Internal Structure of Solids, Liquids, and Gases

A solid holds its shape because its molecules sit locked in fixed positions, vibrating in place but not wandering, bound tightly by strong intermolecular forces. A liquid's molecules are packed almost as closely but can slide past one another, so the material flows and fills the bottom of whatever container holds it rather than keeping a fixed shape. That sliding is possible because thermal energy gives liquid molecules just enough freedom to break and reform contacts with their neighbors constantly, while still staying close enough that the attractive forces between them never fully let go.

A gas is not just a liquid stretched thinner. The real difference is qualitative, not just a matter of spacing. Gas molecules are spaced so far apart, compared to the short range of intermolecular forces, that they barely feel each other at all. They zip along in straight lines until they happen to collide, then bounce off and go straight again, essentially independent travelers rather than a connected crowd. Liquid molecules, by contrast, are always touching, always interacting, never free of their neighbors' pull.

This structural picture explains compressibility directly. A liquid's molecules are already packed close to the limit the intermolecular forces allow, so squeezing them closer takes enormous force and barely changes the volume. A gas has huge empty gaps between its far-flung molecules, so squeezing it just pushes that empty space out, shrinking the volume easily. Melting and boiling trace this same story: ice's rigid lattice loosens into a liquid's mobile but still-crowded molecules, then boiling flings those molecules apart entirely into the sparse, nearly noninteracting arrangement of a gas. This microscopic picture sets up how pressure and density get defined for fluids next.