States of Matter and Particle Models

Objective: Compare particle arrangement and macroscopic shape and volume.

In a solid, particles remain near relatively fixed positions while vibrating or otherwise moving microscopically; a crystalline solid has organized structure, while amorphous solids differ. Liquids flow and take a container’s shape while maintaining approximately fixed volume over modest pressure ranges. Gases spread to fill available volume and are readily compressed because particles are widely separated relative to their sizes. These are models with exceptions and ranges, not claims that solid particles never move or liquids are perfectly incompressible. Plasma is an ionized state found in many high-energy contexts. Macroscopic state depends on temperature, pressure, composition, and intermolecular interactions.

Objective: Relate melting, evaporation, condensation, and sublimation to energy transfer.

Melting changes solid to liquid; freezing reverses it. Vaporization changes liquid to gas, including surface evaporation and bulk boiling under appropriate conditions; condensation reverses gas to liquid. Sublimation changes solid directly to gas under certain conditions. Energy transfer during a phase change alters particle arrangement and interactions, and in an ideal equilibrium transition of a pure substance at fixed pressure, temperature can remain approximately constant. Condensation and freezing generally release energy to surroundings. A phase diagram identifies stable regions at stated pressure and temperature; it does not indicate that new chemical elements form during an ordinary physical phase change.

Objective: Interpret particle collisions and one-variable models.

Gas pressure arises from momentum transfer as particles collide with container walls; in SI it is measured in pascals (Pa = N/m²). In an ideal-gas approximation with fixed amount of gas, pressure, volume, and absolute temperature obey PV = nRT. At fixed temperature and amount, pressure varies inversely with volume; at fixed volume and amount, pressure is proportional to kelvin temperature. Compressing an ideal sample from 4 L to 2 L at constant T doubles pressure. Real gases deviate at conditions where particle size or interactions matter. Use kelvin for proportional temperature calculations rather than treating Celsius zero as absence of thermal energy.