ABSTRACT
In everyday experiences, matter presents itself in several aggregation states. From a macroscopic point of view, they differ from each other as follows: solids have their own shape and volume; liquids have their own volume but take the shape of the container; and gases take the volume and shape of the container. A thermodynamic state is defined by any of the available characteristic functions or, equivalently, by those equations of state which univocally specify any characteristic function. The ideal-gas equations apply only under special conditions, in particular, that they be at temperatures and pressures far from those at which the gas undergoes a phase transition. These occur precisely because molecules interact with each other, whereas the ideal-gas model assumes no interactions among the gas particles, a circumstance that forbids any phase transition.
