This lecture introduces the ideal gas model, explaining how the physical state of a gas changes when its volume is reduced, leading to liquefaction. The lecture covers the assumptions of the model, such as non-interacting molecules and point-like volume. It delves into the concept of pressure, volume, and temperature relationships in gases, deriving the ideal gas law PV = NkT. The presentation also discusses Avogadro's number, the constant of Boltzmann, and the macroscopic effects of gases. Finally, it demonstrates how the ideal gas law is a consequence of Newton's second law by analyzing the variation of momentum in a closed gas system.