen atoms, creating a significant dipole moment. The hydrogen atoms in water are bonded to the oxygen atom and are therefore partially positive (δ+). This allows for strong hydrogen bonds to form between water molecules. Hydrogen bonds are significantly stronger than LDFs or dipole-dipole forces.

The energy required to break these strong hydrogen bonds is much greater than the energy required to overcome the weaker LDFs in methane and carbon dioxide. Therefore, water has a much higher boiling point (100 °C) than methane and carbon dioxide.

The melting and boiling points of simple molecular compounds are typically low due to the presence of weak intermolecular forces. These compounds are composed of individual molecules held together by relatively weak attractions. Unlike ionic compounds or metals, where strong electrostatic forces are present, the forces between molecules in molecular compounds are significantly weaker.

These weak intermolecular forces arise from the relatively small size and simple structure of the molecules. The primary types of intermolecular forces present are:

• London Dispersion Forces (LDF): These forces are present in all molecules, even nonpolar ones. They arise from temporary, instantaneous fluctuations in electron distribution, creating temporary dipoles. LDFs increase with molar mass and surface area.
• Dipole-Dipole Forces: These forces occur between polar molecules, which have a permanent separation of charge. The positive end of one molecule is attracted to the negative end of another.
• Hydrogen Bonding: This is a special type of dipole-dipole force that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. Hydrogen bonding is particularly strong.

Because these intermolecular forces are weak, only a small amount of energy is required to overcome them and change the state of the substance from solid to liquid (melting) or liquid to gas (boiling). Therefore, simple molecular compounds have low melting and boiling points.

Substances with only covalent bonds (molecular compounds) typically have lower melting and boiling points than ionic compounds because the intermolecular forces between the molecules are weaker. In covalent compounds, the intermolecular forces are generally weaker than the electrostatic forces between ions in ionic compounds. These intermolecular forces include:

• London Dispersion Forces (LDF): Present in all molecules, these forces arise from temporary fluctuations in electron distribution. They are stronger in larger molecules with more electrons.
• Dipole-Dipole Forces: Present in polar covalent molecules, these forces arise from the attraction between the positive end of one polar molecule and the negative end of another.

These intermolecular forces are relatively weak compared to the strong electrostatic forces between ions in ionic compounds. Therefore, less energy is required to overcome these weaker intermolecular forces and change the state of the substance from solid to liquid or liquid to gas, resulting in lower melting and boiling points.