Intermolecular Forces vs Thermal Interactions

A tug-of-war between attraction and motion

We have now met the two opposing tendencies that decide how matter behaves: intermolecular forces pull molecules together, while thermal energy keeps them apart by keeping them in constant, energetic motion. The three states of matter — gas, liquid, solid — are simply the outcome of this balance between molecular attraction and thermal energy.

Why weakly-interacting molecules stay gaseous

When the forces between molecules are very weak, the molecules will not stick together to form a liquid or solid unless their thermal energy is first reduced by cooling. This is why gases generally do not liquefy on compression alone: even though squeezing a gas brings its molecules much closer together — letting intermolecular attraction act at its strongest — the molecules still carry enough thermal energy to keep breaking free. Only once the temperature is also lowered, cutting down that thermal energy, can such gases be liquefied easily.

The trend across the three states

Moving from gas to liquid to solid, intermolecular interactions increasingly dominate over thermal energy; moving the other way, from solid to liquid to gas, thermal energy increasingly dominates over intermolecular interactions:

$\text{Gas} \;\rightarrow\; \text{Liquid} \;\rightarrow\; \text{Solid}\qquad(\text{predominance of intermolecular interactions})$ …