Intermolecular forces and physical properties

The three types

• London forces: between all molecules; an instantaneous dipole induces a dipole in a neighbour. Stronger with more electrons and greater surface contact.
• Permanent dipole-dipole: between polar molecules, in addition to London forces.
• Hydrogen bonds: when H is bonded to N, O or F; the δ+ H is attracted to a lone pair on N, O or F of another molecule. The strongest intermolecular force. Draw the lone pair and use a dashed line; the O-H...O arrangement is close to 180°.

Boiling point trends

• Alkanes: boiling point rises with chain length (more electrons); branched isomers boil lower (less surface contact).
• Alcohols boil much higher than alkanes of similar Mr because of hydrogen bonding.
• Hydrogen halides: HCl < HBr < HI because London forces increase with electrons; HF is highest because of hydrogen bonding.

Water's anomalies

Hydrogen bonding gives water an unusually high melting and boiling point for its size, and makes ice less dense than liquid water: in ice the molecules are held in an open lattice by hydrogen bonds.

Solubility

• Ionic compounds dissolve in water because the ions are hydrated: water's δ- O is attracted to cations and δ+ H to anions.
• Alcohols dissolve because they hydrogen bond with water (solubility falls as the hydrocarbon chain grows).
• Halogenoalkanes are barely soluble: they cannot hydrogen bond with water strongly enough to replace water's own hydrogen bonds.
• Non-polar substances dissolve in non-polar solvents, where similar London forces form.

Worked example

Explain why pentane (bp 36 °C) boils higher than 2,2-dimethylpropane (bp 10 °C).

1. Both are C5H12 with the same number of electrons.
2. Pentane is a long chain with more surface contact between molecules.
3. 2,2-dimethylpropane is nearly spherical with less contact.

Answer: Pentane has stronger London forces, so more energy is needed to separate its molecules.

Check yourself

1. Which substance forms hydrogen bonds between its molecules?
   • NH3
   • CH4
   • HCl
   • CH3Cl
   Show answer

   NH3. Hydrogen bonding needs H bonded directly to N, O or F.

2. Which hydrogen halide has the highest boiling point?
   • HCl
   • HBr
   • HF
   • HI
   Show answer

   HF. HF molecules form hydrogen bonds, which outweigh the greater London forces of HI.

3. Why is ice less dense than liquid water?
   • Hydrogen bonds hold the molecules in an open lattice
   • Ice contains air bubbles
   • Covalent bonds lengthen on freezing
   • Water molecules get smaller in the liquid
   Show answer

   Hydrogen bonds hold the molecules in an open lattice. The open, hydrogen-bonded structure of ice has more space between molecules.

4. Which isomer of C6H14 has the lowest boiling point?
   • Hexane
   • 2-methylpentane
   • 3-methylpentane
   • 2,2-dimethylbutane
   Show answer

   2,2-dimethylbutane. The most branched isomer has the least surface contact and weakest London forces.

5. What causes London forces?
   • Permanent charges on ions
   • Instantaneous dipoles inducing dipoles in neighbouring molecules
   • Shared electron pairs
   • Hydrogen bonded to oxygen
   Show answer

   Instantaneous dipoles inducing dipoles in neighbouring molecules. Electron density moves, creating a temporary dipole that induces another.

6. Why is 1-chlorobutane almost insoluble in water?
   • It is ionic
   • It reacts with water instantly
   • It forms hydrogen bonds with itself
   • It cannot form hydrogen bonds with water strong enough to replace water's hydrogen bonds
   Show answer

   It cannot form hydrogen bonds with water strong enough to replace water's hydrogen bonds. Water molecules prefer to stay hydrogen bonded to each other.

7. Why does ethanol (Mr 46) boil much higher than propane (Mr 44)?
   • Ethanol has many more electrons
   • Ethanol molecules form hydrogen bonds
   • Propane is ionic
   • Ethanol is a giant covalent substance
   Show answer

   Ethanol molecules form hydrogen bonds. Similar Mr means similar London forces; the O-H group adds hydrogen bonding.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.