Separate molecules without breaking them.

All atoms and molecules experience dispersion attractions from fluctuating electron distributions. Polar molecules also have dipole–dipole attractions. Hydrogen bonding occurs when H bonded to N, O or F interacts with a suitable lone pair on another species.

Stronger attractions generally mean more energy is needed to vaporise a substance, increasing boiling point and reducing vapour pressure at the same temperature. Molecular size and shape also matter. Ice’s open hydrogen-bonded structure explains why ordinary ice is less dense than liquid water.