Particulate Models of Solids, Liquids, and Gases

A particulate model is just a drawing of what the particles in a substance are doing: how far apart they sit and how much they move. In a solid, particles are packed close and locked into fixed positions, only vibrating in place, so a solid holds both its shape and its volume. In a liquid, particles are still close together, close enough that you can't squeeze the liquid much, but they can slide past each other, so a liquid keeps a fixed volume while flowing to match the shape of its container. In a gas, particles are far apart compared to their own size, moving fast in straight lines until they collide, with almost no attraction holding them together. That is why a gas expands to fill any container, taking on neither a fixed shape nor a fixed volume.

Crystalline solids, like salt or quartz, arrange their particles in a repeating, orderly lattice, so they have sharp melting points because every particle needs roughly the same amount of energy to break free. Amorphous solids, like glass or rubber, have particles frozen in a disordered, liquid-like jumble, so they soften gradually over a range of temperatures instead of melting sharply.

Heating a solid adds energy that first makes particles vibrate harder without changing arrangement, then at the melting point gives them enough energy to slide past each other as a liquid, and at the boiling point gives them enough energy to escape attraction entirely and spread out as a gas. This particle picture is the foundation for the gas laws and intermolecular force ideas that follow: how far apart particles sit and how strongly they attract each other is exactly what determines pressure, compressibility, and phase behavior.