Structure explains the trend.
All molecules experience dispersion forces. Polar molecules also have dipole interactions, and suitable O–H or N–H groups can form hydrogen bonds. Stronger intermolecular attractions generally require more energy to separate molecules, increasing boiling point.
Within a homologous series, a longer chain generally increases dispersion forces and boiling point. Branching often lowers boiling point among comparable isomers by reducing surface contact. Melting points also depend on crystal packing, so their trends are less simple.
Short alcohols interact strongly with water through hydrogen bonding. As their non-polar carbon chain grows, water solubility tends to fall. Boiling does not normally break the covalent bonds inside molecules.
Spot the functional group
Condensed structural diagrams. Bonds show connectivity, not measured bond angles. Compare the molecular formula with the arrangement of atoms.
Explore: Compare structures and predict how intermolecular forces affect solubility and boiling point.
Explain why ethanol has a higher boiling point than ethane.
- Identify hydrogen bonding between ethanol molecules.
- Relate stronger attraction to greater energy required for vaporisation.
Answer: Ethanol forms intermolecular hydrogen bonds; ethane has only dispersion forces, so more energy is needed to separate ethanol molecules.
Assumed knowledge
Like charges repel; polar bonds can produce partial charges.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Explain boiling point and volatility trends.
- Relate chain length and hydrogen bonding to water solubility.