Definition of the state principle
For a simple compressible system, values for any two independent intensive properties determine the values of all other intensive properties.
Definition of simple compressible systems
Systems of commonly encountered pure substances are called simple compressible systems.
Description of state from properties
Choice of properties
The intensive state of a simple compressible system at equilibrium is described by its intensive properties, including temperature, pressure, specific volume, density, specific internal energy, and specific enthalpy.
Properties such as velocity and elevation are excluded because their values depend on arbitrary datum choices, such as zero values at the surface of the earth. For the state principle, these properties are not relevant.
Dependence of properties
Not all of the relevant intensive properties are independent.
Some are related by definitions – for example, density is 1/v and specific enthalpy is u + pv (Eq. 1).
Others are related through expressions developed from experimental data.
Some intensive properties may be independent in a single phase, but become dependent when there is more than one phase present.
Application
Among alternative sets of two independent intensive properties, (T, v) and (p, v) are frequently convenient.
Temperature and pressure are not always an independent set.