interparticle attractions

Interparticle attractions are the forces of attraction that hold a substance's particles to one another — and their strength is really what decides whether that substance is a solid, a liquid, or a gas.

• Strong attractions lock particles into fixed positions, close together — that's a solid. Weaker ones let particles drift within a limited space — a liquid. Nearly absent attractions let particles fly off in every direction — a gas.
• These forces are extremely sensitive to distance: even a small increase in the gap between particles makes the attraction drop off sharply, which is why heating a substance — which pushes its particles slightly further apart — is often enough to change its state entirely.
• The same idea explains why some substances melt at everyday temperatures (like ice) while others need enormous heat (like iron): it comes down to how strong their interparticle attractions are to begin with.

Every time you watch ice melt, water boil, or a solid freeze, you're really watching a tug-of-war between interparticle attraction and the particles' own energy of motion.