States, Phases, and Physical Changes

Aside from gases, solids and liquids are two other common states of matter, states of aggregation, for ensembles of molecular entities (atoms or molecules). Compared to gases, particles occupy less than 0.1% of the volume of a gas, solids and liquids are condensed phases, where atoms occupy 57% and 64% of the volume, respectively. The particles in a solid and in a liquid interact, through intermolecular forces, and they differ in the degree of order (high in a solid, low in a liquid) and the degree of motion (low in a solid, high in a liquid). In this chapter, readers are introduced to the terminology and energetics of phase changes. The impact of temperature and pressure in exerting physical changes, phase changes, is discussed along with the use of pressure-temperature diagrams to represent these relationships.