Intermolecular forces
Within a molecule the covalent bonds are strong; between molecules the forces are much weaker, and those weaker intermolecular forces decide melting points, boiling points and whether something is a gas, liquid or solid at room temperature.
There are three, in rising strength. London dispersion forces (a type of van der Waals force) exist between all molecules, from momentary uneven electron distributions; they grow with molecular size. Dipole–dipole forces act between polar molecules. Hydrogen bonding — the strongest — occurs when H is bonded to N, O or F; it is why water has such a high boiling point for its size.
Why boiling point tracks the forces
Boiling separates molecules from one another, so it must overcome the intermolecular forces (not the covalent bonds). The stronger those forces, the more energy needed, and the higher the boiling point. This is why larger molecules (more dispersion force) boil higher than small ones, and why hydrogen-bonded liquids like water and ethanol boil far above what their size alone would predict.
Water's anomaly, explained
Water (M = 18) boils at 100 °C while methane (M = 16) boils at −161 °C. Almost the same size, wildly different boiling points — because water hydrogen-bonds and methane only has weak dispersion forces. Intermolecular forces, not molecular mass, are the deciding factor.
The solid state
Solids are either crystalline (a regular, repeating lattice — a sharp melting point) or amorphous (disordered, like glass — softening over a range). Crystalline solids come in four types, each defined by what holds the lattice together:
| Type | Held by | Properties |
|---|---|---|
| Ionic | electrostatic ion attraction | hard, brittle, high mp, conduct when molten |
| Molecular | intermolecular forces | soft, low mp, non-conducting (e.g. ice, iodine) |
| Covalent network | a lattice of covalent bonds | very hard, very high mp (diamond, quartz) |
| Metallic | a sea of delocalised electrons | conducting, malleable, variable mp |
A worked example
Ammonia (NH₃) boils at −33 °C but phosphine (PH₃), a bigger molecule, boils at −88 °C. Why does the smaller molecule boil higher?
- Boiling point depends on the intermolecular forces that must be overcome.Boiling separates molecules, not atoms within them.
- NH₃ has N–H bonds, so it hydrogen-bonds; PH₃ cannot (P is not N, O or F), so it has only weaker dipole/dispersion forces.Hydrogen bonding needs H bonded to N, O or F.
- The hydrogen bonds in ammonia are stronger, so more energy is needed to boil it — despite it being smaller.Stronger intermolecular forces raise the boiling point.
Ammonia hydrogen-bonds and phosphine does not, so ammonia's stronger intermolecular forces give it the higher boiling point even though it is the smaller molecule.
Hydrogen bonding and its consequences
When hydrogen is bonded to a small, very electronegative atom — nitrogen, oxygen or fluorine — the resulting dipole is strong enough to attract a lone pair on a neighbouring molecule. This hydrogen bond is the strongest of the intermolecular forces, and it explains a string of anomalies: water and HF boil far higher than their size suggests, alcohols mix freely with water, and — uniquely — ice is less dense than liquid water.
Ice floats because hydrogen bonds hold the molecules in an open, ordered lattice with more empty space than the liquid — which is why lakes freeze from the top down, letting life survive beneath.
Vapour pressure and boiling
In a sealed container some liquid molecules escape into the vapour while others return; at balance this sets the liquid's vapour pressure. Heating gives more molecules the energy to escape, so the vapour pressure rises with temperature. A liquid boils when its vapour pressure equals the external (atmospheric) pressure — which is why water boils below 100 °C on a mountain, where the air pressure is lower.
Boiling is a pressure match
Boiling point is not fixed — it is the temperature at which vapour pressure equals the surrounding pressure. Lower the pressure and the boiling point falls; raise it (as in a pressure cooker) and it rises.