Intermolecular Forces – IGCSE Chemistry Definition and Key Facts

Intermolecular forces are a key concept in the Cambridge 0620 syllabus for explaining the physical properties of simple molecular substances. Candidates who confuse intermolecular forces with covalent bonds consistently lose marks on Paper 4 structure-and-bonding questions.

The 0620 definition

Intermolecular forces are the weak forces of attraction that act between molecules. They are distinct from the strong covalent bonds that hold atoms together within a molecule.

The critical distinction

	Covalent bond	Intermolecular force
What it holds together	Atoms within a molecule	Separate molecules to each other
Strength	Strong	Weak
Broken when substance melts/boils?	No	Yes
Type of force	Shared electron pair	Attraction between molecules

When water boils at 100 °C, the intermolecular forces between H₂O molecules are overcome, allowing molecules to escape as gas. The O–H covalent bonds within each H₂O molecule remain intact — the steam is still H₂O, not separated H and O atoms.

How intermolecular forces explain properties

Low melting and boiling points

Simple molecular substances (water, methane, carbon dioxide, iodine) have low melting and boiling points because the intermolecular forces between molecules are weak and require little energy to overcome.

Substance	Formula	Boiling point	Why
Methane	CH₄	−161 °C	Very weak intermolecular forces; small molecule
Water	H₂O	100 °C	Stronger intermolecular forces (hydrogen bonds)
Iodine	I₂	184 °C	Larger molecule, stronger intermolecular forces

Larger molecules generally have stronger intermolecular forces and therefore higher boiling points. This explains the boiling point trend in the alkanes: methane < ethane < propane < butane.

Do not conduct electricity

Simple molecular substances do not conduct electricity in any state because they have no free ions or delocalised electrons. The molecules are neutral.

Often gases or liquids at room temperature

Because the intermolecular forces are weak, many simple molecular substances are gases (O₂, CO₂, CH₄) or low-boiling liquids (water, ethanol) at room temperature.

Solubility

Many small polar molecules dissolve in water. Non-polar molecules tend to dissolve in non-polar solvents.

Intermolecular forces and boiling point trends

The 0620 syllabus expects candidates to explain boiling point trends using intermolecular forces:

Alkanes (homologous series): As the chain length increases, the molecule is larger and has more surface area. Intermolecular forces become stronger, and the boiling point increases. Methane is a gas; pentane is a liquid; long-chain alkanes are waxy solids.

Halogens (Group VII): F₂ and Cl₂ are gases, Br₂ is a liquid, I₂ is a solid. Intermolecular forces increase as the molecules get larger.

Intermolecular forces vs giant structures

Structure	Forces overcome on melting	Boiling point
Simple molecular	Weak intermolecular forces	Low
Giant ionic	Strong ionic bonds	High
Giant covalent	Strong covalent bonds	Very high
Metallic	Strong metallic bonds	High

Worked exam question

Methane (CH₄) has a boiling point of −161 °C. Silicon dioxide (SiO₂) has a boiling point of 2230 °C. Both contain covalent bonds. Explain this difference. [3]

Methane is a simple molecular substance with weak intermolecular forces between the CH₄ molecules [1]. Only these weak forces need to be overcome when it boils, so the boiling point is low [1]. SiO₂ has a giant covalent structure with strong covalent bonds extending throughout the structure; many strong bonds must be broken, requiring much more energy [1].

Common exam mistakes

1. Saying “the covalent bonds in water are broken when it boils” — only the intermolecular forces between molecules are broken during boiling. The O–H bonds stay intact.
2. Using the term “intermolecular forces” for ionic or metallic compounds — these terms apply specifically to forces between molecules.
3. Not linking boiling point trends to molecular size and intermolecular force strength.