ABSTRACT
The thermodynamic state of a system can be usually defined by four observable properties called variables of state. They are the composition, pressure, volume and temperature. However, in some conditions, it is not necessary to know all these variables to perfectly define it. This is because there may exist an equation which relates them to one another. It is the equation of state. The state principle is a fundamental postulate of thermodynamics. It is a postulate based only on experiment. It leads directly to the concept of an equation of state. It stipulates that for any real system, there exists a positive integer n such that if n intensive thermodynamic state functions are fixed, then all other intensive thermodynamic state functions are fixed. According to the nature of the studied systems, there exist several different state equations.
