This lecture covers the concept of liquid-vapor equilibrium, focusing on the relationship between volume, pressure, and temperature in a fixed system. It explains how the volume occupied by a gas changes as temperature increases, leading to the transition from liquid to gas state. The lecture also discusses diagrams illustrating the behavior of gases and liquids at different pressures and temperatures, emphasizing the critical point and saturation conditions. Additionally, it explores molar volume variations and the average volume of gases in equilibrium.