Introduction to state postulate

Since every thermodynamic system contains some matter with energy in its various forms, the system can be completely described by specifying the following variables.

• The composition of the matter in terms of mole numbers of each constituent.
• The energy of the system.
• The volume of the system, and
• The measurable properties, such as pressure and temperature.

By specifying these quantities, the state of the system is defined. Once the system is in a given state, it possesses a unique state of properties like pressure, P , temperature, T , density, etc. All the properties of a system cannot be varied independently since they are interrelated through expressions of the following type

For example, the pressure, temperature and molar volume ( ) of an ideal gas are related by the expression P = RT . Here R is a constant. Only two of the three variables P , and T can be varied independently. Question is that for a given thermodynamics system, how many variables can be varied independently.