Chapter 18: Thermal Properties of Matter

The kinetic-molecular model provides a theoretical framework for understanding gas pressure as resulting from molecular collisions, leading to key relationships between temperature and molecular kinetic energy through the Boltzmann constant. Students learn to calculate root-mean-square speeds and understand how molecular mass affects gas behavior. The chapter extensively covers heat capacities using the equipartition theorem, explaining why monatomic gases have different heat capacities than diatomic gases and how rotational and vibrational energy modes contribute to thermal energy storage. Maxwell-Boltzmann speed distributions reveal how molecular speeds vary within a gas sample, with most probable, average, and root-mean-square speeds providing different statistical measures of molecular motion. This distribution explains phenomena like evaporation and effusion. The material concludes with phase behavior, introducing phase diagrams that map solid, liquid, and gas regions as functions of pressure and temperature. Critical concepts include the triple point where all phases coexist, the critical point marking the end of liquid-vapor distinction, and direct phase transitions like sublimation. Throughout, the chapter emphasizes how molecular-level interactions and motion determine bulk thermal properties.