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Learn9701Paper 2 · AS Level Structured QuestionsChemical bondingIntermolecular forces
Chemical bonding · Subtopic 3 of 3

Intermolecular Forces

London (instantaneous dipole–induced dipole) forces exist between all molecules; they grow with the number of electrons and surface area. Permanent dipole–dipole forces act between polar molecules. Hydrogen bonds form when H is bonded to N, O or F and is attracted to a lone pair on N, O or F of another molecule; they are the strongest intermolecular force. Hydrogen bonding explains the high boiling point of water, NH₃ and HF, and why ice is less dense than water. Stronger intermolecular forces mean higher boiling points.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH12AQA 7405 Paper 1≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe London forces, permanent dipole forces and hydrogen bonds.
  • Explain trends in boiling points.
  • Explain the anomalous properties of water.
  • Explain solubility in terms of intermolecular forces.
Pause and think

H₂S boils at −60 °C, but H₂O at +100 °C, even though H₂S is heavier. Why?

Section 2 of 14

Core idea

Start hereWhy water is weird

By the trend in its group, water should boil at about −80 °C. It boils at +100 °C. Hydrogen bonds make Earth's oceans possible.

The big idea

Molecules attract each other through London forces (all), permanent dipole forces (polar) and hydrogen bonds (H on N, O, F). Stronger forces mean higher boiling points.

1. Types of force

Which forces are present?

Work down the checklist.

Worked exampleOrdering boiling points

Order by boiling point: propane, ethanol, methoxymethane (all MrM_r 44–46).

Goal
Order and reasons.
Strategy
Identify strongest force in each.
  1. propane: London only\text{propane: London only}
    Non-polar.
  2. methoxymethane: + dipole–dipole\text{methoxymethane: + dipole–dipole}
    Polar C–O, no O–H.
  3. ethanol: + hydrogen bonds\text{ethanol: + hydrogen bonds}
    O–H group.
AnswerPropane < methoxymethane < ethanol.
✓ Checkbp: −42, −24, 78 °C ✓.
Common misconceptionBonds vs forces Boiling separates molecules by overcoming intermolecular forces. Covalent bonds inside the molecules stay intact.
Pause and think

Why does iodine dissolve in hexane but not much in water?

Summary

  • London < dipole–dipole < H-bond.
  • Electrons and shape affect London forces.
  • Water's anomalies.
Key words
London forces
Instantaneous dipole–induced dipole attractions.
Hydrogen bond
Attraction between H on N, O or F and a lone pair on N, O or F.
Permanent dipole
Fixed charge separation in a polar molecule.
HistoryFritz London. In 1930 Fritz London used quantum mechanics to explain why even noble gas atoms attract each other weakly; these forces now carry his name.
Fun factGecko feet. Geckos climb glass ceilings using London forces between billions of tiny hairs on their toes and the surface.
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