Physical Classification of Matter

Matter exists in three familiar physical states -- solid, liquid and gas -- depending on how tightly its particles are held together. A solid holds a fixed shape and volume; a liquid has a fixed volume but takes the shape of its container; a gas has neither a fixed shape nor a fixed volume.

Crucially, these are not permanent labels for a substance -- they describe its condition. By changing temperature and pressure suitably, the same substance can be converted from one physical state to another: ice (solid) can be melted to water (liquid) and boiled to steam (gas), and the reverse changes (condensation, freezing) work the same way. This is …